UV spectral synthesis for iron group elements in A supergiants.

Sylvia R. Becker

Universitätssternwarte München, Germany

UV spectra of A supergiants contain many weak lines from iron group elements. We outline an attempt to synthesize these spectra using the observed lines from the Kurucz lists. The resulting spectra should help to disentangle the weak line background from noise and should thus improve quantitative analyses based on the stronger spectral features. This work is in progress and we expect to be able to present first results at the conference.

CHEMO-SPECTRAL EVOLUTION OF THE MILKY WAY AND OF SPIRAL DISKS

S.Boissier(1), N.Prantzos(1).

(1) Institut d'Astrophysique de Paris, France.

Chemical and spectro-photometric evolution of galaxies are indissociable each one from the other. Actually, a lot of constraints are available for the Milky Way disk (abundances, SFR, gas distribution, color profiles and integrated luminosities in various wavelengths). Moreover, we can also test models against properties of nearby spiral disks, for which photometry profiles, as well as gaseous abundances at several radii and CO and HI distributions are available. These data constrain severely evolutionary models computing in a coherent way the chemical evolution of spirals (abundances, gas and stellar masses, SFR) and the spectro-photometric one (magnitude and color profiles, integrated spectrum).

We shall present a detailed chemo-spectro-photometric evolutionary model and its application to the Milky Way and to several nearby spirals.

Diffuse Warm and Hot Gas in I Zw 18

Dominik J. Bomans

Astronomisches Institut, Ruhr-Universität Bochum, Germany

I will present a deep ROSAT HRI, long-slit echelle data, and archival HST imaging of I Zw 18. The deep HST images reveal loops and filaments of kpc size, some of which were not cataloged before. Combined with the dynamics of the ionized gas as derived from the echelle data, the imaging data give strong evidence for a large-scale (kpc-size) bipolar outflow, perpenticular to the mayor axis of the HI gas of I Zw 18. The ROSAT HRI image resolves for the first time the X-ray morphology of I Zw 18. Since the X-ray emission traces the SW H-alpha loops, we conclude that I Zw 18 is currently venting hot gas and therefore metals into its halo. We will discuss the relevance of our findings to the very low oxygen abundance of the warm ionized gas in I Zw 18.

Oxygen and helium abundance gradients in the Galaxy from photometry of HII regions

J. Caplan¹, L. Deharveng¹, R. Costero² & M. Peña²

¹Observatoire de Marseille
²UNAM, Mexico

A Fabry-Perot spectrophotometer was used to obtain integrated line fluxes of Galactic HII regions through large diaphragms rather than slits. Fluxes of [OII] $\lambda$ 3726 and $\lambda$ 3727, [OIII] $\lambda$ 4363 and $\lambda$ 5007, HeI $\lambda$ 5876, H$\delta$, H$\beta$ and H$\alpha$ were measured in thirty-five HII regions ranging from 6 to 17 kpc in distance from the Galactic centre. This homogeneous set of observations, reduced in a consistent manner, is combined with stellar data to determine the regions' distances, reddening corrections, electron densities and temperatures as well as their ionic abundances O⁺/H$^+\!$, O⁺⁺/H⁺ and He⁺/H$^+\!$. This allows us to discuss our Galaxy's oxygen abundance gradient and also the enrichment of helium relative to oxygen.

Metallicity gradients in the Galactic Disk

Giovanni Carraro¹, Laura Portinari¹ & Yuen K. Ng²

¹ Department of Astronomy, Padova University, Italy
² Astronomical Observatory, Padova, Italy

We discuss observational evidences for the existence of radial and/or vertical metallicity gradients (measured with the index [Fe/H]) inside the galactic disk, and compare them with predictions from chemical evolution models and population synthesis. We conclude that a radial abundance gradients exists amounting to -0.09 dex/kpc, whereas non clear conclusion can be drawn for the existence of a vertical gradient.

Spectrophotometric Analysis of HII Regions in NGC4258

Marcelo Castellanos¹, Angeles I. Diaz¹, Elena Terlevich² and Maria Luisa Garcia Vargas³

¹Depto. de Física Teórica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
²Institute of Astronomy, Madingley Road, Cambridge CB30HA, UK
³GRANTECAN, S.A., C/ Vía Láctea s/n, 38200 La Laguna. Tenerife. Spain

We present a spectrophotometric analysis of nine HII regions in the spiral galaxy NGC4258. The main results can be summarized as follows:

i) From an empirical calibration method based on the sulphur emission lines (Diaz and Perez-Montero, this meeting) we have derived oxygen abundances which turn out to be between 0.3 and 0.7 times solar, substantially lower than previously derived by using standard calibration techniques based on the optical oxygen lines.

ii) We have also derived the ionization parameter for each of the regions by using several emission line ratios.

iii) A prominent Wolf-Rayet feature at 4686 Angstroms is observed in the brightest region analyzed, that reveals the presence of about 35 WN stars.

iv) We have used the photoionization code Cloudy 90 (Ferland 1996) to constrain the effective temperature of the ionizing stars. The values obtained for all the regions are around 35,000K. Therefore, the presence of WR stars does not seem to increase the mean effective temperature of the ionizing clusters.

Beryllium Abundance in Lithium-Rich Giants

B. V. Castilho

Universida de São Paulo, Brazil

About 2% of the red giants show lithium abundances significantly larger than expected by dilution due to convection, and in some of them, the lithium abundance reaches values similar (and even larger) than the ISM value logN(Li) = 3.0.

Lithium and Beryllium are burned in stars in the deep (hot) layers of the stars, being preserved in the external layers. A shallow mixing of the external layers will deplete lithium, and a deeper one will deplete also Be. The Be abundance determination in Li-rich giants could give important information about the origin of the Li observed in these stars. If Be is shown to be depleted, the original Li must have been depleted too, so that, the observed Li in Li-rich giants must be produced in the stellar interior.

We present the observations of the BeII 313.0nm for two Li-rich giants and a Li-normal giant, plus $\alpha$ Cen A and B, obtained with the CASPEC spectrograph at the ESO 3.6 telescope, with a resolving power R=32000 (Castilho et al., in preparation); and discuss the Be abundances derived through spectrum synthesis.

Castilho, B.V., Spite, F., Barbuy, B., de Medeiros, J.R. , and Gregorio-Hetem, J. in preparation

New Constraints on the Primordial Lithium Abundance

C.Charbonnel¹

¹Laboratoire d'Astrophysique de Toulouse, CNRS UMR 5572, France

Recent data of the LiBeB elements in halo and in some globular cluster stars throw a new light on the problem of the lithium depletion in Population II stars. We present new results on the LiBeB abundances obtained with stellar models including the best available physics, and discuss the constraints on the actual depletion of these light elements in Pop II stars. This leads us to make a parameter-free evaluation of the lithium primordial abundance. The major uncertainty is due to systematic errors in the observational data.

FIELD BLUE STRAGGLERS: THE CHEMICAL COMPOSITION REVISITED.

I.V. Chernyshova

Department of Astronomy, Odessa State University, Shevchenko Park, 270014 Odessa, Ukraine
chern_irina@yahoo.com

Field blue stragglers (FBS) are counterparts of cluster blue stragglers. They were selected by Olsen (1980, A&AS 39, 205) from specific Strömgren indices among bright metal-deficient early F dwarfs.
Some stars from this list were observed with high-resolution and high S/N CCD spectra. First results were already published in 1996, A&A 310, 277. Synthetic spectrum technique was applied for the specification of rotational velocities and chemical composition of program stars. Special attention was payed on FBS with high rotation and broad shallow lines with aim of comparison of their chemical abundances and abundances of $\lambda$ Boo stars calculated with the same methods. In general, 17 chemical elements were investigated. All metals show moderate deficiency. Most of the stars show normal abundance of sodium. With the exception of HD35863 the "normal" lithium abundance also was found in HD27523, HD45042 and HD88923.
Main question was "What is FBS?" We discussed three hypotheses that could be applied for explanation of their nature: 1) they are really blue stragglers with prolonged evolution; 2) they are normal stars which were born in metal-deficient medium; 3) they are an extension of $\lambda$ Boo stars towards lower temperature.

Dust and Molecules at High Redshift

Françoise Combes

Paris Observatory, France

In the last years, progress has been very rapid in the domain of molecules at high redshift, and we now know in better detail the molecular content in several systems beyond z=1 and up to $z \sim 5$, either through millimeter and sub-millimeter emission lines, continuum dust emission, or millimeter absorption lines in front of quasars. The first discovery in 1992 by Brown and van den Bout of CO lines at z=2.28 in a gravitationnally lensed starburst galaxy, strongly stimulated searches of other systems, but these were harder than foreseen, and only 5 other systems have been discovered in CO emission. Redshifts range between 2 and 5, the largest being BR1202-0725 at z=4.69. Most of these systems, if not all, are gravitationnaly amplified objects. Some have been discovered first through their dust emission, relatively easy to detect because of the negative K-correction effect. The detection of all these systems could give an answer about the debated question of the star-formation rate as a function of redshift. The maximum of star-formation rate, found around z=2 from optical studies, could shift to higher z if the most remote objects are hidden by dust.

Absorption of quasar millimetric light by molecules is another tool to probe cold gas at high redshift, taking advantage of very high spatial ($\le$10^-3 arcsec) and spectral (30m/s) resolutions. Up to now four absorption systems have been discovered in the millimeter range, and up to 20 molecular lines have been detected in a single object (HCO⁺, HNC, HCN, N₂H⁺, C¹⁸O, CS, H₂CO, CN, etc....). Molecular abundances, excitation temperatures can be studied in detail in these remote galaxies. This technique can sample column-densities between N(H₂)= 10²⁰ and 10²⁴ cm^-2. The diffuse components allow us to obtain an upper limit on the cosmic black body temperature as a function of redshift. The high column density component allow to observe important molecules not observable from the ground, like O₂, H₂O and LiH for example.

The Role of Starbursts in the Chemical Evolution of Galaxies

T. Contini^1,2, R. Coziol³, S. Considère⁴ and E. Davoust⁵

¹ Tel Aviv University, Israel
² ESO Garching, Germany
³ Observatório Nacional, Brasil
⁴ Observatoire de Besançon, France
⁵ Observatoire Midi-Pyrénées, France

Recent results on the chemical properties of Starburst Nucleus Galaxies (SBNGs) and their implications on galaxy evolution will be reviewed. We will show that SBNGs are young galaxies still in the process of formation probably related to an active phase of star formation in the Universe a few Gyrs ago. The link between metal and dust productions in starburst galaxies will be emphasized, together with a discussion on cosmological implications like the importance of dust extintion at high redshifts.

THE RGB BUMP AS A METALLICITY INDICATOR FOR GLOBULAR CLUSTERS

S. Desidera

Department of Astronomy, University of Padova, Italy

The RGB bump is a peak in the luminosity function of the Red Giant Branch (RGB). It has been recently observed in many globular clusters. Its position along the RGB depends on the metallicity of the cluster, suggesting a possible use as a relative metallicity indicator.

In this paper we focuse our attention mainly on metal rich clusters, for which the metallicity measurements are more uncertain while the detection of RGB bump is easier.

We present a compilation of the available data as well as new identifications and we derive calibrations of this metallicity indicator taking as a reference the available metallicity scales. Using these calibrations we derive metallicity of a sample of metal rich clusters of poorly known composition.

Emipirical calibrations of nebular abundances: The sulphur abundance parameter

Angeles I. Díaz and Enrique Pérez-Montero

Depto. de Física Teórica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain

We present an empirical calibration of nebular abundances based on the intensities of the strong [SII] and [SIII] lines in the red part of the spectrum. This calibration has two important advantages over the commonly used one based on the optical oxygen lines: it remains single-valued up to abundances close to solar and is rather independent of the degree of ionization of the nebula.

Measuring Chemical Abundances with the VLT instruments

Sandro D'Odorico

ESO, Garching bei München, Germany

The VLT , with it four 8m telescopes and 12 instruments permanently on line, will be a unique machine to study stars, galaxies and the ISM in the optical and infrared wavelength regions. Three instruments stand out as the ones which are expected to be used more intensively for quantitative studies of chemical abundances: UVES, an echelle spectrograph for the 300-1100nm region, the multi-object visual spectrograph GIRAFFE and the infrared echelle spectrograph CRIRES. The expected capabilities of these instruments in terms of resolving power, S/N ratio and limiting magnitudes are given and the potential impact of the future observations on a few, key chemical abundance issues is discussed.

Possible existence of molecules in the shells of quasars 1556+3517 and 0840+3633

Dubrovich V.K., Lipovka A.A.

Special Astrophysical Observatory RAS, S.Petersburg branch, Russia

New class of low- ionization BAL quasars was suggested recently by Becker et al. (1997). This population is represented by three objects: 1556+3517, 0840+3633 and 0059-2735 with redshifts z= 1.48 , 1.22 and 1.62 respectively. Spectra of these quasars are heavily attenuated at optical wavelength in region less than 2800 A in the quasar restframe. The number of ions and neutral atoms were found in their spectra (in particular FeI and MnI characterized by ionization potentials 7.9 and 7.4 eV). Large optical depths of neutral Fe and Mn in spectrum of 1556+3517 on the one hand, and unusually strong absorption in lines and continuum as well, on the other hand, lead to conclusion on low degree of ionization in their shells. It means that strong ionized radiation of their cores is shielded by dense internal part of the shell. It suggests the possible existence of steady molecules with high (about 10 eV) dissociation energies in the shells. CO molecule has dissociation energy E_D = 11 eV ( it is more than value of ionization potential of many atoms), and for this reason the possible existence of CO in the quasars shells, discussed in this paper, become very attractive. In this case one can identify the line at 2063 A, attributed to CrII, as a line of Cameron band of CO molecule. Estimation of the expected optical depth value in rotational radio lines of this molecule is made by using its optical spectrum, and find to be > 1. The fluxes from quasar 1556+3517 at frequencies 1.4 and 5 GHz are 30.6 and 27.0 mJy respectively (Becker, et al. 1997). Spectral index in this case is -0.1, that lead to expected flux F = 20 mJy at the frequency of line J'J = 2,1 shifted at (1+z): 92.9 GHz. For quasar 0840+3633, the flux at 1.4 GHz is 1.3 mJy. If we take -0.1, we will obtain the expected flux F = 0.8 mJy at frequency of observation 103.8 GHz. Such fluxes can be observed with modern radiotelescopes. For observations with IRAM 3mm receiver, for example, the time of observation should be approximately 1 minute for QSO 1556+3517 and 10 hours for 0840+3633.

References:

Becker R.H., et al., Astrophys.J. (1997) v.479, p. L93

The effect of the environment on the metallicity of dwarf galaxies in clusters

P.-A. Duc

University of Cambridge, Institute of Astronomy, UK

A correlation between metallicity and luminosity has now been well established for giant and dwarf galaxies. However several studies have shown that dwarf galaxies in clusters appear to have Oxygen abundances higher than expected. These deviations from the metallicity-luminosity relation suggest that the latter is affected by the environment. Among the possible explanations are confinement by the intracluster medium or the reprocessing of pre-enriched material expelled from giant stripped galaxies at the origin of the so-called tidal dwarf galaxies. I will illustrate these phenomena using the results of a multi-wavelength survey of dwarf galaxies in the Hydra cluster. The morphology,stellar and gaseous content and metallicity of a complete sample of HI-rich dwarfs have been studied.We found that on average the Hydra dwarfs seem to have a higher metallicity than in the field. This excess may possibly be explained by the scenario involving tidal interactions. This would indicate that part of the population of dwarf galaxies in clusters may be of tidal origin.

CNOSi Abundances in the Orion Nebula Derived from HST FOS UV-Optical Spectroscopy

R. J. Dufour

Rice University, Houston, TX, USA

During the six years that the Faint Object Spectrograph (FOS) was part of the HST instrument repertoire, it was used to obtain UV-optical spectra of numerous extragalactic H II regions in spiral and irregular galaxies which enabled several detailed studies of the UV lines of carbon, nitrogen, and silicon to be studied with high S/N for the first time. The results of several investigations showed that generally C/O $\propto$(O/H)^0.4 and that the gaseous-phase Si/O was generally $\sim$1/2 Solar in the nebulae.

During this period FOS observations of five locations in the Orion Nebula were also obtained, but have yet to be fully analyzed in a manner similar to the extragalactic H II region studies. In this paper, I analyze these FOS spectra using analysis techniques similar to those in the extragalactic studies and incorporating modern atomic data, with the aim of deriving new average CNOSi abundances for the Orion Nebula more appropriate for comparison with the recent extragalactic H II results from identical UV-optical line ratios. These results are intended to improve on the previous general CNOSi abundances for Orion which were largely based on IUE observations. Comparisons will also be made with the results from the recent comprehensive optical study of Orion Nebula abundances by Esteban et al. (1998) that are based on high resolution measurements of both forbidden and permitted emission lines.

Dynamical Effects on Chemical Evolution

Mike Edmunds

Department of Physics and Astronomy, University of Wales, Cardiff, UK

The flow of gas and dust into, out of, or inside galaxies can strongly affect their chemical evolution. Fortunately it is now possible to predict the general behaviour by simple analytic arguments. This paper will outline the systematic effects of gas flows on chemical abundances, element ratios and dust masses. Comparison will be made with both observations and recent SPH dynamical modelling of the formation of disk galaxies. Mechanisms for generating high abundances in galactic nuclei (and perhaps quasars) will be discussed. Suggestions for future investigation will be made, in particular for abundance gradients, groups of galaxies, and the study of global chemical evolution through the behaviour of high-redshift systems.

CHEMICAL ABUNDANCES IN THE EXTREME OUTER REGIONS OF DISK GALAXIES

Annette M. N. Ferguson

Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge UK CB3 0HA, ferguson@ast.cam.ac.uk

I will present the first results from an ongoing project to investigate the present-day chemical abundances of the extreme outer parts of galactic disks, as probed by the emission line spectra of a new sample of HII regions. Deep H$\alpha$ images have revealed the existence of recent massive star formation, traced by HII regions, out to, and beyond, two optical radii in several galaxies (defined by the B-band 25th magnitude isophote). Optical spectra of these newly-discovered HII regions reveal gas-phase abundances of O/H $\sim$ 10-15%  of the solar value, and N/O $\sim$ 20-25%  of the solar value, at radii of 1.5-2 R₂₅. Clear evidence also exists for diminished dust extinction (A_V $\sim$ 0-0.2) in these parts. Within the limits of the current dataset, the radial abundance variations are consistent with single log-linear relationships, although the derived slopes can often differ considerably from those found if only inner disk HII regions are used to define the fit. Nitrogen-to-oxygen ratios appear to be consistent with a combination of primary and secondary production of nitrogen. The mean level of enrichment in extreme outer galactic disks is similar to that measured in some high redshift damped Lyman-$\alpha$ absorbers, suggesting that outer disks at the present epoch are relatively unevolved.

The N/C ratio in high redshift radio galaxies

R A E Fosbury¹, J Vernet², M Villar-Martin³, M H Cohen⁴, A Cimatti⁵, S di Serego Alighieri⁵

¹ST-ECF, Garching bei München, Germany
² ESO, Garching bei München, Germany
³Department of Physics and Astronomy, University of Sheffield, UK
⁴California Institute of Technology, Pasadena, USA
⁵Osservatorio Astrofisico di Arcetri, Firenze, Italy

Deep Keck spectropolarimetry has been obtained of a sample of powerful radio galaxies with z   2.5 +/- 0.5. In addition to a clear 2200A dust extinction signature and an inverse correlation between the degree of continuum polarization and the Lyman alpha emission line strength (relative to CIV), we find an intriguing positive correlation between the NV/CIV ratio and the polarization. The line ratio, which varies over almost a dex in objects having a very similar ionization state, is most likely to indicate an abundance ratio. We speculate that the correlation with polarization represents a connection between nitrogen enhancement and dust production following a major starburst event connected with the birth of the quasar.

Predicting Spectral Properties of DLA Galaxies

U. Fritze - v. Alvensleben, U. Lindner, C.S. Möller, K.J. Fricke

Universitätssternwarte Göttingen, Germany

Comparison of our chemically consistent models for spiral galaxies with observed DLA abundances shows that at high redshift DLA galaxies may well be the progenitors of normal spiral disks of all types from Sa through Sd. Towards lower redshifts ${\rm z \leq 1.5}$ however, early type spirals drop out of DLA samples due to low gas content. We use the spectrophotometric aspects of our unified spectral, chemical, and cosmological evolution models to predict expected luminosities, colours, SFRs, etc. for DLA galaxies at various redshifts and compare to the few optical identifications available.

Abundances in Spiral and Irregular Galaxies in the Local Universe

Donald R. Garnett

University of Minnesota, Department of Astronomy, USA

I will condense into a few short minutes a review of the vast quantity of data on abundances in spiral and irregular galaxies in the nearby universe, outside the Local Group. Nearly all of this data consists of measurements of abundances in H II regions. Optical spectroscopy of H II regions yields relatively complete information on O/H. Where high quality spectroscopy exists for a given spiral galaxy, the dispersion in O/H is found to be quite small: of order $\pm$0.1 dex, which is often smaller than the observational uncertainties for individual points. The implication is that abundance inhomogeneities are quite small on small spatial scales, although there is some evidence that large-scale inhomogeneities may exist. Abundance gradients per unit disk scalelength may have a small range of values in spiral galaxies; barred spirals appear to have shallower gradients than non-barred spirals. The average O/H in spiral and irregular galaxies increases with galaxy mass/luminosity; massive spirals have higher abundances at the same value of disk surface brightness than lower mass spirals. Finally, I will discuss accumulating data on C and N abundances in H II regions that is providing interesting new information on the evolution of both spirals and irregulars.

Chemical abundances of local disk stars

Bengt Gustafsson

Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden

Recent progress as regards abundances in local disk stars will be discussed. The possibilities and the shortcomings in studying nucleosynthesis and galactic evolution using such data will be illustrated. The possibilites of progress when local data are combined with data of other types, e.g.  for irregular blue galaxies, will be demonstratad with the build-up of carbon as one example.

Chemical Evolution of the Intergalactic Gas

Martin Haehnelt

MPI für Astrophysik, Garching bei München, Germany

I will give an overview over constraints from metal absorption systems in QSO spectra on the chemical evolution of the intergalactic gas. I shall give special emphasis to two points:

• The controversy on the metallity of the Lyalpha forest and implications for the enrichment history of the IGM.
• The detection of OVI and the increase of the SiIV/CIV ratio with increasing redshift and their implications for the re-ionization of the universe.

The Detection of Extragalactic ¹⁵N: Consequences for Nitrogen Nucleosynthesis and Chemical Evolution

Christian Henkel

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Carbon, nitrogen, and oxygen, the CNO elements, are among the most abundant species after hydrogen and helium. The dominent stellar production site of the nitrogen isotope ¹⁵N is, however, not yet known. Here we report detections of ¹⁵N from observations of interstellar hydrogen cyanide (HCN) toward the Large Magellanic Cloud and, tentatively, toward the core of the (post-) starburst galaxy NGC4945. The ¹⁴N/¹⁵N isotope ratios drastically differ from those measured in the disk and center of the Milky Way. Strongly supporting the idea that ¹⁵N is predominantly of `primary' nature, hydrostatic production in massive stars should be its dominant source.

Chemical Evolution and Abundance Ratios in Dwarf Irregular Galaxies

G. Hensler

Institut für Theoretische Physik und Astrophysik, University of Kiel, Olshausenstr. 40, D-24098 Kiel, Germany

Because of their low gravitational energies dwarf galaxies are greatly exposed to energetical influences by the interstellar medium, like e.g. stellar radiation, winds or explosions, or by their environment. While the metallicity depletion in dwarf galaxies can be explained in general by supernova-driven galactic winds, the reason for their low N/O ratios at low O abundance is not yet completely understood. Stellar yields enrich the different gas phases with elements that are characteristic for the stellar progenitors. Phase transitions are necessary for a mixing of elements, but depend sensitively on the thermal and dynamical state of the interstellar medium. Models of chemical evolution start usually with a high N/O ratio at low O abundance according to a metal enrichment by ancient stellar populations with common yields, but can only reproduce the N/O-O peculiarity by the application of multiple starbursts. Their galactic winds are invoked to reduce O selectively. Chemodynamical models of dwarf galaxies, however, demonstrate that strong evaporation of clouds by the hot supernova gas leads to an almost perfect mixing of the interstellar gas. These models can successfully account for the observed N/O-O values in a self-consistent way without the necessity of starbursts, if new stellar yields are taken into account which provide additional primary and secondary N production from massive stars.

Alpha-elements Differential Abundances in M33 B-supergiants

A. Herrero, M.I. Monteverde

Instituto de Astrofisica de Canarias, Spain

We present the recent results of our programme aimed at obtaining abundances of O, Si and Mg in B supergiants located at different distances from the center of M33. After the technique presented in Monteverde et al. (1997, ApJ 474,L107) to derive differential O abundances in these stars with respect to their galactic counterparts, which allowed us to obtain the first extragalactic stellar radial O gradient, we are analyzing the spectra to obtain the stellar parameters and Si abundances.

HEAVY ELEMENT ABUNDANCES IN THE MOST METAL-DEFICIENT BLUE COMPACT GALAXIES

Yuri I. Izotov¹, Trinh X. Thuan²

¹Main Astronomical Observatory, Ukrainian National Academy of Sciences, Goloseevo, Kiev 252650, Ukraine
²Astronomy Department, University of Virginia, Charlottesville VA 22903, USA

The heavy element abundances in the most metal-deficient blue compact galaxies (BCGs) are discussed. None of the heavy element-to-oxygen abundance ratios (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12 + log O/H $\leq$ 7.6 (Z $\leq$ $Z_\odot$/20). This constancy implies that all these heavy elements have a primary origin and are produced by the same massive (M $\geq$ 10 $M_\odot$) stars responsible for O production. The dispersion of the C/O and N/O ratios in these galaxies is found to be remarkably small, being only $\pm$ 0.03 dex and $\pm$ 0.02 dex respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest-metallicity BCGs, and hence against production of these elements by intermediate-mass (3 $M_\odot$$\leq$ M $\leq$ 9 $M_\odot$) stars at very low metallicities. The absence of evidence for a time-delayed production of C and N also implies that galaxies with 12 + log O/H $\leq$ 7.6 are undergoing now their first burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr.

We discuss the apparent discrepancy between the N/O abundance ratios measured in BCGs and those in high-redshift damped Ly$\alpha$ galaxies, which are up to one order of magnitude smaller. We argue that this large discrepancy may arise from the unknown physical conditions of the gas responsible for the metallic absorption lines in high-redshift damped Ly$\alpha$ systems. While it is widely assumed that the absorbing gas is neutral, we propose that it could be ionized. In this case, ionization correction factors can boost the N/O ratios in damped Ly$\alpha$ galaxies into the range of those measured in BCGs.

Present and Forthcoming UV Missions

Edward B. Jenkins

Princeton University Observatory, Princeton, NJ 08544-1001, USA

Over the past 30 years, UV instruments have improved by many orders of magnitude in some key parameters that govern their ability to perform useful observations. A review of the characteristics of present and future UV missions will be presented, with a special emphasis on properties that are relevant to abundance determinations. New concepts presented at a conference on Ultraviolet-Optical Space Astronomy in early August (Boulder, CO) may also be summarized.

IMAPS Observations of Variations in D/H in the Interstellar Medium

E. B. Jenkins, T. M. Tripp, P. R. Wozniak

Princeton University Observatory, Princeton, NJ, USA

U. J. Sofia

Dept. Astron. & Astrophys., Villanova University, Villanova, PA, USA

G. Sonneborn

Code 681, NASA Goddard Space Flight Center, Greenbelt, MD, USA

R. Ferlet, A. Vidal-Madjar, G. Hébrard

Institut d'Astrophysique de Paris, CNRS, Paris, France

C. Gry, O. Dupin

Laboratoire d'Astronomie Spatiale, CNRS, Marseille, France

M. Lemoine

Dept. Astron. & Astrophys., University of Chicago, Chicago, IL, USA

The Interstellar Medium Absorption Profile Spectrograph (IMAPS) recorded the L$\delta$ and L$\epsilon$ D and H absorption features in the spectra of $\delta$ Ori, $\gamma^2$ Vel and $\zeta$ Pup during the ORFEUS-SPAS II mission in late 1996. The resolution of $5\,{\rm km~s}^{-1}$ provides a substantial improvement over previous spectra recorded by the Copernicus satellite. The atomic deuterium column densities derived from our observations are compared to re-evaluations of N(H I) from L$\alpha$ features in IUE archival spectra to yield determinations of D/H. The D/H toward $\delta$ Ori seems to be lower than the general result of $1.5\times 10^{-5}$ reported for lines of sight to cool stars within the local region, while the numbers we derive for $\gamma^2$ Vel and $\zeta$ Pup appear to be equal to or slightly greater than this value (more precise statements will be given in the poster paper). Our findings about a variability in D/H are consistent with independent conclusions by Vidal-Madjar, et al. (astro-ph 9807004) for the line of sight to G191-B2B.

Abundance Determinations Through Astrophysical X-Ray Spectroscopy

Steven M. Kahn

Department of Physics and Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, UNITED STATES

The conventional X-ray band (0.1 - 10 keV) is especially rich in emission and absorption features from nearly all charge states of all cosmically abundant metals (carbon through iron). X-ray spectroscopic observations therefore provide a promising new frontier for abundance determinations in a wide variety of astrophysical systems, ranging from clusters of galaxies to the coronae of nearby normal stars. As yet, progress in this field has been rather slow because of instrumental limitations. However, this will change dramatically in the next few years with the impending launches of three major X-ray observatories, AXAF, XMM, and Astro E, all of which include high resolution, high sensitivity spectroscopic experiments. I will review existing X-ray abundance determinations in three different fields (stellar coronae, young supernova remnants, and the gaseous haloes of elliptical galaxies), which collectively illustrate both the power and some of the problems with this technique. I will also describe and compare the capabilities of the three new missions, and comment on both the near-term and far-term future of this field.

The effect of mergers on abundances

Guinevere Kauffmann

MPI für Astrophysik, Garching bei München, Germany

According to hierarchical theories of structure formation, the structures observed today were formed by the gravitational amplification of small perturbations in an initially gaussian dark matter density field. Small scale overdensities were the first to collapse and the resulting objects merged under the influence of gravity to form larger structures such as clusters. Galaxies formed within dense halos of dark matter, where gas was able to reach high enough overdensities to cool, condense and form stars. In this picture, galaxies do not undergo "closed-box" chemical evolution. Instead, metals can be transferred between the stars, cold gas and the hot gas halos of the galaxies. It is assumed that metals are ejected out of disk galaxies during supernova explosions and these metals enter the hot gas component. The fact that metals are more easily ejected from small galaxies leads to the establishment of a mass-metallicity relation for the disk systems. The early rapid enrichment occurring in the dwarf galaxy progenitors of the Milky Way provides a natural solution to the classic G-dwarf problem. We demonstrate that massive ellipticals are more metal rich because they are formed from the mergers of bigger disks. Finally, the models can be used to study the enrichment history of the intracluster gas. We show that more than 80 percent of the metals were ejected by galaxies with circular velocities less than 250 km/s at redshifts greater than 1.

Non-LTE abundances from Magellanic Cloud B stars

A.J. Korn, B. Wolf

Landessternwarte Königstuhl, 69117 Heidelberg, Germany

Recent results of CASPECtroscopic abundance determinations of B stars in the Magellanic clouds are presented. The sample stars include both young globular cluster (NGC 1818, 2004 (both LMC), 330 (SMC)) and field stars on and close to the main sequence.

The method for deriving the stellar parameters (Balmer profiles, Si II/III/IV ionisation equilibria, weak and strong OII lines) was thoroughly revised to be capable of treating the metallicity of the underlying atmosphere (which was shown to have a significant effect on the stellar parameters and therefore on the abundances to be derived) in a self-consistent manner. LTE ATLAS 9 model atmospheres (Kurucz 93) and the DETAIL/SURFACE NLTE line formation code (Butler and Giddings 85) are used.

Abundances for He, C, N, O, Mg, Al and Si are derived. From the metals an average underabundance of [m/H]=-0.5 (-0.8) is deduced for the Large (Small) Magellanic cloud. No significant abundance difference is found between cluster member and field stars within the respective cloud.

Search for a High-redshift Damped Lyman-alpha Absorber with NICMOS

Varsha P. Kulkarni, John M. Hill

University of Arizona, Steward Observatory, Tucson, AZ 85721, U.S.A.

Ray J. Weymann, Lisa Storrie-Lombardi

Carnegie Observatories, Pasadena, CA 91101, U.S.A.

Marcia Rieke, Glenn Schneider, Rodger Thompson

University of Arizona, Steward Observatory, Tucson, AZ 85721, U.S.A.

Buell Jannuzi

National Optical Astronomy Observatories, Tucson, AZ 85726, U.S.A.

High-redshift damped Lyman-alpha absorbers in quasar spectra are believed to be the progenitors of present-day galaxies. However, their exact nature is not yet clearly understood, since their detection in emission has proved very difficult. We present high-resolution HST NICMOS images of a quasar field known spectroscopically to have a damped Lyman-$\alpha$ absorber at redshift $z \approx 1.9$. The images were obtained in broad and narrow-band near-IR filters, aimed at detecting the absorber in rest-frame continuum and H-$\alpha$emission. Some of the images were obtained using the coronagraph on NICMOS camera 2 with the goal of increasing the sensitivity for detection of the damped Lyman-$\alpha$ absorber by decreasing the light from the background quasar. The analysis of the NICMOS images is presented, including a discussion of subtraction of the point spread function. The final broad-band images are used to constrain the morphology and environment of the damped Lyman-$\alpha$absorber. The narrow-band images offer constraints on the star-formation rate in the damped Lyman-$\alpha$ absorber. Our study, which benefits from the unique combination of high near-IR sensitivity and high spatial resolution offered by NICMOS, can thus give important information about chemical evolution of galaxies at high redshifts.

Element Abundances in Damped Lyman-alpha Quasar Absorbers

Varsha P. Kulkarni, Jill Bechtold, and Jian Ge

University of Arizona, Steward Observatory, Tucson, AZ 85721, U.S.A.

The damped Ly-$\alpha$ (DLA) absorbers in quasar spectra represent an earlier stage of chemical evolution compared to present-day galaxies. However, the nature of stellar populations dominating the DLA absorbers (e.g. halo-like or disk-like) is not fully understood. We report results of our spectroscopic survey of DLAs with low and moderate redshifts, carried out at the Multiple Mirror Telescope. We have so far obtained high-S/N spectra of about 10 quasars with DLAs at z < 1.8, to measure or put limits on the abundances of Zn, Cr, Fe, Mn, Ni, Si, S etc. at these redshifts. The goal is to use Zn as the metallicity indicator (since it is almost undepleted on to interstellar grains), and to use the relative abundances of the other elements to try to discern the nucleosynthesis and dust-depletion patterns. Our data, combined with observations from the literature at higher redshifts, will help to put better constraints on the evolution of metallicity and dust content in galaxies.

X-Ray Study of Hot Interstellar Medium in Early-Type Galaxies

Matsushita Kyoko

Tokyo Metropolitan University, Japan
Email: matusita@phys.metro-u.ac.jp

We have analyzed ASCA data of about 30 early type galaxies, and studied their X-ray emitting ISM (InterStellar Medium) properties. Our study has been motivated by the apparently very low metallicity of the ISM, which cannot easily be reconciled with theoretical predictions. By carefully examining the abundance ratios and uncertainties in the Fe-L complex, we have concluded that the ISM abundances in X-ray luminous galaxies are in fact about 1 solar. Therefore, the severe discrepancy between the ISM and stellar abundance has been relaxed. The ISM metallicity of X-ray fainter galaxies are uncertain, but at least SNe Ia contribution to the ISM abundance is smaller than in the X-ray luminous ones.

We have also discovered that X-ray emissions from X-ray luminous galaxies are very extended, and expressed with two beta models of different angular scales. This means that the X-ray luminous ellipticals are central galaxies of some larger-scale potential structures. We show that presence/absence of such a larger-scale potential can consistently account for several unsolved problems with the ISM.

Chemical Abundances in High-z Star-Forming Galaxies

Claus Leitherer

Space Telescope Science Institute, Baltimore, USA

An increasing number of galaxies at z >2 with stellar absorption lines from massive stars has been detected in recent years. These lines provide direct evidence for active star formation during the past 100 Myr. I will discuss the nature of the underlying stellar population. Attempts will be made to estimate the metallicity of the stars. I will compare the restframe UV spectra of high-z galaxies to those of local template galaxies. Future developments in the field will be pointed out.

Massive Star Abundances

D.J.Lennon

ING, La Palma, Spain

Abundances as derived from early-type stars are currently gaining credibility as standard abundances against which ISM depletions are measured. Similarly, the use of massive stars in the study of abundance gradients in the Galaxy is also gaining credence and provide important new constraints for chemical evolution models. Blue supergiants are now being used to probe the chemical composition of both spiral and dwarf irregular galaxies in the Local Group, producing very detailed information on abundance ratios in these galaxies. We briefly review these results and discuss the application of these techniques to galaxies in the nearby and distant universe.

Interpreting the redshift evolution of DLA galaxies

Ulrich Lindner, Uta Fritze - von Alvensleben, Klaus J. Fricke

Universitäts-Sternwarte Göttingen, Geismarlandstraße 11, D-37083 Göttingen, Germany

We have extended our chemical and cosmological galaxy evolution model to calculate the abundance evolution for altogether 16 different elements in spiral galaxies in a chemically consistent way which is a considerable step towards a more realistic galaxy modelling. We have compiled from the literature all available data on element abundances in DLA systems and the comparison with our model calculations yields the following results.

The conformity between observed and calculated abundances over the redshift range from $\rm z \sim 4.5$ through $\rm z \sim 0.4$indicates that DLA galaxies may well evolve into the full range of present-day spiral galaxies from Sa through Sd.

Comparison of our chemically consistent models with models using only solar metallicity input physics shows that differences in the redshift evolution are small for some elements but large for others. For those elements with large differences the chemically consistent models provide significant better agreement with observed DLA abundances.

For typical spiral galaxies the star formation histories of our models clearly bridge the gap between high redshift DLA systems and the nearby spiral galaxy population. The slow redshift evolution of DLA abundances is understood in terms of the long star formation timescales in (proto)galactic disks. The large scatter of observed abundances in DLAs of similar redshift is rather explained by the range of SFRs among early and late type spirals.

Abundance determinations of photoionized gaseous nebulae

X.-W. Liu

Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Heavy element abundances of photoionized gaseous nebulae are traditionally derived from the analysis of collisionally excited optical and UV lines. Abundances thus derived however have an exponential sensitivity to the adopted electron temperature. For collisionally excited lines of low critical densities, the results are also sensitive to the adopted electron density. The advent of large format, high quantum efficiency CCDs, together with the high quality effective recombination coefficients now available for many heavy element ions have opened up the possibility of obtaining accurate abundances using optical recombination lines from heavy element ions. Although they are much weaker and more difficult to measure than collisionally excited lines, the emissivities of radiative recombination lines from H, He and heavy element ions have only a weak, similar, power-law dependence on temperature and are essentially independent of density under typical nebular conditions. Thus ionic abundances derived from the intensities of heavy element recombination lines relative to a hydrogen recombination line, such as H$\beta$, are almost independent of the temperature and density structure of the nebulae under study, and consequently should be more reliable.

A long-standing problem in nebular studies has been that the heavy elemental abundances derived from optical recombination lines are often (although not always) higher than those derived from UV and optical collisionally excited lines, by factors up to 10. The discreepancies have been often attributed to the presence of large temperature and/or density inhomogeneities which lead to underestimated collisional line abundances. Planetary nebulae (PN) are envelopes ejected by low- and intermediate-mass stars in their late evolutionary stage and are the chief sources of carbon and s-process elements in the Universe. Because of their particularly simple structures, i.e. a generally symmetric nebula excited by one centrally located star, PN provide the best astrophysical laboratories to study the abundance determination problem.

We have been undertaking a systematic optical recombination line abundance survey for a large sample of Galactic PN. The deep spectra obtained yield accurate fluxes for multiple recombination lines from ions of C, N, O, and Ne. For half of them, we have also obtained 2.4-45$\mu$m and 43-197$\mu$m IR spectra using the SWS and LWS on board ISO. The major goal of the programme is to derive accurate recombination line C, N, O and Ne abundances for a large sample of PN and to compare the results to those derived from IR, optical and UV (from the IUE archival data) collisionally excited lines of different excitation energies and critical densities, in order to probe the underlying physics causing the abundance determination problem. In this contribution, we present some results from this programme.

Abundance Constraints from Interstellar Dust

John S. Mathis

University of Wisconsin-Madison, USA

Recent observations have created a ``crisis'' - better viewed as ``opportunity'' - in understanding interstellar dust. These determinations are: (a) gas-phase abundances of many elements in the diffuse ISM, and (b) stellar abundances through analyses of spectra; (c) new infrared observations of the weakness of O-H , N-H, and C-H absorptions in the diffuse ISM. They have collectively reduced the amount of solid materials (especially carbon) in the ISM, while establishing that the total abundance of elements (gas + solid) in the local ISM is sub-solar by about 0.12 - 0.2 dex (25-35%). The ISM abundance is determined from four separate lines of reasoning: (a) H II regions; (b) gas-phase abundances of elements not in diffuse dust (e.g., N); (c) early-type stars that have just formed out of the ISM; (d) later-type stars that are comparable with the Sun.

Well-established requirements for the extinction per H must be met with the available solid-state abundances: (a) the carbon demands of the 2175 Å feature and the observed Unidentified Infrared Bands (similar materials?), and (b) the abundances required for continuum extinction in both the visual and 1200 Å regions. My view of major problems currently facing our understanding of the physical nature of diffuse dust will be mentioned.

Abundance gradients in the Galaxy: a clue to Galaxy formation

F. Matteucci

Dipartimento di Astronomia, Università di Trieste / SISSA, Italy

I will review the current status of abundance gradient determinations along the the disk of the Galaxy and discuss various chemical evolution models predicting abundance gradients and their temporal evolution. I will show that abundance gradients are strong constraints for the mechanism of formation of the Galactic disk.

Abundances in the Galactic Bulge

Andrew McWilliam

Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101, U.S.A

The proximity of the Galactic bulge permits study of individual stars which trace all phases of its history. Study of the bulge is important not just because it is a major component of the Galaxy, but because it is our closest spheroidal system; a census of its stars and composition can be used to guide integrated-light studies of external bulges and giant elliptical galaxies.

Parameters for chemical evolution models of the bulge, such as the star formation rate, the initial mass function, the yield and age, can be constrained with knowledge of the detailed chemical composition. Moreover, study of the Galactic bulge composition provides a means to test our chemical evolution models and predicted supernova element yields, which have been guided by abundance results for the local Galactic disk and halo populations.

I will review results of some recent Galactic bulge composition studies, discuss potentially interesting diagnostic element abundance ratios, and highlight a few of the current problems.

Stellar Populations in a High Density Environment: Elliptical Galaxies in the Coma Cluster

Dörte Mehlert¹, Ralf Bender¹, Roberto P. Saglia¹, Gary Wegner²

¹Universitätsternwarte, München, Germany
²Dartmouth College, New Hampshire, USA

Being one of the densest galaxy cluster, Coma is the ideal place to study galaxy structures as a function of environmental density in order to constrain theories of galaxy formation and evolution. Hence we obtained spectra with good S/N and spatial resolution for a magnitude limited sample of 35 E and S0 Coma galaxies within a field of about 5 square degree around the cluster's center. We derived gradients of the line indices Mg, Fe, H$\beta$ as well as rotation curves and the velocity dispersion profiles. Using Worthey's (1994) stellar population synthesis models, we derived ages and metallicities as well as element ratios ([Mg/Fe]). Additionally we investigated eventually existing correlations between the gradients of the element indices and other galaxy/cluster parameters. The results support the idea that early type galaxies formed in processes that include both stellar merging and gaseous dissipation.

Abundance Measurements with the Next Generation of Radio and (Sub)millimeter Instruments

Karl M. Menten

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Radio- and (sub)millimeter-wavelength observations of ionic, atomic, and in particular molecular lines have been extensively used for determining elemental abundances and isotopic ratios in the interstellar medium of the Milky Way and external galaxies as well as in the circumstellar envelopes of red giant and supergiant stars. A number of new instruments that are currently being planned or under construction will yield new or greatly improved abundance measurements. These include the Square Kilometer Array Interferometer (SKAI) for the meter- to centimeter-wavelength range, the Millimeter Array/Large Southern Array (MMA/LSA) for the millimeter and submillimeter regime, and the Stratospheric Observatory for Infrared Astronomy (SOFIA), which will operate at submillimeter and infrared wavelengths. All of these facilities will have much larger collecting areas than existing instruments. For example, key observations expected from SKAI and SOFIA include determinations of the [D/H] ratio toward numerous lines of sight throughout the Galaxy from measurements of the 92 cm ``spin flip'' line of DI and the J = 1-0 ground-state rotational transition of the HD molecule at 112 $\mu$m, respectively. The high sensitivity and spatial resolution of the MMA/LSA interferometer will allow accurate isotopic abundance measurements of the molecular gas in many galactic nuclei, yielding important information on its production history.

Abundances in Interstellar Gas

David M. Meyer

Northwestern University, USA

Ultraviolet absorption-line studies of the atomic gas in diffuse interstellar clouds with the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope are yielding accurate measurements of the elemental abundances in the local ISM. A key development based on observations of interstellar oxygen and krypton is that the overall (gas plus dust) metallicity of the local ISM now appears to be about 2/3 of the solar system value. There are no statistically significant variations in the measured O and Kr abundances from sightline to sightline and no evidence of density-dependent O and Kr depletion from the gas phase. These results will be discussed in concert with recent GHRS findings on other elements such as carbon, nitrogen, and sulfur.

Chemical Evolution of Dwarfs Galaxies

Mercedes Mollá and Jean-René Roy

Département de Physique and Observatoire du mont Migantic;
Université Laval, Québec, G1K 7P4 Qc, Canada.

We have attempted to reproduce the flat abundance distribution observed in NGC  1313 and other magellanic and irregular galaxies by using a multiphase chemical evolution model. We found that it is not possible to reproduce it and at the same time, be consistent with observed radial distributions of the surface gas density and star formation profiles.

The conclusion is that a more complicated galactic evolution model is neccessary to explain the apparent chemical uniformity of the disk of NGC 1313 and other low-mass irregular galaxies.

We show a new chemical evolution model, developed to treat this problem, including radial flows, and possibly mass loss due to supernova explosions and winds. It is based in the multiphase model which has been previously used successfully to depict other spiral galaxies along the Hubble morphological sequence. The preliminar results will be analyzed and discussed.

The Metal Systems in the Line of Sight Towards GB1759+7539

P. J. Outram¹, F. H. Chaffee², and R. F. Carswell¹

¹ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, U.K.
²W. M. Keck Observatory, 65-1120 Mamalahoa Hwy., Kamuela, HI 96743, U.S.A.

We present observations of the high-redshift QSO GB1759+7539 (z_em=3.05) obtained with HIRES on the Keck 10m telescope. The spectrum has a resolution of FWHM=7kms^-1, and a typical S/N $\sim$25 in the Ly$\alpha$ forest region, and $\sim$ 60 long-ward of the Ly$\alpha$ emission.

Twelve heavy-element absorption systems were identified, including damped Lyman-alpha (DLA) systems at z_abs=2.62 and 2.91. The C, N, O, Al, Si, P, S, Mg, Fe, and Ni absorption features of these systems were studied, and the elemental abundances calculated for the weak unsaturated lines. The systems have metallicities of $Z_{2.62}\simeq1/20 Z_{\odot}$ and $Z_{2.91}\simeq1/45 Z_{\odot}$. Both systems appear to have a low dust content. They show an over-abundance of $\alpha$-elements relative to Fe-peak elements, and an under-abundance of odd atomic number elements relative to even. N was observed in the z_abs=2.62 system, and found to be under-abundant relative to O, in line with the time delay model of primary N production. C II* was also seen, allowing us to determine an upper limit to the cosmic microwave background temperature at z=2.62 of T_CMB <12.0K.

Chemical Evolution of the Magellanic Clouds

B.E.J. Pagel (1)

(1) Astronomy Centre, Sussex University, UK

Previous models for the chemical evolution of the Magellanic Clouds have assumed either a steepened IMF compared to the solar neighbourhood or preferential expulsion of oxygen and $\alpha$-particle elements by selective galactic winds. These assumptions were largely motivated by a belief that the O/Fe ratio in the Clouds is substantially lower than in the Galaxy, but the difference appears to have been exaggerated: Galactic supergiants have a similar O/Fe ratio as Cloud supergiants, there is no corresponding effect in Mg and other $\alpha$-elements and a combination of data from planetary nebulae, H II regions and supernova remnants indicates an O/Fe ratio more or less equal to solar.

Consequently new analytical models for the chemical evolution of the Magellanic Clouds have been developed in collaboration with Grazina Tautvaisiene of the Institute for Theoretical Physics and Astronomy, Vilnius, Lithuania, assuming chemical yields and time delays identical to those we previously assumed for the solar neighbourhood. We include infall, non-selective galactic winds and burst-like modes of star formation represented by discontinuous variations in the star formation rate per unit gas mass. We find adequate agreement with age-metallicity relations and element:element ratios within their substantial uncertainties, whereas our LMC model turns out to give an excellent fit to the anomalous Galactic halo stars discovered by Nissen & Schuster (1997). It also gives an enhanced SNIa/SNII ratio compared to the solar neighbourhood, due to the assumption that the SFR has declined in the past 1 to 2 Gyr.

Galactic H II Region Abundances

Manuel Peimbert

Instituto de Astronomia, Universidad Nacional Autonoma de Mexico

A review of the chemical abundances of galactic H II regions is presented. The C and O abundances derived from recombination lines are higher than those derived from forbidden lines, arguments in favor of the recombination abundances are given. The galactic abundance gradients are discussed. The M8, M17 and Orion abundances are compared with those of the Sun and B stars of the solar vicinity as well as with primordial abundances.

Chemical Abundances of Planetary Nebulae in Galaxies

L. Morbidelli (1), M. Perinotto (2) & M. Dopita (3)

(1) CAISMI-CNR c/o Arcetri Observatory, I-50125 Firenze, Italy
(2) Dipartimento di Astronomia, Universita' di Firenze, I-50125 Firenze, Italy
(3) Mount Stromlo & Siding Spring Observatories, Univ. of Canberra, Australia

An effort has been made to update the knowledge of chemical abundances in Galactic Planetary Nebulae, the corresponding gradients and the enrichment of the Interstellar Medium. A comparison with the PNE population in LMC and in other Galaxies is attempted.

Abundances in Damped Lyman Alpha Systems

Max Pettini

Royal Greenwich Observatory, Cambridge, UK

I will review the current status of abundance determinations at high redshifts, focussing in particular on damped Lya systems (DLAs) in QSO spectra which are still our best tool for measuring the gas phase abundances of a good selection of elements in distant galaxies. Recent observations confirm that damped systems trace a varied population of galaxies which, in addition to spirals, probably includes dwarfs and low surface brightness galaxies and more generally galaxies with star formation histories which may well differ from that of the Milky Way. It now seems clear that new surveys for DLAs in radio selected QSOs are required for a census of metals which is free from selection effects. While existing samples may provide only a partial view of global chemical evolution, damped systems are playing an increasingly important role in our efforts to understand the nucleosynthetic origin of several elements.

BORON OBSERVATIONS IN METAL-POOR STARS

F.Primas

ESO, Garching bei München, Germany

The results of the spectroscopical analysis of HST boron spectra in a new sample of 7 stars ranging in metallicity from [Fe/H]=-2.0 to [Fe/H]=-1.0 will be presented and discussed. These new data points enlarge by almost 50% our previous B sample (cf Duncan et al. 1997), and give us the opportunity to draw firmer conclusions as far as production mechanisms and B galactic evolution are concerned.

The targets had been selected on the basis of their lithium and beryllium abundances, available in the literature, in order to take advantage of the fact that the simultaneous knowledge of LiBeB in the same objects is a powerful diagnostic for testing depletion and internal mixing predicted by different stellar structure models. One star shows the remarkable aspect of deficient B, probably deficient Be, and completely normal Li. No stellar destruction mechanism can explain this. Rather, chemical inhomogeneities in the halo could be the cause.

Extragalactic stellar abundances: Oxygen in extreme A-type supergiants

N. Przybilla^1,2, K. Butler¹, K. A. Venn³, R. P. Kudritzki^1,2, J. K. McCarthy⁴, and A. Kaufer⁵

¹Institut für Astronomie und Astrophysik, Universitätssternwarte, München, Germany,
²Max-Planck-Institut für Astrophysik, Garching bei München, Germany,
³Department of Physics and Astronomy, Macalester College, St. Paul, MN 55101, USA,
⁴Palomar Observatory, California Institute of Technology, M/S 105-24, Pasadena, CA 91125, USA,
⁵Landessternwarte Heidelberg-Königstuhl, Heidelberg, Germany

A-type supergiants are the optically brightest stars in irregular and spiral galaxies and allow the direct determination of stellar abundances. We present preliminary results on the abundance determination for Oxygen for objects of the extreme luminosity class Iae in the Galaxy and M31. Special emphasis is given on modelling the strong OI triplet $\lambda\lambda7771-5$ which will be the preferred feature in extragalactic objects. Future applications using FORS on the VLT will help to investigate the nucleosynthesis history of young populations beyond the Local Group.

NLTE study of oxygen abundances in solar-type stars

Johannes Reetz

University Observatory, Munich, Germany

We determined oxygen abundances of $\sim 85$ solar-type stars in the neighborhood of the Sun spanning a metallicity range from [Fe/H] = -2.3 to +0.4. The analysis of the OI7773 triplet, the [OI]6300 line and the ultraviolet OHA-X band is performed differentially to the Sun using spectroscopically determined effective temperatures and metallicities from Axer et al. (1995) and Fuhrmann (1998), respectively. Fuhrmann determines reliable stellar parameters and magnesium abundances from spectra obtained with the FOCES spectrograph (Pfeiffer et al. , 1998). The NLTE effect for the OI7773 triplet is typically 0.15dex in solar-type stars but increases to $\sim 0.3$dex in F-type stars with [Fe/H] > -0.4. A new method to test the adopted collision strengths for the NLTE model atom of OI is presented. The results are:

• The solar oxygen abundance is $\log \varepsilon = 8.80\pm0.06$ as derived from the ultraviolet and infrared OH bands, from [OI]6300, [OI]6363, OI7773 and OI8446.
• For stars that kinematically belong to the thin disk we find in the average a solar Fe/O abundance ratio ($-1.29\pm 0.1$dex). There is no trend in [Fe/O] for stars with metallicities higher than the Sun - neither on the [Fe/H] nor on the [O/H] scale.
• [O/Fe] $\sim$ 0.8 is the upper limit for halo stars in our sample. A slight increase of [Fe/O] and [Mg/O] with increasing [O/H] is indicated in the halo. The underabundance of Mg against oxygen in metal-poor stars agrees with the relative SNII yields of Mg and O given by Nomoto et al. (1997).
• For 17 stars with both spectra of [OI]6300 and OI7773 available no systematic discrepancy between the oxygen abundances is found.
• For 9 stars we determined oxygen abundances from the OHA-X band (observed with CASPEC) and find a noticeable scatter of [O/H]$_{\rm\scriptsize OH}$ - [O/H]$_{\rm\scriptsize 7773}$, but no significant systematic discrepancy.

Chemical Evolution at the Galaxy Cluster Scale

Alvio Renzini

ESO, Garching bei München, Germany

Clusters of galaxies are the largest structures for which the abundance and mass of the heavy elements can be measured, including their fraction in the diffused intracluster medium (ICM) as well as that locked into stars and galaxies. The relative fraction of metals in the ICM and galaxies is an important boundary condition for chemical evolutionary models at the galactic scale, while the total amount of metals provides a measure of the average past metal production rate in clusters, hence on the average past star formation rate. Several lines of evidence offer also the possibility of estimating the age of the stellar populations in cluster galaxies, hence of pinpointing the epoch at which much of the heavy elements were released along with the accompaining heating of the intracluster medium. Similarities and differences between clusters and the general field will then be addressed, with the aim of setting contraints on the global star formation activity as a function of cosmological time.

The z=1.6748 C I Absorber Toward the QSO PKS 1756+237

Katherine Roth

University of Hawaii, Institute for Astronomy, USA

The QSO PKS 1756+237 ($z_{\rm em}=1.725$) has two strong low-ionization metal-line absorption systems at $z_{\rm abs}=1.461$ and 1.675. Spectra obtained in May 1997 using the Keck HIRES (8.3 km s^-1 FWHM) revealed the existence of weak Ni II, Fe II and C I absorption in addition to the usual strong lines in the higher redshift absorber. The presence of these lines is surprising given the relatively low H I column density reported for this absorber ($\log(N_{HI})=19.12$) by Turnsheck et al. 1979 (ApJ 230, 330). Our data also cover Ly$\alpha$ (1215.7 Å) at $\lambda_{\rm obs} =3250$ Å, and although noisy the data show evidence for a significantly higher neutral column density, $\log(N_{HI})\approx 20.3$ which is more compatible with weak metal lines normally associated with damped Ly$\alpha$absorption systems. The $z_{\rm abs}=1.6748$ system metallicity is > 5-10% solar with a sub-solar Ni/Fe abundance ratio, presumably indicative of dust. This is only the fourth known C I absorber and the susceptibility of C I to destruction by UV light also argues in favor of appreciable dust content (Meyer and York 1987 ApJ 319, 45L). The absence of excited-state fine-structure C I lines is compatible at our level of noise (S/N = 20) with the redshifted Cosmic Microwave Background Radiation at a temperature of 7.29 K. We have also obtained deep, high resolution (0.3'' FWHM) images in H+K' with the UH 2.2m Tip-Tilt system of the QSO field in an attempt to identify the system responsible for the absorption.

Support for this work was provided by NASA through Hubble Fellowship grant # HF-01076.01-94A.

2-dimensional optical spectrophotometry with PMAS

Martin M. Roth

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany, email: mmroth@aip.de

2D-spectroscopy, also known as integral field spectroscopy, has been used on an experimental level for several years, mainly for 2-dimensional investigations of extended objects such as velocity fields in galactic nuclei, or ionization structure of AGN and so forth. With the advent of modern 8-10 class telescopes providing an unprecedented image quality for ground-based observations, more powerful 2D-spectrographs have been proposed and are being designed at this time. It has been discovered only recently, that 2D-spectroscopy has a unique potential for spectrophotometry of faint sources that are superimposed on a bright background, e.g. point sources in galaxies, which are becoming accessible with the new generation of large telescopes -- similar to the advantage of CCD photometry over classical aperture photometry in crowded stellar fields. Applications include the measurement of emission line fluxes of H II regions in distant galaxies, and of planetary nebulae in galaxies of the local group and beyond, spectroscopy of supergiants in the same distance range, supernovae in faint stages of their light curve, and many others. Some of these applications may become important tools for the study of abundances and chemical evolution through spectrophotometric observations of stars and gaseous nebulae.

PMAS, the Potsdam Multiaperture Spectrophotometer, is designed to work as a high efficiency optical spectrophotometer with simultaneous 2-dimensional spatial resolution. The technical concept and observational properties will be discussed, along with several examples of applications.

Backyard Cosmology: Making Abundance Observations of Nearby Galaxies Interesting

Evan D. Skillman

Astronomy Department, University of Minnesota, USA

Within the context of observations of nearby (primarily Local Group) galaxies, I will address the challenge in the title.

Emphasis will be placed on understanding the mass-luminosity- surface brightness-metallicity relationships, and simple ideas concerning the chemical evolution of dwarf galaxies.

New Galactic abundance gradients from early-type stars - the Milky Way and beyond.

Stephen Smartt

Isaac Newton Group of Telescopes,Santa Cruz de La Palma, Spain

I will present new results on abundance gradients and spatial variations in the Galactic disk derived from spectral analyses of hot main-sequence B-type stars, B-type supergiants, and A-type supergiants. Our model atmosphere techniques will be outlined, and shown to produce accurate abundance determinations in our target objects, and that using massive early-type supergiants in many other Local Group systems is feasible for probing their composition in unprecedented way. There are three aspects in particular that I wish to highlight:

• Using an extensive data set of B-type main-sequence stars lying in the solar neighbourhood and the Galactic anti-centre. we have derived, for the first time, a stellar Galactic oxygen abundance gradient - now consistent with that derived from HII regions and PN (Smartt & Rolleston 1997 ApJ, 481, L47) in the region 6 < R_g < 18kpc. We have further shown the correlation between spiral arm structure, and PopulationI metallicity (Smartt et al. A&A 123, 310). Stellar analyses offers the opportunity to probe the abundance variations of many more elements than are available through nebular methods, and I will present new results for the elements C, N, Mg, Si. For C, Mg and Si these are the first extensively sampled gradients ever produced.
• We have just analysed a collection of newly identified massive stars sitting slightly out of the disk, but in the inner 3-4kpc of the Galactic centre. These consist of main-sequence and supergiant B-types, and also some A-type supergiants. A surprising levelling off of the oxygen gradient is found, while the other $\alpha$-processed elements (e.g. Mg, Si) show continuing steep rises. Again this illustrates the wide abundance variety that we can cover with early-type stars.
• Showing the feasibility of analysing AB-type supergiant model atmospheres naturally leads us to a method of mapping the abundance variations, ratios and gradients in a host of Local Group galaxies such as the spirals (M31 and M33) and the dwarf Irregulars (e..g NGC6822 and IC1613). These supergiants are visually the brightest blue objects, and at typical magnitudes of B = 16-19^m are within the spectroscopic reach of 10m telescopes at high resolution. Combining the absorption line spectra of A and B-types we can sample a wonderfully rich range of elements (C,N,O,Mg,Si,Na,Ca,Ti,Cr,Mn,Fe,Ni,Sr,Zr); and, very importantly, the O/Fe abundance ratio within much of the Local Group as a key indicator of it's chemical evolution. We have just begun this project and first results of a survey of M31 supergiants will be discussed, along with the plans for the future 8-10m generation.

The Interstellar Abundances of the s-Process Elements Cadmium and Tin

U. J. Sofia ¹ and D. M. Meyer ²

¹Whitman College, USA
²Northwestern Univ., USA

We have determined interstellar gas-phase abundances of tin and cadmium in diffuse clouds toward 14 and 5 stars respectively. The data used were high signal-to-noise absorption spectra of the Cd II 2145 Å and Sn II 1400 Å transitions observed with the HST's Goddard High Resolution Spectrograph.

Tin and cadmium are primarily produced by the s-process in intermediate and low mass AGB stars. When compared to the solar abundances, an intrinsic (gas + dust) abundance of these elements in interstellar clouds should provide an empirical determination of s-process yields and the cumulative addition of this material into the ISM over the lifetime of the Sun. We present our preliminary analysis of the intrinsic interstellar abundances of Sn and Cd in our sample and the implications for s-process enrichment.

Our results for interstellar Sn/H indicate that tin is definitely enriched with respect to hydrogen as compared to the Sun even when only taking into account the gas-phase Sn. The data suggest that tin is exchanged between the gas and dust phases of the diffuse ISM, so a direct measurement of the intrinsic abundance of tin cannot be made with our absorption spectra. We use Ge, an element in the same column of the periodic table as tin, to infer the intrinsic Sn ISM abundance. The Cd abundance measurements show that this element is not exchanged between the gas and dust phases of the diffuse ISM. The interstellar gas-phase Cd/H is just below the solar ratio. Since the Sun appears to have higher metal abundances than the ISM, this may imply that interstellar Cd abundances, like Sn, have been enriched since the formation of the Sun.

Early Metal Formation and the Intergalactic Medium

Antoinette Songaila

University of Hawaii, Institute for Astronomy, USA

The surprising discovery of widely distributed metals in the high redshift Lyman alpha forest has allowed us to probe the very earliest star formation, which is believed to have produced this contamination. There is widespread agreement among observers and modellers that the higher column density clouds with $3\times 10^{14}~{\rm cm}^{-2} < N({\rm H~I}) < 10^{17}~{\rm cm}^{-2}$ have a C/H metallicity in the range -3 to -2.5, with Si/H being about 2 to 3 times higher. The absolute metal density in the diffuse IGM has been estimated at $\Omega_{\rm metals} = (3.3 \pm 0.8) \times 10^{-7}\,h_{65}^{-1}\,(1+2q_{\circ}z)^{0.5}$ at z = 3. A remaining fundamental question is how uniformly the IGM was polluted, and, in particular, if metal contamination extends to the low density voids in the IGM which produce the lowest column density Lyman alpha forest clouds. Such pollution would require very efficient energy release from the early galaxies which would result in higher temperatures in the IGM than current models assume. We find that C IV/H I remains roughly constant down to $N({\rm H~I}) \sim 3\times 10^{13}~{\rm cm}^{-2}$, well into the void regions, though this result is still controversial. If substantiated, it could explain recently noticed discrepancies between observations of the Lyman alpha forest and models, particularly for the lowest column density absorbers, which in the models have too little velocity broadening to be consistent with observation. It is quite possible that the energy input of the early star formation is the key to refining the models.

Increasing Abundance of infrared observing facilities

N. Thatte, R. Genzel

Max Planck Inst. für extraterrestrische Physik, Garching, Germany

Near infrared instrumentation at 4 and 8 meter class telescopes has grown rapidly in the past few years with the advent of 1024² and 2048² detector arrays. We present a brief review of high resolution spectroscopic capabilities available world-wide, with an eye on new developments, e.g. spatially resolved spectroscopy offered by integral field instruments.

Observing at near-infrared wavelengths has its own quirks. We discuss observing strategies to get past the limits of high background (including both thermal background and background due to OH sky emission lines), variations in atmospheric transmission, and detector constraints. Special attention is given to the subject of OH suppression/avoidance, which can significantly increase the sensitivity in the J, H and short K windows. This is especially important for (fainter) high redshift targets, where the rest frame visible diagnostic lines are shifted into the near infrared.

Supernova Nucleosynthesis: Models vs. Observations

Friedrich-K. Thielemann

Department of Physics and Astronomy, Univ. of Basel, Switzerland

This review concentrates on nucleosynthesis processes in general and their applications to late stages of stellar evolution and supernovae. After a brief discussion of hydrostatic burning stages in stellar evolution follows the fate of massive stars, i.e. the nucleosynthesis in type II supernovae (SNe II). Given the still existing problems with self-consistent hydrodynamic collapse and explosion calculations, our nucleosynthesis predictions are based on induced shocks with typical energies of the order of 10⁵¹ erg.

1.

One of the major issues is whether the average SNe II yields can reproduce the alpha element to Fe ratios ([x/Fe], x=O, Ne, Mg, Si, S, Ar, Ca) at metallicities [Fe/H]<-1.

2.

The Fe-group elements witness more details of the explosion mechanism, like (a) mass cuts between the proto neutron star and ejecta, (b) the neutron excess in the innermost ejected layers, and (c) explosion energies. Three specific obervables of supernova explosions ^56,57Ni (^56,57Co) [via light curve and gamma-ray observations] and ⁴⁴Ti [via gamma-ray observations] play a key role in the understanding of the explosion mechanism as a function of the progenitor mass.

3.

Variations in the above mentioned parameters (a-c) change the predictions for these unstable isotopes, observable in individual supernova explosions, but they might also explain the interesting trend of other Fe-group element abundance ratios, like [Cr/Fe], [Mn/Fe], [Ni/Fe], and [Co/Fe], of halo stars in the very low metallicity range -4<[Fe/H]<-2.5. They can be explained by a changing mix of nucleosynthesis products from explosive Si-burning with an alpha-rich freeze-out and incomplete Si-burning.

4.

Finally, we discuss the production of heavy elements in the r-process up to Th and U and connections to supernovae or other possible stellar sources.

Later in galactic evolution emerge type Ia supernovae (SNe Ia). Here the major uncertainties involved in the Chandrasekhar mass models are related to

1.

the companion star of the accreting white dwarf (which determines the accretion rate and the carbon ignition density),

2.

the flame speed of the deflagration fron after central C-ignition, and

3.

the density at which a possible transition from deflagrations to detonations occur.

We present several constraints for these three quantities as they apply to the "average" progenitor systems which provide the dominant contribution to SNe Ia ejecta and represent the major source of Fe-group nuclei in galactic evolution. These constraints are derived from nucleosynthesis calculations, and their effect on nuclei like e.g. ⁴⁸Ca, ⁵⁰Ti, ⁵⁴Cr, ^54,58Fe, and ⁵⁸Ni, for relatively slow deflagrations with a variety of deflagration speeds and ignition denisities, by requiring that the resulting Fe-group abundances are consistent with galactic chemical evolution. We also show the impact of new shell model calculations for electron captures on Fe-group nuclei on such conclusions.

Chemical enrichment of the intracluster medium

Daniel Thomas

Institut für Astronomie und Astrophysik, Scheinerstraße 1, 81673 München, Germany

Since the mass of iron in the intracluster medium (ICM) is tightly connected to the E/S0-luminosity (Arnaud et al. 1992), it is generally assumed that ellipticals are the main contributors to the ICM enrichment. On the other hand, observations indicate a steepening of the luminosity function at the faint end (e.g. Driver et al. 1994), and even the existence of intergalactic stars (e.g. Mendez et al. 1997), pointing to the possibility of additional sources for the origin of ICM elements. Moreover, hierarchical clustering models predict that more than 40 per cent of the ICM metals come from dwarf galaxies (Kauffmann & Charlot 1998). Abundance ratios observed by ASCA (Mushotzky et al. 1996) set strong constraints on the enrichment history of the ICM. Based on an evolution code which is calibrated on the solar neighbourhood (Thomas, Greggio & Bender 1998), the chemical evolution of the ICM is computed. In order to get a constraint on the main enrichment process, the resulting ICM abundance patterns are explored for different relative contributions from the various sources like giant Es, dwarf galaxies, and intergalactic stars.

Constraints on chemical evolution of dwarf Irregular galaxies from colour-magntiude diagrams

Eline Tolstoy

ESO, Garching bei München, Germany

The extremely accurate CMDs now available from HST observations of nearby dwarf irregulars have allowed us to put some interesting constraints on the chemical evolution that can have taken place in these very low metallicity systems. The Red Giant Branch, the Helium Burning Red Clump, and the Helium Burning Blue Loop features in a Colour-Magnitude diagram allow us to constrain the metallicity of old, intermediate and young stars in a galaxy, and thus obtain limits on possible chemical evolution scenarios. I will discuss these issues with reference to Leo A. This galaxy has almost primordeal metallicity (2.4% solar) and a fairly well determined star formation history going back several gigayears, from a very detailed HST colour-magnitude diagram.

Planetary Nebulae and the chemical evolution of ³He and ¹²C/¹³C.

Monica Tosi

Osservatorio Astronomico di Bologna, Italy

³He is one of the few elements produced by primordial nucleosynthesis and, as such, represents an important test of Big Bang Theories. It is observable only in relatively recent objects (namely, the solar system, the local ISM, a few PNe and several galactic HII regions), all of which show abundances of the order of ³He/H=10^-5, except the PNe for which values up to a factor of 100 higher have been derived. The interpretation of this apparent inconsistency and a description of the overall evolution of this light element from the Big Bang to the present epoch requires a detailed modelling of its galactic evolution.

By computing a series of chemical evolution models taking into detailed account the stellar nucleosynthesis of ³He, Galli et al (1997) suggested that the observed abundances of this element can all be justified and reconciled with each other, if a mechanism of deep-mixing occurs in the interior of the vast majority of low mass stars. Such mechanism has been proposed independently by several authors (e.g. Boothroyd & Sackman, Charbonnel, Hogan, Weiss, etc) and destroys most of the existing ³He during the red giant evolutionary phase. Since the deep-mixing mechanism affects non only the stellar yields of ³He, but also those of other more abundant elements, for instance by significantly reducing the ¹²C/¹³C ratio predicted by standard nucleosynthesis computations, we have followed a two-folding approach to check the self-consistency of this scenario.

On the one hand, we have computed the corresponding galactic evolution of the carbon isotopes and compared the resulting predictions with the available observational constraints. Here we present the results of these new computations, showing that a percentage of at least 75% of low mass stars affected by the deep-mixing is required not only to reproduce all the observed ³He abundances, but also to best fit the time and space distribution of the ¹²C/¹³C isotopic ratio.

On the other hand, we have observed at IRAM a number of PNe (with and without detected ³He lines) to measure their ¹²C/¹³C and check if the distribution of the resulting ratios with mass of the stellar progenitor is consistent with the percentages derived from the chemical evolution models. The uncertainties in the derivation of both the carbon ratios and the progenitor masses are too large to infer firm results from these measures, but we show that they are consistent with those from the chemical evolution models.

Metallicity and massive stars at $z\sim 0$ and at high redshift : the evolution of the ionising background

David Valls-Gabaud^1,2 and Joël Vernet³

¹ Observatoire de Strasbourg, 11 Rue de l'Université, F-67000 Strasbourg, France
² Royal Greenwich Observatory, Madingley Road, Cambridge, CB3 0EZ, UK
³ ESO, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

Both at low and at high redshifts, quasars appear to be less numerous than galaxies, and provide a smaller contribution to the ionising background radiation. Using a detailed modelling of the evolution of the ionising background from z=0 to 5, we discuss its implications for a variety of observables, from the HeII Gunn-Peterson effect to the luminosity density of galaxies and quasars as a function of redshift. We analyse in detail the pattern of abundances and ion ratios (CIV, SiIV, SiIII, OVI) at both high and low redshift, using Keck and HST observations respectively, and confirm our earlier prediction that the background is dominated by young massive stars at $z \leq 1.5$ and $z \geq 3.5$. We discuss the implications of this analysis and provide further tests to measure the shape and amplitude of the ionising background.

Abundance determination of New Planetary Nebulae close to the Galactic Center

G.C. Van de Steene

Mount Stromlo and Siding Spring Observatories, Canberra, Australia
e-mail: gsteene@mso.anu.edu.au

George H. Jacoby

National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726, U.S.A., e-mail: gjacoby@noao.edu

Planetary Nebulae (PNe) are observable at large distances throughout the Galaxy. They serve as probes of kinematics and abundances in the Galactic Bulge, and thus, are important in unraveling the history.

We identified 95 new planetary nebula candidates, beyond the 34 previously known, in the central 4 degrees of the Galactic Bulge (Acker et al. 1992, Strasbourg-ESO Catalogue of Galactic Planetary Nebulae), in [SIII]$\lambda$9532 using the KPNO 0.6-m Schmidt. The brightest 45 candidates of this unique, large sample were confirmed spectroscopically as planetary nebulae at the ESO 1.52-m, and 22 fainter candidates were confirmed with the CTIO 4-m. The velocity dispersion is consistent with the stellar velocity dispersion in the Bulge and reaffirms the likelihood that these planetary nebulae are not foreground objects.

So far, half of the PNe were observed in the radio continuum to obtain their radio flux and angular diameter, and to estimate the extinction and total absolute H$\beta$ flux. The knowledge of these quantities is necessary to determine the abundances and central star properties via photo-ionization modelling with the method described in van Hoof & Van de Steene (1998, MNRAS submitted).

We will present the latest results of the photo-ionization modelling.

Damped Lyman $\alpha$ systems at low redshift towards HE1122-1649 and HE0515-4414

Ana de la Varga and Dieter Reimers

Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany.

Very few DLA at z<1.1 have been analyzed up to now and enlarging the sample is essential in order to understand the nature of DLA. We have studied the properties of two DLA at z=0.68 towards HE1122-1649 and z=1.15 towards HE0515-4414. Metal abundances are found to be rather low (of 1/10 and <1/20 of solar value) and comparable to the abundances in high z DLA. Therefore, the metallicity increase with decreasing z seems to be much more moderate than expected in cosmic chemical enrichment models and the enlarged sample is even compatible with no metallicty evolution. The relative abundance pattern for the DLA at z=1.15 towards HE0515-4414 is compatible with considerable amounts of Galactic-type dust. On the contrary, for the system at z=0.68 towards HE1122-1649, the comparable amounts of Fe, Ti, and Zn suggest that the system has a very low metal abundance and negligible dust content. Its abundance pattern resembles somewhat that in metal-poor Halo stars. Constraints for N/Fe and N/Si also hints at z=0.68 DLA as a galaxy with recent galaxy formation. Only weak absorption from highly ionized species associated to low z DLA have been detected and it probably originates in regions distinct from the low ionized gas. The low-ion profiles show a very complex structure: the system at z=0.68 spans over $\sim$ 300 kms^-1 and the z=1.15 DLA presents several substructures over more than 700 kms^-1, the largest extent in the velocity space found up to date for a DLA. Possible correlations between the profile kinematics and metallicity are discussed. These new systems added to the available sample confirm the assertion of DLA being a heterogeneous population.

The detection of the 2200 Angstrom dust feature in high redshift radio galaxies.

J Vernet¹, R A E Fosbury², M Villar-Martin³, M H Cohen⁴, A Cimatti⁵, S di Serego Alighieri⁵.

¹ ESO, Garching bei München, Germany
²ST-ECF, Garching bei München, Germany
³Department of Physics and Astronomy, University of Sheffield, UK
⁴California Institute of Technology, Pasadena, USA
⁵Osservatorio Astrofisico di Arcetri, Firenze, Italy

Deep Keck spectropolarimetry has been obtained of 9 powerful radio galaxies with redshift between 2.3 and 3.6 giving a spectral coverage from Ly alpha to beyond CIII]. In addition to revealing new correlations between NV/CIV line ratio and polarization, we clearly detect the 2200 angstrom dust feature. All objects in our sample exhibit a common continuum shape with a maximum around 1400 angstrom (which corresponds to the minimum of the dust extinction curve) and a dip around 2200 angstrom. Fitting the data by a power law absorbed by a standard galactic extinction law in the ragio galaxy rest frame gives an E(B-V) of $\sim 0.1$ and an approximatively flat SED in $F_\lambda$. This suggest a 'scattering atmosphere' mechanism where most of the light we see comes from regions where the optical depth is $\sim 1$. We will discuss implications of this result for the star formation history and rates, dust chemistry and ISM chemical composition.

Effects of the Environment on Star Formation and Abundances in Galaxies

J.M. Vilchez

Instituto de Astrofisica de Canarias and Instituto de Astrofisica de Andalucia, Spain.

The influence of the environment on the processes that regulate star formation and galaxy evolution is a key parameter in the study of the origin and evolution of galaxies. New evidences have reinforced the idea that galaxies located in high density environments, such as groups and clusters, show different star formation rates and evolutionary histories as compared to field galaxies. A compilation of the most recent results concerning environmental effects on star formation and chemical evolution is reviewed, with a special emphasis on the analysis of the evolution of star-forming galaxies.

The Small-Scale Structure of the Orion Nebula - Evidence for the Presence of High Density Condensations

J. R. Walsh, M. Rosa

ESO, Garching bei München, Germany

HST FOS small aperture spectra covering the wavelength range 1600 to 8500Å have been obtained over one of the Trapezium Partially Ionized Globules (PIGS) and a narrow filament in the core of M 42. The spectra have been compared with the surrounding background and the electron density and temperature and abundances have been derived. For the PIG we found a very high density ($\sim$5$\times$10⁵ cm^-3), such that much of the forbidden line emission occurs at the critical density; as a result the derived electron temperature is high since the forbidden line emission cannot effectively cool the gas. The filament displays the same effect but the density is more modest, but still enhanced relative to the mean surroundings. This suggests a natural explanation of T_e fluctuations in Orion - that they are in fact attributable to the cululative effect of high density, spatially unresolved, globules in the line of sight.

We have drizzled the set of WFPC2 narrow band images of M 42 to produce photometrically accurate ratio maps in order to study the possible presence of high density condensations with similar line ratios to the condensations observed with FOS. We find that upto 5% of the summed emission from the Orion core can arise in dense globules and discuss the effect this has on abundance determinations in general.

Elliptical Galaxy Chemical Evolution

Guy Worthey

St. Ambrose University, Davenport, USA

Elliptical galaxies are the most chemically evolved objects in the universe. The average metal abundance of an elliptical galaxy is not much greater than that of the sun, decreasing with radius as d[Fe/H]/d log R = -0.3 dex per decade. Light elements (at least N, Na, and Mg) are enhanced relative to Fe and Ca in elliptical nuclei. In the case of Mg, for which the best data exist, the light-element enhancement appears global, not a nucleus-only phenomenon. However, there is a clear trend with galaxy size: large galaxies have a stronger light element enhancement than small ones. Of the several possible explanations for this, we favor one in which the IMF was mildly top-heavy in larger galaxies, causing more Type II supernova light-element enrichment. The abundance distribution of elliptical galaxies is strongly peaked, like that of the solar neighborhood or even more extreme.