Comunicato Stampa

UVES Analyses the Universe: A First Portfolio of Most Promising Results

06 Aprile 2000

Astronomers working with a major new instrument at the ESO Very Large Telescope (VLT) at the Paranal Observatory have had a first taste of what is bound to become a research bonanza. Recent test observations with the Ultraviolet-Visual Echelle Spectrograph (UVES) have demonstrated the exceptional science potential of this powerful facility.

The first long-exposure test spectra of stars, galaxies and quasars obtained with UVES have thrilled the astronomers by their extraordinary quality. Some have already yielded important results, from unprecedentedly accurate chemical analysis of individual stars in nearby galaxies to abundance measurements of light elements created during the Big Bang. Others provide new insights into the composition of the gas in a galaxy in the early Universe, less than 3 billion years after the Big Bang [1]. One particular set of observations measured an upper limit of the uranium content in a very old star - and thereby a lower limit to the age of the universe.

UVES starts operations

UVES began normal operations on April 1, 2000, at the Nasmyth B focus of KUEYEN, the second 8.2-metre VLT Unit Telescope, and it is now fully at the disposal of the scientists. The observing time available until the end of September 2000 was assigned to a large number of front-line research projects by the ESO Observing Programmes Committee. For UVES, as for all other VLT instruments, all observations are executed by ESO staff. In "service observing" mode, they choose the targets from a pre-compiled list, taking into account programme priority and sky conditions. In "visitor mode", the ESO astronomer executes the observations according to the sequence chosen by the visiting astronomer who is also present at the telescope.

UVES is the third instrument after FORS1 and ISAAC to enter into regular use at the VLT. It is followed by FORS2, the second version of FORS that was successfully commissioned earlier this year (c.f. ESO Press Photos eso0005), and which also started operations on April 1, at the Cassegrain focus of KUEYEN.

UVES achieved "First Light" on September 27, 1999, c.f. ESO Press Release eso9944. This event was followed by three intense weeks of "Commissioning Observations", by a second tune-up in December 1999 and finally by a very successful period of "Science Verification" in February 2000, c.f. the dedicated UVES SV webpage (SV data are being released starting on April 10).

A very powerful instrument

The analysis of the test observations has demonstrated that UVES is presently the most powerful astronomical instrument for high-resolution spectroscopy available to the world-wide astronomical community. Its high intrinsic efficiency, at all wavelengths from ultraviolet to red light (e.g., 13% at 360 nm, 18% at 600 nm; [2]), coupled with the large light collecting power of the 8.2-metre KUEYEN telescope, makes it perfectly suited for high-resolution spectral observations of faint stellar and extragalactic objects.

UVES achieves "resolving powers" up to 80,000 in the UV-blue spectral region and 115,000 in the red region [3].

UVES has several unique capabilities. First, it can work in parallel at blue and red wavelengths by using a dichroic beam splitter that divides the light from a celestial object at the entrance of the instrument. This observational mode is as efficient as single-spectral-region operation and it implies that it is possible to record light simultaneously over a very broad wavelength interval - up to 500 nm at a time. Secondly, the available CCD detectors register the spectra over 23 million pixels (as compared to 4 million pixels available in similar spectrographs elsewhere). This results in a much better sampling, i.e., a more detailed view, of the spectral features at any given resolving power.

First science results from UVES

The instrument operated smoothly during the entire three weeks of the first commissioning. Altogether, only 7 hours were lost due to technical problems of either the instrument or the telescope, a small figure at this stage of operation of a new instrument at a new telescope. A large number of scientific observations were obtained with the aim of testing the limiting capability of UVES for various types of research programmes.

At the same time, the data reduction pipeline was successfully tested; it contributes effectively to the extensive data processing needed to prepare the complex spectra for accurate astrophysical evaluation. Since February 2000, many of these observational data, with a total of more than 90 hours of pure integration time, have been included in the Public Data section of the ESO Science Archive facility.

The analysis of the first scientific data has been carried out by a team of ESO astronomers and by members of the UVES Instrument Science Team [4] who have made available the results to illustrate the impressive capabilities of this powerful instrument as reported below. They cover a wide range of current front-line research areas and they provide an most promising preview of the many observational possibilities with this new instrument.

Short summaries of the first science with UVES are presented here. The titles provide weblinks to detailed reports on each of the eight research topics, attached below.

* A. The beryllium abundance in extremely metal-poor stars

Spectral lines from the light element beryllium in the ultraviolet part of the spectrum close to the atmospheric limit for ground-based observations have been detected in a 3-hour exposure of a very metal-poor, 11-magnitude galactic star. Further observations of this type of a larger sample of stars will provide crucial information about the primordial abundance of this element, and hence about the element-building processes in the early universe.

Together with lithium (Li) and boron (B), beryllium (Be) belongs to a small group of elements (the "light elements") that are not formed by nuclear reactions in stellar interiors (where, on the contrary, they are destroyed). The main mechanism responsible for their production, and the only one in case of beryllium, involves spallation reactions between high energy cosmic rays and abundant species such as carbon (C), nitrogen (N) and oxygen (O) in the interstellar medium.

Knowledge of the abundance of beryllium in the Milky Way Galaxy allows to trace its chemical history and that of cosmic rays; it also provides a further test of the current theories of Big Bang nucleosynthesis.

The abundance of beryllium can be obtained from measurements of the Be II (single ionized beryllium) resonant lines at wavelength 313.1 nm, just above the atmospheric cut-off around 300 nm, caused by ozone molecules. The atmospheric opacity at this ultraviolet wavelength is high and very little light gets through to ground-based telescopes. Together with the low efficiency of current instrumentation at this wavelength, this has until now made measurements of this important beryllium line extremely difficult, even in relatively bright celestial objects.

Moreover, some of the most interesting targets in this research field are relatively faint, old and metal-poor stars which formed from low-abundance interstellar gas in the early phases of the evolution of the Galaxy and hence trace the pristine abundance of beryllium. So far, only two measurements have been made of the beryllium abundance in stars with very low metallicity indices of [Fe/H] ~ -3 (the iron-to-hydrogen abundance [Fe/H] = log(Fe/H) star - log(Fe/H) sun), and both were only possible after an enormous observational effort involving many hours of exposure.

It has been predicted that UVES will play a unique role in this research area, since its overall efficiency at this short wavelength is four times higher than the most powerful facility available so far.The first tests with UVES provide a clear confirmation of this. In this preliminary test, two extremely metal-poor stars, LP 815-43 (visual magnitude V = 10.91) and CD -24°17504 (V =12.18), with metallicity indices around -3.1 and -3.5, respectively, were observed. The total exposure time was 3 and 2.5 hours, respectively. A signal-to-noise (S/N) ratio of around 100 was achieved in the added-up spectra for the first and around 80 for the second star, both at the high resolving power of about 45,000.

The beryllium features are detected at the 2-sigma level in the higher-S/N spectrum of LP 815-43, with a measured equivalent width of the bluer BeII line of 2 mÅ. This would imply log(Be/H) + 12

This observation provides a very valuable first test and paves the way towards more systematic investigations of a variety of stars in the coming season. Longer exposures than those used during these first tests will increase the accuracy of the abundance determinations.

With UVES it will now be possible to investigate if the stellar beryllium content continues to decrease with decreasing metallicity or whether there exists a plateau of more or less constant beryllium abundance at very low metal abundances. Whatever the case, the coming observations will provide crucial information about the conditions in the young universe.

* B. The isotopic lithium abundance in a metal-poor halo star

An unexpectedly low 6 Li/ 7 Li-ratio of 0.02 is measured in a very-high resolution, very high signal-to-noise spectrum of a 10-mag metal-poor star, obtained during a 3-hour exposure. This new result indicates that it will be necessary to obtain many more measurements of this key isotopic ratio before it can provide significant cosmological information, an observational task for which UVES is very well suited

The isotopic abundances of lithium (Li) in metal-poor stars - objects that were formed from interstellar gas in the early phase of the universe, some 12 - 14 billion years ago - provide key information about the amount of lithium produced by nucleosynthesis in the Big Bang, and by reactions between cosmic rays and nuclei in the interstellar gas.

To disentangle the relative contribution of these two processes, and to correct the measured abundances for lithium depletion by thermonuclear reactions with protons at the bottom of the convection zone in the stellar atmospheres, isotopic abundances are needed for a large sample of stars.

In stellar spectra, the measurement of the low-abundance, light 6 Li isotope is based on the increased width and asymmetry of the 7 Li doublet spectral line at wavelength 670.8 nm, introduced by the isotopic shift of 6 Li of about 0.016 nm (corresponding to 7.1 km/sec).

It is a critical measurement which requires both high spectroscopic resolution and very high signal-to-noise ratios. The signature of 6 Li in stellar spectra has so far been recognized in four stars only; two are halo stars, and two are metal-poor disk stars, with an isotopic ratio at a level of 6 Li/ 7 Li = 0.05.

UVES is potentially capable of extending these observations to fainter stars and to stars with a wide range of metallicities. Such observations will make a very significant contribution to our understanding of the formation of the light elements in the universe and their cosmic chemical evolution.

On the technical side, this program is a critical test of the spectrophotometric calibration of UVES. Flat-fielding procedures must provide a very accurate correction of the pixel-to-pixel variations in the detector (which can be up to 20% at red wavelengths), in order to reach signal-to-noise ratios in the detected stellar continuum that are higher than 300, as needed for this type of observation.

The open circles correspond to the observed data and the fully drawn line to a model atmosphere synthesis with isotopic ratio 6 Li/ 7 Li = 0.02; see the text. The inserted profile of a nearby thorium line from the calibration lamp represents the instrumental profile of UVES ; it has Full-Width-Half-Maximum (FWHM) of 0.056 mÅ, corresponding to a resolution of R = 112,000.

The stellar line broadening function needed for such a study is determined from several other lines available in the recorded UVES spectrum. An important byproduct of this spectrum is detailed knowledge about the turbulent motions in the atmosphere of the star.

The UVES observation of the metal-poor ([Fe/H] = -2.2) halo star G271-162 (V = 10.35) at a spectral resolution ~ 110,000 was included in the commissioning programme to prove this instrumental capability. This star is representative of a group of about 50 halo turn-off stars that are too faint to be studied with high-resolution spectrographs at 4-m class telescopes.

A total of around 3 hours of integration was obtained in excellent seeing through a slit opened to 0.3 arcsec. The S/N of the summed, reduced spectrum is higher than 600.

It is obvious that the profile of the Li line in G271-162 is considerably broader than the instrumental profile. This is due to Doppler-broadening in the stellar atmosphere, caused by thermal and gas-turbulent motions of the absorbing Li atoms.

Furthermore, the line is clearly asymmetric. This is primarily due to the doublet structure of the 7 Li absorption; the two components have a theoretical relative strength of 2:1. However, any presence of 6 Li adds to the asymmetry of the line. From a detailed model atmosphere study of the data shown, it is concluded that 6 Li is marginally detected at the abundance ratio 6 Li/ 7 Li = 0.02.

This is a relatively low value when compared to previous stellar determinations and to the value measured on Earth and in meteorites, 6 Li/ 7 Li = 0.08. This first result thus clearly points to the need to extend such measurements to a large number of stars, before the value of the isotope ratio and its scatter can be well understood. Only then may the measured ratios provide useful information, not only about the individual stars, but also at the cosmological level.

* C. Tracking the age of the universe with radioactive elements in stars

A 4-hour exposure of the spectrum of an old 13-mag star allows a firm upper limit to be placed on the uranium abundance in its atmosphere. When used with the measured thorium abundance, this translates into a minimum age of 12 billion years of the universe. Future observations of this and similar stars with longer integration times will be able to set tighter limits to this age determination.

Radioactive elements which have very long decay-times, such as thorium (Th) and uranium (U), can be used to accurately date the age of stars, in a way that is quite similar to that used in archaeology by means of Carbon-14 dating.

The oldest objects presently known in the universe are low-mass, very metal-poor stars in the halo of the Milky Way Galaxy. These stars are of course younger than the universe. Thus, if it would be possible to put more stringent constraints on the ages of such stars, a more precise lower limit to the age of the Universe can be derived.

The radioactive element dating method does not make use of stellar models such as, e.g,. the globular cluster dating method to estimate the minimum age of the universe. Therefore, it provides an independent estimate and the comparison of these estimates allows a cross check of both methods.

Thorium and uranium are heavy elements that produce faint absorption lines in the extreme blue and UV part of the spectrum. With the hitherto available instruments, it was extremely difficult to observe such lines in faint, metal-poor stars.

CS22982-52 (V = 13.2, [Fe/H] = -3.1) is a very old star that was born at a time when the Milky Way Galaxy was very young and contained 1000 times less iron than does the Sun, while several other elements, including very heavy elements (to which U and Th belong), are not so depleted compared to the Sun. This star is therefore a perfect test-case for the radioactive decay dating method.

The very faint uranium line is discernible as a slight "depression" in the recorded spectrum. In order to estimate the critical upper limit of the uranium abundance in the star's atmosphere, several synthetic spectra with different logarithmic abundance of uranium (from "none" to -2.31 on a scale where the logarithmic hydrogen abundance is 12) have been computed (continuous thin lines in both diagrams). The abundance values are indicated - it can be seen that the model with [U/H] ~ -2.5 is clearly below the observed spectrum, i.e., this value is a very conservatory upper estimate of the uranium abundance. The strong lines on either side of the U II line are due to iron (Fe I), while the weaker features are blends of lines from various elements.

The star was observed for a total of 4 hours with UVES and the S/N ratio in the final spectrum is 130 at the uranium line near 386 nm. A spectrum of this high quality could never have been achieved without the combination of the large collecting power of the 8.2-m KUEYEN telescope and the extremely good sensitivity of UVES in this ultra-violet spectral region.

A very conservative upper-limit of [U/H] = -2.5 for the uranium abundance (relative to hydrogen) was established by comparing the observations with spectrum synthesis calculations with different uranium abundances; see the diagramme. The abundance of thorium was also measured from the line at 419 nm and checked on weaker lines in the 330 - 380 nm region.

From the known decay times, this upper limit of the uranium abundance, together with the thorium abundance measurements, translates into a minimum value of 12 billion years for the age of the Universe .

* D. Spectra of individual stars in other galaxies

In an unprecedented observational feat, high-quality spectra of two 18-mag giant stars in the Sagittarius Dwarf Galaxy of the Local Group of Galaxies were obtained from which the abundances of 20 different elements were determined. The deduced fractions of heavier elements were unexpectedly high, a clear sign that this galaxy has experienced more intense chemical reprocessing during its evolution than previously believed, on the basis of less detailed observational data.

The measurement of chemical abundances in individual stars in Local Group galaxies makes possible the study of their history of star formation.

Stars are the witnesses of the chemical evolution of galaxies, reflecting in their atmospheres the chemical composition of the interstellar gas in the galaxy at the time when they were born. Stellar photometry and the resulting colour-magnitude (CM) diagrammes of Local Group dwarf galaxies have been used to deduce the star formation histories in these systems, suggesting that they, unlike our own Galaxy, have been the sites of several strong star formation bursts .

This result is in agreement with the prediction of theoretical models that chemical evolution proceeds at a different pace in different environments, e.g., in the large Milky Way Galaxy vrs. the much smaller dwarf galaxies. However, until now there it was not possible to test this result by means of a detailed comparison between the well-known properties of stars in the Milky-Way with those of stars in other galaxies.

Only direct spectral measurements of the metallicity of stars in their host galaxy would allow to disentangle the similar effects of age and metallicity in the CM diagrammes. However, these stars are so faint that their spectra cannot be observed in sufficient detail with 4-metre class telescopes.

By analysing high-resolution spectra of individual stars, like those that can now be recorded by UVES, it will be possible to obtain accurate values of the metallicities for the Local Group galaxies. High-resolution spectra also allow to measure the relative abundances of different elements, hereby providing unique information about the nucleosynthesis processes that dominate different phases of the chemical enrichment in the host galaxy.

UVES has been tested on this crucial issue during the commissioning period. Spectra of unmatched quality were obtained for stars in 3 Local Group galaxies, two giants in the Sagittarius Dwarf galaxy, two supergiant stars in NGC 6822 and a cluster giant star in the Large Magellanic Cloud.

The analysis of the two stars in the Sagittarius Dwarf galaxy has now been completed. Abundances were determined for 20 elements: O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Y, Ba, La, Ce, Nd and Eu. While earlier photometric studies indicated a metallicity much lower than the solar one, the present spectral analysis found that the overall metallicity of the stars is only two times less than solar. The abundance pattern is essentially solar (no alpha elements or oxygen enhancement), with the noticeable exception of the very heaviest species (Ba to Eu) which are enhanced by a factor that is similar to what is observed in the Magellanic Clouds.

This implies that this dwarf galaxy has experienced a higher level of chemical reprocessing on a longer timescale than was previously assumed . The new finding is a clear demonstration of the crucial importance of high-quality spectra for this type of investigation.

Future systematic observations with UVES will certainly shed new light on the evolution of these systems.

* E. Hunting black holes in the nuclei of galaxies

By measuring the exact shape of spectral lines emitted from gas moving around the centre of the spiral galaxy NGC 7782 and using a dynamical model for these motions, it has been possible to determine the mass of the presumed black hole at this centre. An unusually small, upper limit of about 50 million solar masses was derived. Future observations of galactic nuclei of this type will permit interesting demographic studies, for instance the relation(s) between the mass of central black holes and those of galaxy bulges.

It is believed that super-massive black holes are present at the centres of most galaxies and that they constitute the hidden engines of the observable phenomena related to the Active Galactic Nuclei . But which fraction of galaxies do possess such holes and what is the distribution of masses and the relation to the other galaxy parameters, as the mass of the bulge, the total mass? This is another fertile research field to which ESO's new spectrograph can provide very valuable contributions.

An UVES spectrum was obtained of the nucleus of the spiral galaxy NGC 7782, which is located in the southern constellation Pisces (The Fishes) at a distance of about 250 million light-years (74.8 Mpc). It unveils the presence of a central gaseous component that revolves around a mass concentration at the centre of the galaxy.

The two-dimensional shape of the spectral emission lines, c.f. ESO Press Photo eso8709, provides detailed information about the gas velocities (ordinate) at a given position in the galaxy (abscissa). At the high spatial and velocity resolution achievable with UVES, it is possible put constraints on the mass of the central supermassive black hole in this galaxy. The upper limit is estimated at 50 million solar masses, a relatively small value for such an object. With the achievable accuracy, UVES would certainly be able to detect and measure even less massive black holes.

This observation illustrates how the high-resolution capabilities of UVES, combined with the large light-collecting power of the VLT, makes it a prime instrument to extend demographic studies of super-massive black holes in galaxies, for instance in order to better define the relation between their mass and those of the spheroidal components of the host galaxy. This relation offers strong clues to galaxy formation and evolution.

* F. A first glimpse at the intergalactic medium in the redshift interval z = 1.5 - 2.0

On its way to us, the light from a remote quasar traverses many intergalactic clouds that leave their signatures in the quasar spectrum in the form of absorption lines. Most prominent are those of hydrogen, especially the Lyman-alpha spectral line. The study of these lines allow to determine the mean density of the intervening clouds at a given redshift, i.e. distance and hence epoch of the universe. With its unsurpassed ability to observe detailed ultraviolet spectra of even quite faint objects, UVES has provided a first glimpse of the intergalactic medium during the so far unexplored, but crucial epoch between approx. 8 and 10 billion years ago (redshift interval 1.5 - 2.0), at a time when intense star formation took place.

The physical state of the intergalactic medium (IGM) at high redshift is important for the understanding of the formation and evolution of large-scale structures and individual galaxies which mostly formed at an early stage of the universe.

The absorption line systems imprinted on the spectra of high-z QSOs provide a unique and powerful tool to study the IGM in the universe, up to the distances of the farthest known quasars at z ~ 5. In particular, absorption line systems of lower hydrogen column density in the Lyman-alpha forest (i.e., with N(HI)

The properties of the Lyman-alpha forest at different redshifts allows the study of various cosmological parameters such as the baryon density and the density parameter Omega, and thus have direct bearing on the cosmological theories on the evolution of the early universe.

The Lyman-alpha forest and the metal systems (i.e., denser Lyman-alpha absorptions to which many metal lines are also associated) are thought to be photoionized by the metagalactic UV background and contaminated by the metals produced in the early stages of galaxy evolution. As such, they also provide a unique opportunity to probe the intensity and shape of the metagalactic UV background as well as the type of star formation scenarios which may account for the observed metal abundances.

The epoch in the universe that corresponds to redshifts between 1.5 and 2.5 is one of the most interesting, because it is characterised by intense star formation. However, it remains largely unstudied until now because the line "signatures" of luminous matter (both stars and emitting gas) fall mainly in the little-studied IR region and, in particular, the key resonance absorption lines of the gas are seen in the UV region of the spectrum (300 - 400 nm) for which no efficient, high-resolution spectrograph has so far been put into operation at very large telescopes.

But now, the high UV efficiency of UVES opens new research possibilities in this field. Already in the commissioning phase, several QSOs at redshifts around 2 were extensively observed.

The analysis of these test data has been completed and the results about the forest and the metal absorption systems now provide the first detailed information about the Intergalactic Medium in this redshift range.

Various examples of these results are shown in eso0013f through eso0013h . The specific implications are explained in the associated captions.

* G. UVES takes a deep look at the intergalactic gas in the direction of the Hubble Space Telescope Deep Field South

UVES has delivered the best spectrum ever obtained of the 17.5-mag quasar at the centre of this sky field that has been extensively studied with the HST. It covers the entire spectral range from the atmospheric UV-limit to the near-infrared (305 - 1000 nm) and shows a large number of spectral lines from intervening gas clouds in this direction. When compared with other data, this now allows a detailed study of the distribution of matter far out in this direction, corresponding to a look-back time of about 8 billion years. According to current ESO policy, these data for HDF-South have been made public worldwide".

The Hubble Deep Field South (HDF-S) is a region of the sky in which the Hubble Space Telescope (HST) has provided the extremely deep images at wavelengths from the UV to the infrared. The photometric information on the galaxies in this field, complemented by low-resolution spectroscopy on the brightest ones, is being used to reconstruct the distriedshiftof luminous matter with redshift.

The HDF-South field is centered on the V = 17.5 quasar J2233-606 with emission redshift z = 2.238. The absorption spectrum of the quasar can be used to extract the properties of the intergalactic gas in the QSO line-of-sight up to its emission redshift which corresponds to an age of the Universe of approx. 8 billion years. The UV spectrum of the QSO has already been observed by the STIS instrument of the HST, while 4-m class telescopes have been used to get high-resolution spectra at blue-visual and red wavelengths. UVES has observed the complete spectrum of the quasar in the range 305 - 1000 nm with a total integration time of almost 8 hours. The data have been reduced by the UVES Commissioning team and are available at the corresponding web pages of the Space Telescope European Coordinating Facility. With an average resolution of 45,000 (i.e., 6.5 km/sec) and a S/N-ratio in the range 40-80, the spectrum is of unique quality. It will permit a detailed study of the properties of the Lyman-alpha forest and of the metal systems, including their ionization state and abundances (see Report F above).

For the first time, astronomers have at their disposal high-quality data to establish a relationship between the luminous matter (galaxies) and the absorbers, both in the QSO environment and at various redshifts along the line of sight to the QSO.

* H. First accurate measurements of the oxygen and zinc abundance in a damped system at z > 3

Observing the spectrum of a 17.5-mag quasar at redshift 4.1, it has been possible to analyse the metal content of a system of absorbing lines associated with a galaxy, so faint that its emission escapes detection, at redshift 3.4, corresponding to an epoch at about 2 - 3 billion years after the Big Bang. The abundances of several metals, as well as of oxygen, zinc (first time at such a great distance) and other elements were measured; they suggest an episodic or low star formation rate at this early stage of the evolution of the galaxy. Damped Lyman-alpha systems (DLA) are the class of absorption systems in QSO spectra that show the largest gas column density (N(HI) > 2x10 20 cm -2). They represent the majority of the neutral gas at high redshifts and are widely believed to originate in the progenitors of present-day galaxies.

By studying the low-ionization metal absorption lines associated with the damped line systems, the absolute and relative abundances of several key elements can be measured. They reveal the amount of nuclear processing that has taken place in these systems and which of these are the dominant ones.

These abundance measurements at high redshifts are the most accurate estimates of metal abundances we have for galaxies in the early stage of galaxy formation in the universe.

Note

[1] 1 billion = 1,000 million.

[2] These figures indicate the percentage of the photons from a celestial object entering the UVES spectrograph slit that are effectively registered by the detectors. For comparison, astronomical spectrographs constructed in the early 1990's only reached efficiencies of the order of 5% and 10% in these wavebands, respectively. The very effective performance of UVES thus signifies another important gain (in addition to that caused by the large mirror of KUEYEN), allowing comparatively fainter objects to be observed, or shorter integration times in the case of brighter objects.

[3] The "spectral resolution" indicates the amount of spectral detail that is registered. The number is calculated as the wavelength of observation, divided by the smallest wavelength difference at which two spectral lines can still be resolved. A resolution of 115,000 at the wavelength of H-alpha, a prominent hydrogen emission line at 656.2 nm in the red spectral region, thus corresponds to the possibility of recording individual spectral features that are only 0.006 nm apart (or a velocity difference of about 0.26 km/sec).

[4] The UVES Instrument Science Team was constituted at the end of 1992 and is composed by Bengt Gustafsson (Uppsala Observatory, Sweden), Herman Hensberge (Royal Observatory, Bruxelles, Belgium), Paolo Molaro (Trieste Astronomical Observatory, Italy) and Poul Erik Nissen (Chairman, Aarhus University, Denmark). The team members followed the development of the instrument from the early design phase to its installation at Paranal, providing timely advice on all science-related matters. Antoinette Songaila (Hawaii, USA) and Francesco Bertola (Padova, Italy) also contributed to the first evaluation of the data.

Ulteriori Informazioni

A. The Beryllium Abundance in Extremely Metal-Poor Stars

An extensive discussion of these results will be given in a research paper by Francesca Primas and Vanessa Hill (ESO Garching), Paolo Molaro and Piercarlo Bonifacio (Trieste Astronomical Observatory, Italy), now in preparation.

B. The Isotopic Lithium Abundance in a Metal-Poor Halo Star

An extensive discussion of these results is given in the paper by Poul Erik Nissen (Aarhus University, Denmark), Martin Asplund (Uppsala Astronomical Observatory, Sweden), Vanessa Hill and Sandro D'Odorico (ESO Garching), in press in the European research journal Astronomy & Astrophysics.

C. Tracking the Age of the Universe with Radioactive Elements in Stars

An extensive discussion of these results will be given in a research paper by Bengt Gustafsson and Michelle Mitzuno-Wiedner (Uppsala University Observatory, Sweden), Vanessa Hill and Francesca Primas (ESO Garching), Piercarlo Bonifacio and Paolo Molaro (Trieste Astronomical Observatory, Italy), and Poul Erik Nissen (Aarhus University, Denmark), now in preparation.

D. Spectra of Individual Stars in Other Galaxies

An extensive discussion of these results is given in the paper by Piercarlo Bonifacio, Paolo Molaro, Paolo DiMarcantonio and Paolo Santin (Trieste Astronomical Observatory, Italy), and Vanessa Hill and Luca Pasquini (ESO Garching), submitted to the European research journal Astronomy & Astrophysics .

E. Hunting black holes in the nuclei of galaxies

An extensive discussion of these results will be given in a research paper by Francesco Bertola, Michele Cappellari, Enrico Corsini, Alessandro Pizzella and Marc Sarzi (University of Padova, Italy) and José-Gabriel Funes, S.J. (Vatican Observatory), now in preparation.

F. A First Glimpse at the Intergalactic Medium in the Redshift Interval z = 1.5 - 2.0

The first analysis of the data shown here has been carried out by Taesun Kim and Sandro D'Odorico (ESO Garching) and Stefano Cristiani (ST/ECF, Garching). The corresponding research paper is in preparation.

H. First Accurate Measurements of the Oxygen and Zinc Abundance in a Damped System at z > 3

An extensive discussion of these results is given in the paper by Paolo Molaro, Piercarlo Bonifacio, Giovanni Vladilo, Miriam Centurion, Paolo Dimarcantonio and Paolo Santin (Trieste Astronomical Observatory), and Sandro D'Odorico (ESO Garching), submitted to Astrophysical Journal.

Connect with ESO on social media