12 Questions on Star and Massive Star Cluster Formation

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12 Questions on Star and Massive Star Cluster Formation

An ESO Workshop

Garching (Germany), July 3 to 6, 2007

organized by: Tom Wilson and Markus Kissler-Patig

Contact: star07@eso.org 

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I. How are the stellar and cluster initial mass functions related and how are they influenced by the star formation history?

Coordinated by Marina Rejkuba

Initial mass function (IMF) is the fundamental ingredient to the population synthesis models and chemical enrichment models. Given the fact that most of the star formation is going on in clusters, the investigation of the stellar initial mass function should be closely correlated with that of the cluster initial mass function. The standard stellar IMF is typically described by the power-law function with the Sal- peter slope alpha=2.35. The flattening of the IMF slope for lower mass stars is relatively well established and there is still some debate about the possible steepening for high mass stars. Overall, the standard IMF is a segmented power-law. The initial young (em- bedded) cluster mass function is also a power-law distribution, with a slope of beta=2.0 in most of the studies. The universality of this slope is still somewhat debated. It is of interest to connect these two distributions and to show how/whether the star formation history influences them. In particular the star formation in smaller, dwarf gal- axies, does not form very massive stellar clusters, while they are more abundant in larger galaxies. The question is then how is this dependence of the initial cluster mass function reflected on the apparent universality of the stellar IMF.

II. What are the effects of stellar feedback ?

Coordinated by Nate Bastian and Ralf Klessen

Massive stars release enormous amounts of energy and momentum into their surrounding environment. This may contribute significantly to ISM turbulence on various scales, ranging from individual star-forming regions to the disk of the Milky Way as a whole. For example, high-mass stars emit UV photons, creating bubbles of hot, ionized gas around them. Such HII regions are likely to quench further star formation in their interior. By the same token, however, stellar feedback processes may trigger the birth of new stars somewhere else in the Galaxy. We ask: What are the effects of stellar feedback on subsequent star formation? When and on what scales does feedback predominantly quench star formation, when does it trigger star formation? Shifting our focus to individual young star clusters, we realize that protostellar feedback provides enough energy to rapidly remove any remaining gas from the cluster which was unused in star-formation. This leaves the cluster severely out of equilibrium. Observations point to this effect in destroying > 90% of young clusters within the first few 10s of Myr. This rapid disruption (i.e. infant mortality and infant weight-loss) is an efficient way to disperse young stars into the field. We therefore ask: How does the early evolution of star clusters depend on what types of stars are present and on what feedback processes are acting?

III. What is the demographics of star formation in our Galaxy and others?
or more specifically, What is the statistical distribution of star formation environments and is it universal?

Coordinated by Tom Megeath

Star formation occurs in a diversity of environments. The canonical examples of this diversity are the isolated young stars in the Taurus dark clouds and the dense cluster of young stars in the Orion nebula. However, observations of our galaxy and others have shown us that Taurus and Orion are only two examples in a continuum of environments ranging from isolated cores, to small groups, to large clusters, and finally to super star clusters and (in earlier epochs) globular clusters. This continuum can be found in a single star forming cloud; indeed, recent Spitzer observations show that giant molecular clouds forming rich clusters of low and high mass stars also contain large populations of isolated stars. These observations motivate several specific questions: what is the dis- tribution of star formation environments in our Galaxy, i.e. what fraction of stars in our galaxy form in isolation, groups, clusters, or super star clusters? How does this distribu- tion differ between low and high mass stars? How is the star formation environment best parameterized: number of stars in a cluster, density of stars, quantity of massive stars (and UV radiation)? How is the distribution of star formation environments in our galaxy related to the stellar clustering law observed in other Galaxies? Is there a univer- sal distribution of star formation environments? This is a fundamental question which must involve both the galactic and extragalactic communities. While extragalactic studies provide superior statistics on large clusters and super star clusters, smaller clusters and isolated low mass stars can only be detected in our galaxy. Only through a careful discussion of the observations, methodologies and biases can we proceed to a unification of the extragalactic and galactic observations with the ultimate goal of deriving a universal distribution of star formation environments.

IV. How did star formation proceed in Globular Clusters?

Coordinated by Francesca D'Antona

In these latest years it became more and more evident that most Globular Clusters are not 'simple stellar populations' as we thought they were. There is one 'very peculiar' cluster, Omega Cen, in which the different s stellar metallicities indicate that the star formation process is possibly similar to that of a small galaxy, as it includes chemical enrichment from supernova ejecta. Most of the other globular clusters (examined so far) do not show detectable metallicity differences, but they show some famous chemical anomalies, e.g. the anticorrelations sodium vs. oxygen or magnesium vs. aluminum. These anomalies interest even 50% of the stars and are present also among the unevolved (turnoff) stars. They are then attributed to a 'second stellar generation'. Further signs of this second generation are the famous 'second parameter' -if it is identified with the helium content- and the existence of double main sequences, in omega Cen and NGC 2808 (and probably also in other GCs), the bluer one being terribly enhanced in helium (formally Y~0.40, implying a huge deltaY/deltaZ). Today's modelling of these two (or more) stellar generations is at a handwaving stage, in spite of spectacular observations, both spectroscopic and photometric, and of the complex computation of stellar evolution pursued so far. The main problems, relevant also for the proposed meeting, are: 1) identification of the parents of the second stellar generation (massive Asymptotic Giant Branch stars (AGBs)? Massive rotating stars?) 2) the IMF of the first generation stars must be very unusual (peaked at the mass range from which the second generation is born), or the cluster must have ways of losing in great part the low mass stars of the first stellar generation. Maybe a solution is possible by looking at the super star clusters now forming in low metallicity dwarf galaxies? The question until now has remained mostly within the stellar community, either working on the stellar populations in our galaxy both from spectroscopic and photometric points of view, or working on stellar modelling and nuclear processing in intermediate mass stars and massive stars (see recent literature). This community needs the help of the star formation and star cluster formation communities, to understand theoretically the globular clusters formation process, and, observationally, to get informations to identify the progenitors of the second stellar generation. For example, if the second stellar generation is made up from the winds of massive AGBs, there must be a phase in the cluster life, at ages in between 30Myr and 100Myr, at which these winds have been collecting in the clusters' cores forming stars. Can infrared observations of young, very massive, star clusters, in galaxies in which cluster formation is still going on, reveal this second phase of star formation?

V. How can we merge the galactic and extragalactic views of massive star formation regions derived from images and SEDs to a coherent physical picture?

Coordinated by Juergen Steinacker

In view of the importance of massive stars for the galactic and extragalactic astronomy, it is striking that we still do not have a working massive star formation model. High-resolution imaging and spectroscopy using upcoming instruments as well as the inclusion of radiative pressure into simulations now make it possible to tackle this key question of astrophysics again on galactic and extragalactic scales. The approach and the methods differ in both communities. While extragalactic research has the advantage of a less extincted view and a more complete sample of MSF regions, it lacks the superb resolution obtained in observations of nearby galactic regions. Therefore, statistical methods and the analysis of SEDs or low-resolution images are widely used in extragalactic MSF research. Galactic research concentrates on local structure analysis using advanced image analysis tools. Merging both views seems to be a promising: With the knowledge about the structure details in nearby SF regions, the SED analysis methods used for distant regions can be evaluated, while extragalactic researchers can correct the galactic views by identifying the few sources we have as "(un-)typical" SF regions when compared to the large sample of SF regions that is available. The formation of filaments hosting massive cloud cores can be better understand with the external view of extragalactic research. In turn, special image analysis tools that are now used in galactic research may be well-suited to be used for upcoming better-resolved images of MSF regions in nearby galaxies.

VI. What governs proto-stellar mass accretion?

Coordinated by Dirk Froebrich

Almost the entire final mass of a star is accumulated during the protostellar phase of its evolution. Despite this being the important link between the core/clump mass function and the initial stellar mass function, it is still one of the most poorly understood phases during star formation. Knowledge of the physical processes that govern mass accretion will help us to understand the time evolution of the internal structure of protostellar envelopes as well as all observable quantities of these objects. Inevitably connected to the mass accretion process is the mass ejection into jets and outflows, and hence the injection of mass and energy into the surrounding material, i.e. the self regulation of star formation. There are a variety of important open questions to be answered. Is mass ac- cretion governed by turbulence and/or magnetic fields? At which point during the pro- tostellar phase is the accretion disc formed, i.e. the feedback processes start? Is there a general law governing mass accretion, or are the initial conditions and the environment (isolated and clustered mode of star formation) the important driving forces behind that process?

VII. Are mergers of smaller clusters a relevant step in the formation of very massive, gravitationally bound clusters?

Coordinated by Bernhard Brandl

The stellar masses of most globular clusters are larger than the most massive giant molecular clouds (GMCs) known in the local Universe. Even more massive clouds would have been required to form these clusters by cloud collapse, taking the IMF and the generally low star formation efficiencies into account. Either there are different mechanisms at work to grow and support these super-GMCs, or very massive cluster can be built up gradually by merging with less massive subclusters. But is this assembly of very massive, gravitationally bound clusters in dense and turbulent environments a possible scenario? What are there possible observational tests? Would this be a statistically relevant mode of cluster formation or just some oddball?

VIII. How does spiral structure affect star formation?

Coordinated by Preben Grosbol

Since the classification of disk galaxies (see e.g. Hubble 1926; van den Bergh 1960), it was noted that star forming regions often are concentrated in spiral arms. The density wave theory for spiral structure (Lin & Shu 1964; Roberts 1969) suggested that this may be associated to large scale shocks in the gas driven by such waves. Both new high resolution hydrodynamic calculations (see Dobbs, Bonnell & Pringle 2006; Kim & Ostriker 2006) and K-band observations of stellar clusters aligned with spiral arms (Grosbol, Dottori & Gredel 2006) suggest that spiral density waves are important for the triggering and formation of stars and clusters in disk galaxies. Spectroscopy of deeply embedded cluster in NGC 2997 indicates them to be very young (i.e. < 10 Myr), reach absolute magnitudes of -13 in the K-band, and form through a continuous process as suggested by Dufton et al. (2006) for 3 Galactic clusters. The difference between stochastic and shock triggered star formation is an important issue to be considered in this context. Although star formation caused by encounters or merges of galaxies may be more spectacular, spiral structure could be as important as it is more frequent and works over longer periods.

IX. How important is "primordial" mass segregation in the context of massive star cluster formation and evolution?

Coordinated by Richard de Grijs and Hans Zinnecker

Observations of young clusters in the local Universe clearly show that almost every single cluster is significantly mass segregated, out to radii well outside their cores. This is particularly puzzling for the youngest star clusters, given that their ages are often only a fraction of the time-scales required for dynamical effects to become significant on cluster-wide scales. It has therefore become generally accepted that at least some of the observed mass segregation seems to be intimately linked to the process of star formation itself. The effects of mass segregation in both young and old clusters clearly complicate the interpretation of an observed stellar luminosity function (LF), and its associated present-day mass function at a given position within a star cluster in terms of its underlying initial mass function (IMF). Without reliable corrections for the effects of mass segregation, hence for the structure and dynamical evolution of the cluster, it is impossible to obtain an accurate estimate of the IMF from the observed LF. Similarly, the derivation of physical cluster properties from integrated photometry or spectroscopy also hinges on the assumption of a well-behaved underlying stellar mass function. If there is significant, possibly "primordial", mass segregation within a cluster at the youngest ages, and hence a possible spatial dependence of the IMF (i.e., in the sense of preferential formation of the more massive stars in higher-density environments), this will have important consequences for the accuracy of the physical output parameters.

X. What is the relationship between the properties of star clusters and the environments from which they form?

Coordinated by Kelsey Johnson and Ian Bonnell

We know that most stars form in stellar clusters and that often clusters themselves can be in groups. The question is why do stars form in groups and how does this environment affect the star formation process. Does a clustered star formation simply mean that many independent stars form in a given volume of space or, alterna- tively,do the forming stars affect one another and thus their final properties. Such inter- actions must be primarily gravitational/tidal or through feedback, as even in the densest regions, direct interactions are very rare. If the clustered environment plays a determi- nistic role in the star formation process, there should be certain observational conse- quences that can be searched for.
The nature of this relationship has broad implications for galactic and extragalactic star formation alike -- from OB associations to super star clusters, from field populations to globular clusters. Star formation is a local process, yet in many cases the properties of the resulting clusters appear to populate a statistical distribution. Exactly what determines the properties of a resulting cluster remains unclear. A number of recent studies, both theoretical and observational, are beginning to shed light on this issue, and this workshop would be an excellent venue to delve into this topic.

XI. What are the similarities and differences between star forming objects at different scales and in different environments, in particular between the formation stages of galactic young high mass stellar associations, galactic young massive star clusters and the super massive clusters in starburst environments (with/without AGN)? To which extent are the properties of these different objects related by simple scaling, and what are the fundamental difference between them? How does linear resolution difference between local and extragalactic sources affect this comparison?

Coordinated by Emmanuel Galliano and Leonardo Bronfman

Modern infrared as well as near-future sub-millimetre instruments open a new era of research for the study of extra-galactic star-formation. We are not anymore limited to the global properties of star-forming galaxies. After Hubble resolved the building blocks of star-bursting regions in Super-Star Clusters in the 90s, we start to get insight on the early stages of formation of these clusters, the embedded (very) young massive clusters (EYMCs). These forming clusters display obvious similarities with the local forming massive stars.
This calls for a discussion about the similarities and possible fundamental differences between the different star forming objects, both from an observational (phenomenological) and physical point of view. Identifying differences might be extremely relevant for the global understanding of star formation, globular cluster formation, and galaxy formation.
An important point to examine in order to perform a meaningful comparison between the properties of galactic and extragalactic star forming objects relates to the linear resolution at which we can observe them. In other terms, how would we see the galactic clusters and star formation regions from a external galaxy.

XII. Which Physics determine the stellar upper mass limit?

Coordinated by Hans Zinnecker

What is the physical reason for the observed relation between the maximum stellar mass in a star cluster and the star cluster mass for very young star clusters? Is it due to the interplay between the binding energy of the cluster-forming cloud core and the feedback energy from the emerging stellar population? Linked to this question is understanding the physical reason for the empirical upper mass limit for stars near 150 Msun, observed for a number of populations with different physical parameters.

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