12 Questions on Star and Massive Star Cluster Formation Held at ESO Headquarters, Garching, Germany, 3–6 July 2007

Astronomical News

Report on the ESO Workshop on 12 Questions on Star and Massive Star Cluster Formation held at ESO Headquarters, Garching, Germany, 3–6 July 2007

Markus Kissler-Patig1 various reasons. The main one is prob “How are the stellar and cluster initial Tom Wilson1 ably the distance of the studied objects. mass functions related and how are they Nate Bastian 2 Star formation can be studied in great influenced by the star-formation history?” Francesca D’Antona 3 detail close-by, i.e. in our Milky Way where Richard de Grijs 4 distances are expressed in parsecs, and The first question of the workshop was Dirk Froebrich 5 the studied regions enclose typically a posed and introduced by Marina Rejkuba. Emmanuel Galliano 6 few hundred stars. In contrast, suitable It is an indisputable fact that star for Preben Grosbøl1 targets for studying the formation of mas- mation is hierarchical. The empirical evi- Kelsey Johnson7 sive star clusters are only available in dence indicates that the initial mass func- Eric Keto 8 neighbouring galaxies and distances to tion (IMF) of young embedded stellar Ralf Klessen 9 them are rarely smaller than a few million clusters is a universal power law with a Tom Megeath10 parsecs – the studied objects enclose slope of 2.0, and that the stellar IMF Marina Rejkuba1 up to hundreds of millions of stars. The follows a universal segmented power law Jürgen Steinacker11 scales in which the two communities typ- in many different environments. The Hans Zinnecker12 ically think are thus different and the time validity of the power-law form for the IMF had come to try to connect them. Fur was questioned in the discussion, but ther, ALMA – the large sub-mm array – is opinions remain divided. The theoretical 1 ESO now within reach and will boost both arguments for the universal stellar IMF 2 University College London, fields, exactly allowing to connect the two were presented by Ralf Klessen, while United Kingdom scales thanks to an increased sensitivity Hugues Sana and Jorge Melnick showed 3 Osservatorio di Roma, Italy and angular resolution. the pitfalls and problems in empirical de- 4 University of Sheffield, United Kingdom termination of the stellar IMF in clusters 5 University of Kent, United Kingdom The format we have chosen for the work- and in the field. Andrés Jordán discussed 6 Universidad de Chile, Santiago de shop was a new one: we built the pro- the form of the initial old cluster-mass Chile, Chile gramme of the workshop around a num function. 7 University of Virginia, Charlottesville, ber of questions to be addressed in Virginia, USA dedicated sessions. We summarise here The fact that the clusters undergo mass- 8 Harvard-Smithsonian Center for Astro- some (not all) of the discussed points. loss and disruption on short timescales, physics, Cambridge, Massachusetts, All presentations are available for down- leads to the composite nature of the field- USA load from the workshop web site www. mass function. Whether this composite 9 Universität Heidelberg, Germany eso.org /star 07. IMF is steeper, or has the same slope as 10 University of Toledo, Ohio, USA 11 Max-Planck-Institut für Astronomie, Heidelberg, Germany 12 Astrophysikalisches Institut Potsdam, Germany

The Workshop “12 Questions on Star and Massive Star Cluster Formation” was held in Garching from 3 to 6 July 2007. The programme was set up to allow long (and fruitful) discussions around several questions connecting the formation of stars and star clusters. Here we summarise some of the discussions, and encourage interest- ed readers to download the contribu- tions from http://www.eso.org/star07.

In view of the booming fields of star formation and star-cluster formation, we had thought, back in mid-2006, to organise a meeting that would bring these two communities together. To an outsider, star formation and star-cluster formation might Figure 1: The conference delegates look like one and the same thing, yet collected in the entrance hall of the ESO Headquarters in Garching. these two communities rarely interact for

The Messenger 129 – September 2007 69 Astronomical News Kissler-Patig M. et al., 12 Questions on Star and Massive Star Cluster Formation

the star cluster IMF, has been shown to mass cluster stars affect the ISM and the clouds and the distribution of OB stars in depend on the answer whether 20 clus- young sources in their neighbourhood. other galaxies. This analysis suggested 4 ters with 10 MA can produce the same hierarchical structure with no preferred IMF, with the same maximum stellar mass, scale. Mark Gieles showed a minimum 5 as one cluster with 2 × 10 MA. With re- “What is the demographics of star forma- spanning tree analysis of the Small Mag- spect to this point the two major theoreti- tion in our Galaxy and others?” ellanic Cloud where they segregated cal works of Elmegreen and of Weidner sources by age. The youngest stars and Kroupa disagree. Unfortunately, due In studies of galactic star formation, showed clear hierarchical structure while to the often poorly known star-formation young stars have traditionally been sepa- the distribution of the oldest stars was in- history of the observed field, and the rated into two demographic groups: iso- distinguishable from a random structure, problems to establish the frequency of bi- lated stars and clustered stars. This pic- indicating that structure is erased as the naries in the population, the observational ture is evolving for many reasons. Tom stars migrate from their birth sites. In evidence seems to be still inconclusive. Megeath posed this question. He showed summary, the discussion of this question Until the errors of the IMF derivation can Spitzer surveys of the giant molecular motivated the need to find Galactic ana- be reduced, the question remains open. clouds in the nearest 1 kpc revealing the logues of extragalactic clusters and asso- presence of large clusters, small groups, ciations. Furthermore, it demonstrated and large numbers of relatively isolated the need for new methodologies for ana- “What are the effects of stellar feed- stars. These surveys suggest that isolated lysing structure on many different spatial back?” stars and dense clusters are extremes of scales, such as the minimum spanning a continuum. Galactic studies provide the tree or wavelet-based multi-resolution In this session negative feedback effects, opportunity to study physical processes techniques. namely the destruction of clusters, were in great detail and trace the distribution of addressed. During the collapse and frag- low-mass stars which dominate the mass mentation of a molecular cloud into a but are undetected in extragalactic ob- “How did star formation proceed in star cluster, only a modest amount of the servations. Detected extragalactic clus- Globular Clusters?” gas is turned into stars. The stellar winds, ters are much more massive than clusters photoionisation, and supernovae from near the Sun such as the Orion Nebula In recent years, research on these very massive stars inject enough energy into Cluster. Frédérique Motte suggested that old systems has shown that they are the gas to remove it extremely rapidly our Galaxy contains large clusters which not the ‘simple stellar populations’ we (in less than a dynamical crossing time). overlap with the continuum of observed thought they were; this topic was intro- This rapid gas removal can leave a clus- extragalactic ones. Arjan Bik presented a duced by Francesca D’Antona. There ter significantly out of equilibrium. If the programme to obtain infrared spectro- is precise spectroscopic evidence that star-formation efficiency is low enough scopic maps of young embedded clus- chemical anomalies are present, gener- (~ 30 % of the initial fraction of gas turned ters in our Galaxy; infrared spectroscopy ally involving about 50 % of the stars in into stars) the entire cluster can become of the massive stars are needed to deter- each globular cluster (GC). Anomalies are unbound. mine the membership, age and size of found in practically all clusters observed these highly reddened Galactic regions. so far. Anomalies are also present among Nate Bastian reviewed the physics and unevolved stars, so that they cannot be some recent observations of this proc- Finally, there was a discussion of how imputed to ‘in situ’ mixing in giants. In ess. In particular he discussed recent best to characterise the clusters, asso particular, sodium is enhanced and oxy- N-body simulations of the effects of gas ciations and complexes of star formation gen depleted with respect to the normal removal on the early evolution of clusters. in our Galaxy and others. Nate Bastian values in the field population II. This, in He also reviewed recent attempts to introduced the minimum spanning tree the end, means that about half of the quantify the amount of infant cluster mor- analysis (see Figure 2) and applied this to stars in the clusters (a second generation) tality, with most studies finding values Spitzer surveys of nearby molecular are born from matter contaminated by of 60–90 % of young clusters being dis- �

rupted in the first 40 Myr of their lives. 00 °

Sabine Mengel focused on the young 63 �

massive cluster Westerlund 1 in the Gal- 45

axy. She concluded that the star-for � mation efficiency of this cluster was quite 30 � 15 high (> 60 %). Dieter Nürnberger concen- � Figure 2: Example of a trated on another young massive cluster 00 °

Declination minimum spanning in the Galaxy, namely NGC 3603. On the 62 � tree, presented by Nate 10 pc10 basis of multi-wavelength observations 45 Bastian and collabora- he pointed out that this cluster is part of � tors, revealing the hier- 30 archical structure of star a much larger area of star formation. He � 15 also showed examples of how the high- ° formation (Figure cour- 61 23h 20m 15 m 10 m 05m 00m 23h 55m tesy Gutermuth et al., in Right Ascension preparation).

70 The Messenger 129 – September 2007 products of hot CNO burning in stars of time evolution of the objects might come sity environments), this will have impor- the first generation, but not by supernova from competitive accretion. However, tant consequences for the accuracy of ejecta, as the metallicity stays constant. no agreement could be reached on how the physical output parameters. The culprits may be the ejecta of massive to interpret the observational evidence. Asympotic Giant Branch (AGB) stars, as Joana Ascenso challenged this view. pointed out by Paolo Ventura, or even the From a series of Monte Carlo realisations envelopes of massive rotating stars, as “How does spiral structure affect of non-segregated synthetic clusters with presented by Thibaut Decressin. The two star formation?” a standard IMF and a surface density suggestions imply very different time distribution, she performed the ‘standard’ scales for the formation of the second Preben Grosbøl drew the attention back mass-segregation analysis. Surprisingly, stellar generation (a few million years, or to extragalactic objects. Many grand-de- she found a similar degree of ‘primor- many tens of millions of years after the sign spiral galaxies display a concentra- dial’ mass segregation as claimed in the formation of the first-generation stars). tion of H ii regions and OB associations in observational studies of de Grijs and The number ratio of the two generations, their arm regions. The increased num- Gouliermis for young clusters in the Large however, implies that the first generation ber of very young sources in the arms Magellanic Cloud, by Espinoza in the in the pristine globular cluster must have suggests an enhanced star-formation rate Galactic Centre Arches cluster, and by been much more massive (by a factor which could partly originate from a higher Brandner in Westerlund 1. Enrico Ves- of 5–10) than today’s globular clusters, surface density in the arms due an un- perini used extensive N-body simulations and this may provide us with hints about derlying density wave. On near-infrared to show that early mass segregation in how to model the second stage of star K-band images of late-type spiral galax- young clusters may have a dynamical ori- formation. Additional evidence of nuclear ies, one frequently observes bright knots gin. Despite the animated discussion fol- processed matter is the presence of stars aligned along the arms. The colours lowing these thought-provoking presenta- with helium content higher (and even of these knots indicate that they are very tions, the jury is still out on the basic much higher, implying an incredibly high young, massive stellar clusters. The num underlying question of the importance or slope of the He enrichment function dY/ ber and brightness of such sources cor- even the reality of primordial mass segre- dZ) than the standard Big Bang abun- relate with absolute magnitude and spiral gation. dances. Beautiful observations of splitting perturbation of the host galaxy. The align- of the main sequences in two clusters ment is seen mainly in the galaxies with were presented by Antonino Milone, on strong spiral arms, suggesting a trigger- “What is the relationship between the behalf of Giampaolo Piotto’s group. This ing mechanism associated with the un- properties of star clusters and the view was enlarged to consider low-mass derlying density wave. This could support environments from which they form?” dwarf galaxies by Michael Hilker. a scenario in which young, massive clusters are disrupted in an early phase, since The environment of massive star-cluster fainter, aging clusters are not observed formation is determined by a large num- “What governs protostellar mass downstream, as expected due to the rel- ber of parameters, including pressure, accretion?” ative motion between material and wave. density, temperature, turbulence, magnetic fields, metallicity, gas content, galactic The physical processes at work in the rotation (or lack thereof), and triggering main mass accretion phase during the “How important is ‘primordial’ mass mechanisms. Of course, many of these process of star formation were presented segregation in the context of massive star parameters are not orthogonal. The chal- initially by Dirk Froebrich. In particular, cluster formation and evolution?” lenge, posed by Kelsey Johnson by this the question of what are the main physi- question, was to ascertain which of these cal forces and/or initial conditions respon This question was posed by Richard de parameters can essentially be neglected sible for converting the observed core Grijs and led to a controversial discus- and which may have a significant impact mass distribution into the stellar IMF was sion. Observations of young clusters in on the properties of the resulting clus discussed. It became apparent during the local Universe show that almost every ter(s). For example, one might expect me- the discussion that, beside gravity, the in- single cluster is significantly mass seg tallicity to play a role in the properties of itial specific angular momentum of the regated, out to radii well outside their the resulting cluster, yet the few observed core most likely is the main influence on cores. This is particularly puzzling for the ultralow metallicity systems (e.g. IZw18, the time dependence of the mass-accre- youngest star clusters, given that their SBS0335-052) show very diverse modes tion rate onto the central object. In turn ages are often only a fraction of the time- of cluster formation, suggesting that me- this is, together with the structure of the scales required for dynamical effects tallicity (at least down to this abundance) object, what determines the time evolu- to become significant on cluster-wide does not play a dominant role. Detailed tion of the protostellar observables (e.g. scales. If there is significant, possibly ‘pri- studies of the relation between specific

Tbol, L bol). Magnetic fields are considered mordial’, mass segregation within a clus- frequency of globular clusters and galaxy to also be important in determining the ter at the youngest ages, and hence a type may provide important insight. How- accretion rates, which reach much higher possible spatial dependence of the IMF ever, this is a blunt tool, and raises the values than predicted by self-similar (i.e., in the sense of preferential formation challenge of relating the role of macro- collapse solutions. Further impact on the of the more massive stars in higher-den- scopic phenomena to local physics. A re-

The Messenger 129 – September 2007 71 Astronomical News

current theme in the presentations of Figure 3: A massive star-forming re- this session has been the importance of gion, presented by Eric Keto, illustrat- ing massive accretion flows. The fig- star-formation efficiency to cluster forma- ure shows an overlay of contours of tion. However, this then begs the ques- emission from molecular gas on a tion of what primarily determines the star- background image of mid-IR emission formation efficiency. On the macroscopic from Spitzer Space Telescope data. The central cloud encloses a particu- scale, perhaps the most plausible answer 3 larly massive flow of 10 MA/yr into the is pressure, but it is not yet clear what centre of the star cluster seen in the subtleties, which may have a secondary IR. The innermost contour is about 30? role, are hiding in the microphysics; we in diameter (0.9 pc at 6 kpc). must keep in mind that star formation is a local process. Although it is currently the fashion to accumulate large data sets with impressive statistics, in-depth case studies are still critical if we wish to disen- tangle all of the ingredients affecting massive star and star-cluster formation. combined mass of several O stars at forces of radiation and thermal pressure. the cluster centre. Within a few thousand The observational evidence, and more AU of the O stars, the molecular accre- of the physics of the upper stellar mass “Which physics determine the stellar tion flow becomes ionised, but continues limit, are reviewed in the recent Annual upper mass limit?” in toward the cluster centre because the Reviews of Astronomy and Astrophysics escape velocity from the co-op of the article by Zinnecker and Yorke. Finally, Hans Zinnecker, Eric Keto, stars exceeds the ionised sound speed. Carsten Weidner and Hugues Sana set The ionised accretion flow spins up into Overall, the workshop certainly left us out to answer this question. The most an ionised accretion disc at the cluster with more questions than answers, but massive stars in the Galaxy are typically centre. Because the dust that was origi- the format was a success as the work- found in a group of several similar stars at nally in the molecular accretion flow is shop was dominated by long and fruitful the centre of an OB cluster, for example, destroyed by the high temperatures and discussions. The two communities work- the Orion Trapezium. When we look at a densities in the disc, accretion can ing on star formation and star-cluster very young, embedded OB associations continue onto the individual stars in the formation moved a step closer towards still in formation, such as G10.6-0.4, we co-op, unimpeded by the intense radi each other and there is good hope that see a massive molecular accretion flow ation pressure on dust. Secondly, in the they will merge in the epoch of ALMA. into the centre of the cluster. Thus we ex- fully ionised flow there is no outward pect the most massive stars in the Gal- pressure between the hot ionised and axy to form at the centres of such cluster- cold molecular gas to impede the flow. Acknowledgements scale accretion flows (see Figure 3). The Thus the cluster-scale cooperative ac We wish to acknowledge Christina Stoffer, who man- accretion velocities indicate that the flow cretion flow sets up these two conditions aged all the logistics of the workshop, as well as is a ‘cooperative accretion flow’ drawn that allow accretion onto very massive Arjan Bik for his help in the local organising commit- by the cooperative gravitational pull of the O stars despite the presence of outward tee.

Announcement of a Workshop on Science from UKIDSS

17–19 December 2007, ESO Headquarters, Garching, Germany

The workshop will take place a few with UKIDSS, and to share knowledge and an opportunity to discuss the future weeks after the UKIRT Infrared Deep Sky gained in working with the data and ideas direction. Survey (UKIDSS) large Third Data for exploiting the archive efficiently, in Release (DR3). The purpose of the work- an informal atmosphere. The emphasis Registration will be open from 15 Sep- shop is to provide a forum, bringing to- will be on work in progress. The work- tember 2007 at http://www.ukidss.org/ gether European astronomers working on shop will include science and technical esoworkshop. (or planning to work on) UKIDSS data, talks, and tutorials, as well as a sum- to hear about science being undertaken mary of the current status of the surveys,

72 The Messenger 129 – September 2007