Gas Expulsion in Massive Star Clusters? Constraints from Observations of Young and Gas-Free Objects
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A&A 587, A53 (2016) Astronomy DOI: 10.1051/0004-6361/201526685 & c ESO 2016 Astrophysics Gas expulsion in massive star clusters? Constraints from observations of young and gas-free objects Martin G. H. Krause1,2,3, Corinne Charbonnel4,5, Nate Bastian6, and Roland Diehl2,3 1 Universitäts-Sternwarte München, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany e-mail: [email protected] 2 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85741 Garching, Germany 3 Excellence Cluster Universe, Technische Universität München, Boltzmannstrasse 2, 85748 Garching, Germany 4 Geneva Observatory, University of Geneva, 51 Chemin des Maillettes, 1290 Versoix, Switzerland 5 IRAP, UMR 5277 CNRS and Université de Toulouse, 14 Av. E. Belin, 31400 Toulouse, France 6 Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK Received 6 June 2015 / Accepted 13 December 2015 ABSTRACT Context. Gas expulsion is a central concept in some of the models for multiple populations and the light-element anti-correlations in globular clusters. If the star formation efficiency was around 30 per cent and the gas expulsion happened on the crossing timescale, this process could preferentially expel stars born with the chemical composition of the proto-cluster gas, while stars with special composition born in the centre would remain bound. Recently, a sample of extragalactic, gas-free, young massive clusters has been identified that has the potential to test the conditions for gas expulsion. Aims. We investigate the conditions required for residual gas expulsion on the crossing timescale. We consider a standard initial mass function and different models for the energy production in the cluster: metallicity-dependent stellar winds, radiation, supernovae and more energetic events, such as hypernovae, which are related to gamma ray bursts. The latter may be more energetic than supernovae by up to two orders of magnitude. Methods. We computed a large number of thin-shell models for the gas dynamics, and calculated whether the Rayleigh-Taylor instability is able to disrupt the shell before it reaches the escape speed. 5 Results. We show that the success of gas expulsion depends on the compactness index of a star cluster C5 ≡ (M∗/10 M)/(rh/pc), with initial stellar mass M∗ and half-mass radius rh. For given C5, a certain critical, local star formation efficiency is required to remove the rest of the gas. Common stellar feedback processes may not lead to gas expulsion with significant loss of stars above C5 ≈ 1. Considering pulsar winds and hypernovae, the limit increases to C5 ≈ 30. If successful, gas expulsion generally takes place on the crossing timescale. Some observed young massive clusters have 1 < C5 < 10 and are gas-free at ≈10 Myr. This suggests that gas expulsion does not affect their stellar mass significantly, unless powerful pulsar winds and hypernovae are common in such objects. By comparison to observations, we show that C5 is a better predictor for the expression of multiple populations than stellar mass. The best separation between star clusters with and without multiple populations is achieved by a stellar winds-based gas expulsion model, where gas expulsion would occur exclusively in star clusters without multiple populations. Single and multiple population clusters also have little overlap in metallicity and age. Conclusions. Globular clusters should initially have C5 100, if the gas expulsion paradigm was correct. Early gas expulsion, which is suggested by the young massive cluster observations, hence would require special circumstances, and is excluded for several objects. Most likely, the stellar masses did not change significantly at the removal of the primordial gas. Instead, the predictive power of the C5 index for the expression of multiple populations is consistent with the idea that gas expulsion may prevent the expression of multiple populations. On this basis, compact young massive clusters should also have multiple populations. Key words. ISM: bubbles – globular clusters: general – galaxies: star clusters: general 1. Introduction feedback processes are directly proportional to the latter. A cer- tain mass limit is therefore expected, above which the released The formation of stars is associated with radiation, stellar winds, energy is insufficient to remove the remaining gas (compare, and energetic explosions. This feedback is expected to easily e.g., Decressin et al. 2010; Krause et al. 2012). Candidate re- clear away surrounding gas (e.g. Freyer et al. 2003; Acreman gions for this to occur certainly include nuclear star clusters in et al. 2012; Fierlinger et al. 2012; Krause et al. 2013b; Krumholz galaxies, where a super-massive black hole typically adds to the et al. 2014). Indeed, star-forming regions are observed to be ac- gravity of the stars (e.g. Schartmann et al. 2010). But other star 6 tively clearing away gas at an age of about 10 years (e.g. Vo ss clusters, in particular globular clusters (GCs), can also be so et al. 2010, 2012; Preibisch et al. 2012; Galván-Madrid et al. massive and compact that the gravitational binding energy of the 2013; Klaassen et al. 2014), and to be free of dense gas from gas becomes important. about a few Myr after the onset of star formation (e.g., the review If at the time of gas clearance the gravitational potential of by Lada & Lada 2003). such clusters is dominated by this gas, and if the gas clearance The gravitational binding energy of a star-forming region happens sufficiently fast, comparable to the crossing timescale, increases with the square of the total mass, however, whereas then many stars if not all (disruption) are also lost as a result Article published by EDP Sciences A53, page 1 of 16 A&A 587, A53 (2016) of the drastic change in gravitational potential (e.g., Bastian & that the number of embedded star clusters strongly depends Goodwin 2006; Goodwin & Bastian 2006; Bastian et al. 2008; on the adopted star density threshold, complicating conclusions Gieles & Bastian 2008; Baumgardt et al. 2008; Decressin et al. about early star cluster dissolution. They also found, however, 2010; Marks et al. 2012; Pfalzner & Kaczmarek 2013; Pfalzner that class I (younger) objects have, on average, larger densities et al. 2014; Banerjee & Kroupa 2016). This is commonly re- than class II (older) objects, supporting the idea that star-forming ferred to as gas expulsion (Portegies Zwart et al. 2010; Banerjee regions often dissolve. The environment has a strong effect on & Kroupa 2015, for reviews). the evolution of the cluster mass functions (see the review by Gas expulsion is central in explanations for the chemical pe- Longmore et al. 2014). It is therefore difficult to constrain gas culiarities in GCs and is invoked in certain scenarios as a possi- expulsion in this way. bility to eject a large number of less tightly bound first popula- Young (≈10 Myr), exposed star clusters often appear to tion stars born with the pristine composition of the proto-cluster have supervirial velocity dispersions (Gieles et al. 2010). This gas. At the opposite, second population stars showing peculiar has been discussed as evidence for dissolution after gas ex- abundance properties (e.g. the O-Na anti-correlation) and born pulsion (e.g., Goodwin & Bastian 2006). It is, however, ex- in the cluster centre would remain bound after gas expulsion pected that many clusters would have re-virialized by the time (D’Ercole et al. 2008, 2010; Krause et al. 2013a). This is one of observation (Baumgardt & Kroupa 2007; Gieles et al. 2010; possibility to solve the so-called mass-budget problem to explain Portegies Zwart et al. 2010). An interpretation in terms of a large the number distribution of stars along the O-Na anti-correlation. contribution from binaries to the velocity dispersion (compare, Other possibilities exist though, that call for a modified initial e.g., Leigh et al. 2015; Oh et al. 2015) seems more probable mass function (IMF; Prantzos & Charbonnel 2006; Charbonnel (Gieles et al. 2010; Rochau et al. 2010; Cottaar et al. 2012; et al. 2014). The stars ejected from GCs might form a substantial Clarkson et al. 2012; Hénault-Brunet et al. 2012; Cottaar & population of galactic halos (e.g., Schaerer & Charbonnel 2011; Hénault-Brunet 2014). Therefore, it is also difficult to constrain Larsen et al. 2014b). gas expulsion in this way. Young massive clusters (YMCs) are important test cases for the gas expulsion idea because the involved timescales, masses, and energetics may be constrained directly by observations (e.g., 2.2. Light-element anti-correlations in globular clusters Portegies Zwart et al. 2010). Some extragalactic objects appear to be gas-free after 10 Myr or less (Bastian et al. 2014; Hollyhead With the former tailwind from the general research on star clus- et al. 2015) and are thus very interesting objects for comparison ters, gas expulsion assumed a central role for the understanding to theoretical expectations. of GCs (e.g., Goodwin 1997a,b; Fenner et al. 2004; Prantzos We have investigated gas expulsion in the context of mod- & Charbonnel 2006; Parmentier & Gilmore 2007; Baumgardt els that aim to explain the light-element anti-correlations in GCs et al. 2008; D’Ercole et al. 2008; Decressin et al. 2010; Marks with massive progenitors (Krause et al. 2012, Paper I). We found & Kroupa 2010; Vesperini et al. 2010; Schaerer & Charbonnel that gas expulsion would not work for common cluster param- 2011; Marks et al. 2012). In contrast to the vast majority of open eters because the outflow would be disrupted by instabilities (lower mass) clusters studied so far, almost all of the GCs show before escape speed was reached. An unusually strong power chemical peculiarities, most prominently the light-element anti- source, like the coherent onset of Eddington-strength accretion correlations (Charbonnel 2010; Gratton et al.