arXiv:1610.10072v1 [astro-ph.GA] 31 Oct 2016 cetdXX eevdYY noiia omZZZ form original in YYY; Received XXX. Accepted aitosi ih lmnswt eldcmne anti- (see documented Al and well Mg Na, with and O elements between correlations light abundance -to-star in main large distinct variations and have them subpopulations of sequence Space all that Hubble revealed with photometric have spectroscopy Deep confirmed structure. detailed now complex and more are observations much , a be have assembling to instanta- to young formed time in long systems, neously a stellar for homogeneous considered chemically (GCs), clusters Globular INTRODUCTION 1 MNRAS lblrcutr orti h rnerce supernovae enriched iron the proto (see retain young ejecta (SNe) of to ability clusters the globular restricts strongly which lations ⋆ re- ones massive mono-metallic most the remain (e.g. Only elements. GCs group of iron the majority garding the time, same 2012 2004 egySilich Sergiy in dominates process clusters? what massive retention: young gas vs expulsion Gas

c nttt ainld Astrof´ısica de Nacional Instituto -al [email protected] E-mail: 06TeAuthors The 2016 ; ; ω aioe l 2008 al. et Marino ezn ta.2015 al. et Renzini e,M2 ezn5 xii utpe[eH popu- [Fe/H] multiple exhibit 5) Terzan M22, Cen, 000 , 1 – 9 21)Pern uut21 oplduigMRSL MNRAS using Compiled 2018 August 7 Preprint (2016) ezn 2013 Renzini ⋆ n ulem Tenorio-Tagle Guillermo and ; n eeecsteen.A the At therein). references and , artae l 2009 al. et Carretta n eeecsteen.De- therein). references and , piayEet´nc,A 1 20 uba M´exico Puebla, 72000 51, Electr´onica, AP y Optica ´ h rtcllnsotie o lseswt oa n rmrilga primordial these a in and zones words: solar different Key a in with appear clusters clust 5253 rich for compared. NGC gas obtained are para the lines mass the and critical cloud galaxies in The Antennae pre-stellar age the vs and in density mass efficien clusters t central free formation in gas gas separated natal star two well the different are th in regimes considers where and possible clouds one t three forming if be The star must concentrations. dense possible collisions and ejecta are compact very SNe s regimes of and the case of the stellar revised thermalization in nearby are the to cloud regarding due forming concept energy star retain accepted natal to usually the required the within conditions formation the star the restricts Here from strongly formation. which r star them, not population of from did majority stellar clusters over the expla globular pattern in left of to gas those gas of abundance to aiming amount similar the ov element models masses detect with left light all clusters to gas day in peculiar attempts the issue the Recent retain central clusters. and or a expel is populations to formation stellar clusters star stellar of young episode of ability The ABSTRACT n rcse:hydrodynamics Processes: and ; itoe al. et Piotto ei tal. et Bedin aais trcutr lblrCutr uenvePyia D Physical Supernovae — Clusters Globular — clusters star galaxies: - (IMB, 2007 2004 tr (FRMS, h ih yrgnbrigcniin oepanthe a) ( are: explain stars These (AGB) to clusters. branch conditions globular giant asymptotic in burning anomalies hydrogen abundance right the the populations stellar in of decade evolution last and the galaxies. origin of in the results of of challenging discovery context most the remains the problem, clusters of globular this in solve populations stellar to multiple efforts many spite omdatrteedo Nepoin rmteso AGB slow the from explosions ex- SN SN of case and end latter this winds the in after formed by stellar removed second AGB The is the plosions. in gas whereas this po (20Myr-40Myr) stellar scenario time first a some of for formation th ulation the of from over some left retain gas clusters pristine proto-globular young that quires hr r he ancniae htptnilyreach potentially that candidates main three are There h plcblt fteFM n M cnro re- scenarios IMB and FRMS the of applicability The ; ; ata ta.2013 al. et Bastian eMn ta.2009 al. et Mink de ’roee l 2008 al. et D’Ercole rnzs&Cabne 2006 Charbonnel & Prantzos n neatn asv binaries massive interacting and ) ; eoi-al ta.2016 al. et Tenorio-Tagle ;b atrttn massive rotating fast b) ); omto npresent in formation cnro fmultiple of scenarios ehl-asradius half-mass he e trformation star ree kinetic cluster tar ti hw that shown is It . ee pc.The space. meter va significant a eveal ’noa&Caloi & D’Antona rwt similar a with er A h a etover left gas the T ; knwt care with aken ratrafirst a after er E isadmass and cies ersi tal. et Decressin tl l v3.0 file style X nglobular in s s nmultiple in diagrams. ). ata one p- is e 2 S. Silich and G. Tenorio-Tagle winds retained in the potential well of the cluster mixed with Mart´ınez-Gonz´alez et al. 2014; Gupta et al. 2016). Such some matter accreted late in the evolution of the cluster. shells are driven by a high thermal , which is boosted Recently Bastian et al. (2013, 2014) attempted to find when individual stellar winds begin to collide and convert the matter left over from in nearby young their kinetic energy into thermal energy of the injected mat- massive clusters, but did not detect any significant amount ter. We leave the detailed analysis of ef- of gas (down to 1-2 per cent limits of the mass) fects on our model to a forthcoming communication. in their target clusters. This led them to claim that even The paper is organized as follows. In section 2 we in- 6 high-mass clusters with masses 10 M⊙ or more, expel their troduce the models adopted for the initial gas and stellar natal gas within a few Myr after their formation, which density distributions in the star forming cloud and deter- strongly restricts scenarios for multiple stellar population mine the rate of mechanical energy injected by massive stars formation. However observations of nearby galaxies provided into the intra-cluster medium. In this section we also discuss in infrared, millimeter and radio wavelengths revealed sev- the dynamics of bubbles driven by individual stellar winds eral very luminous and compact regions which are believed and individual explosions and derive the condi- to be young stellar clusters still embedded into their natal tions for them to stall before merging with their neighbors. gaseous clouds (Turner et al. 2000, 2003; Beck 2015). In section 3 we split the half-mass radius vs star forming 1D and 3D numerical simulations provided by cloud mass and the central gas density vs star forming cloud D’Ercole et al. (2008) and Calura et al. (2015) show that mass parameter space onto three regions. In the first case, stellar winds effectively remove gas from star forming re- neighboring -driven bubbles merge and upon the colli- gions and completely clean them up of the pristine matter sions their shells fragment leaving the swept-up matter as a no later than 15Myr after their formation. However calcula- collection of dense clumps. Neighboring stellar winds begin tions provided by Krause et al. (2012, 2016) with a 1D thin then to interact directly causing the large overpressure that layer approximation led to the conclusion that in most cases evolves into a cluster wind. The fragmented original gas will the expanding star cluster wind-blown shells are destroyed then be mass-loaded by the cluster wind and finally expelled via Rayleigh-Taylor (RT) instabilities before reaching the out of the star forming volume. In the second case the wind- escape velocity which prevents gas expulsion from the star driven bubbles are unable to merge and are thus not able forming region. to cause the conditions for the generation of a cluster wind Another approach has been recently discussed by and the removal of the leftover gas. In this regime individual Tenorio-Tagle et al. (2015, 2016) who considered a model SNe may engulf neighboring wind sources. The energy input in which the negative feedback provided by stellar winds rate to the SNR then grows with time causing a continuous is suppressed because all massive stars compose close bi- acceleration and eventually the SNR destruction. This im- nary systems. The major implications of this are that the mediately leads to the expelling of SN products out of the large collection of interacting massive binaries are likely cluster. In the third region SN remnants stall before merging to hold the remaining cloud against gravitational collapse with nearby wind-driven bubbles. The natal gas is then con- while contaminating the gas left over from star formation. taminated with the iron group elements and retained in the This would happen without disrupting the centrally con- central zones of the star forming cloud. Here we also discuss centrated density distribution even during the early evolu- how the SFE and the stellar metallicity affect the critical tion of globular clusters. Under such conditions, blast waves lines which separate clusters evolving in different regimes. from sequential supernova explosions are likely to undergo We also show that three clusters with similar masses and blowout expelling the SN products into the ambient inter- ages, two gas-less clusters in the and the stellar medium (ISM) keeping most clusters mono-metallic gas rich cluster in the NGC 5253, are located regarding the iron group elements. in different regions of our half-mass radius vs star forming Here we show that bubbles formed by stellar winds and cloud mass diagram. Finally, in section 4 we summarize our single SN explosions in the central zones of compact and major results and conclusions. massive young stellar clusters may stall before merging with their neighbors. This naturally allows for the retention and enrichment of the gas left over from the formation of a 1G 2 MODEL SETUP and likely would lead to the formation of a second generation 2.1 Stellar and gas density distributions of stars with a peculiar abundance pattern. The fate of the gas left over from star formation depends then not only on In order to study the feedback that recently formed stars the proto-stellar cloud mass and the star formation efficiency provide on their parental gaseous cloud one has to se- (SFE), but also on the gas concentration (or the central gas lect a model for the mass density distribution in the star- density) in the proto-stellar cloud. The major output from forming region. Following recent works (e.g. Krause et al. our study is critical lines which separate the gas expulsion 2012; Calura et al. 2015) we adopt a Plummer density dis- and the gas retention regimes in the star forming cloud size tribution: and in the central gas density vs the star forming cloud mass −5/2 3M r2 parameter space and the dependence of these critical lines ρ (r)= g 1+ , (1) g 4πa3 a2 on the SFE.   Radiative pressure effects are presumably small in − 3N r2 5/2 the dynamics of bubbles formed around single OB stars n (r)= 1+ , (2) ⋆ 4πa3 a2 (Krumholz & Matzner 2009). They may be significant   only for a short while in the dynamics of shells formed where ρg and n⋆ are the gas volume density and the massive around star clusters (e.g. Silich & Tenorio-Tagle 2013; star number density in the star forming cloud, respectively,

MNRAS 000, 1–9 (2016) Gas expulsion vs gas retention in YSCs 3

Mg = (1 − ǫMtot) is the total amount of gas left over from 2.3 Wind-driven bubbles the formation of the first stellar generation, M is the total tot Each massive star composes a wind which sweeps up the mass of the star forming cloud, ǫ is the efficiency of star surrounding medium into a shell and forms a bubble filled formation and N is the total number of massive stars, the with a hot shocked wind gas. If thermal conduction and major feedback agent in any young massive cluster. In the mass evaporation from the wind-driven shell are inhibited case of a Plummer density distribution the half-mass radius by magnetic fields the radiative losses of energy from the is R = 1.3a, where a is the characteristic length scale for hm shocked wind region are negligible (Silich & Tenorio-Tagle the gaseous and the stellar density distributions. We assume 2013). Wind-driven bubbles then evolve in the snow- a standard Kroupa initial mass function (IMF). The number plow regime and the thermal pressure inside of the wind- of single massive stars N with M > 8 M⊙ then scales with driven bubble decreases as the bubble grows larger (see the star cluster mass as (e.g. Calura et al. 2015): Bisnovatyi-Kogan & Silich 1995): 6 N = N0(M⋆/10 M⊙) (3) 7 375(γ − 1)L 2/3 where N = 104, M = ǫM . The mean separation be- P = ⋆ ρ1/3r−4/3. (6) 0 ⋆ tot 25 28(9γ − 4)πN g tween nearby massive stars, ∆ = 2X, is determined by the   condition that within a sphere of radius X there is only one The bubble stalls when this pressure drops to the pres- 3 massive star: 4πX n⋆/3 = 1. In the case of the Plummer sure in the ambient intra-cluster medium. We assume that in density distribution X grows with distance to the star clus- the central zones of star-forming clouds the turbulent pres- ter center: sure dominates over the thermal one. The stalling radius is 2 5/6 then determined by the condition that P = Pt, where the r 1/3 X = a 1+ /N . (4) turbulent pressure P = ρ σ2 (e.g. Smith et al. 2006). The a2 t g   one-dimensional σ is

GM 1/2 2.2 Input energy σ = tot , (7) βRhm Another input parameter is L⋆ - the star cluster mechanical   . L⋆ changes in stellar populations with different where G is the gravitational constant and Rhm is the half- and in recent starbursts is expected to be larger mass radius of the star forming cloud. In the case of a spher- than it was at the epoch when the first stellar clusters (glob- ical cluster with an isotropic velocity distribution β = 7.5 ular clusters) formed. L⋆ scales with the star cluster mass (Smith & Gallagher 2001). as One can calculate now where the bubble around a single 6 massive star stalls: L⋆ = L0(M⋆/10 M⊙), (5) 3/4 − 1/2 40 7β 125(γ 1)L⋆ where the normalization coefficient L0 ≈ 3 × 10 erg × Rstall,w = 3 −1 25G 7(1.3) (9γ − 4)(1 − ǫ)N s for young stellar clusters with solar metallicity, stan-     dard Kroupa IMF and Padova stellar evolutionary tracks 1+ r2/a2 5/4 (Leitherer et al. 1999). For stellar clusters with a low metal- R9/4. (8) M hm licity, analogues to proto-globular clusters, we use the  tot  Calura et al. (2015) prescription for the power Stellar winds do not merge if the stalling radius is smaller which results in an order of smaller star cluster than 1/2 the distance between nearby sources: Rstall,w < X. mechanical luminosity and thus reduces the value of the nor- This condition together with equations (4) and (8) allow one × 39 malization coefficient L0 in equation (5) to L0 = 3 10 erg to determine how compact the parental cloud should be in −1 s . The typical power of individual stellar winds then is order to keep newly born stars buried within the pristine gas Ls = L⋆/N. For supernovae explosions a standard value of left over from star formation: 1051erg was adopted regardless of the star formation epoch. 25G 3/5 9.1(9γ − 4)(1 − ǫ)N 2/5 Note that the number of massive stars N decreases af- R < × hm 7β 125(γ − 1)L ter the onset of SN explosions (it is assumed that the rate    ⋆  of SN explosions follows the Starburst99 model predictions). M The mean separation between nearby massive stars ∆ then tot . (9) N 4/15(1 + r2/a2)1/3 increases with time. Clusters which were not able to form a wind and expel the leftover gas after the first SN explosion Rhm in equation (9) is the half-mass radius of the star form- should retain it when the separation between nearby energy ing cloud and r is the size of the central zone where stellar sources becomes larger. Therefore hereafter we do not con- winds do not merge. The condition that r = 0 determines sider the star cluster mechanical luminosity time evolution. then the critical half-mass radius and the critical central The results obtained below should not be restricted to density in the star forming cloud: the Plummer model. For example, in the case of mass seg- 25G 3/5 9.1(9γ − 4)(1 − ǫ)N 2/5 regation (e.g. Dib et al. 2008) the assumption that the gas R = × hmw,crit 7β 125(γ − 1)L and massive stars are equally distributed does not hold. The    ⋆  mean separation between massive stars in the central zones M of the cluster then would be smaller than that predicted by tot , (10) N 4/15 equation (4). This will reduce the critical radii obtained in the next sections and enhance the gas critical densities, but 3M 3(1 − ǫ)M ρ = g = tot . (11) does not change our major conclutions. w,crit 3 3 4πacrit 4π(Rhmw,crit/1.3)

MNRAS 000, 1–9 (2016) 4 S. Silich and G. Tenorio-Tagle

If the half-mass radius of the star forming cloud is larger which leads to the SN-driven bubble stalling radius than Rhmw,crit and thus the central density is smaller than − 3(γ−1) 1/3γ the critical one, stellar winds merge everywhere inside the 1 β(γ 1)E0R R = 0 × stall,SN 1/γ − 2 star forming volume. Their merging results in the frag- 1.3 " (1 ǫ)GMtot # mentation of individual wind-driven shells which leads to a plethora of dense clumps around the sources and at the r2 5/6γ 1+ R4/3γ . (13) same time allows for the direct collision and thermaliza- a2 hm tion of neighboring winds. The hot gas then rapidly streams   away from the center comprising a cluster wind which can As the SN expands into the rarefied gas left by a pre- be mass-loaded with the fragmented matter (e.g. Silich et al. vious stellar wind, the initial radius R0 could be approx- 2010; Rogers & Pittard 2013). Such a wind eventually cleans imated by the radius of the stalling wind-driven bubble: ≈ up the star forming volume from the natal gas and prevents a R0 Rstall,w (Wheeler et al. 1980). 2G formation (Calura et al. 2015). However, if the half-mass Numerical simulations (e.g. Chevalier 1974; Falle 1981; Tenorio-Tagle et al. 1990) show that about 1/2 of the su- radius of the proto-cluster cloud is smaller than Rhmw,crit pernova remnant thermal energy is radiated away during a and thus the central density is larger than ρw,crit, stellar winds do not merge in the central zone of the cluster. The short transition phase from the adiabatic to the radiative regime. Therefore it was assumed that the initial thermal size of this zone R2G depends on the actual half-mass ra- dius of the proto-stellar cloud and becomes larger as one energy E0 is (Bisnovatyi-Kogan & Silich 1995): considers clouds with radii smaller than Rhmw,crit. 1 γ + 1 E0 = ESN , (14) Inside the central zone with radius r < R2G the 1G stars 2 3γ − 1 contaminate the pristine gas left over after their formation 51 with different H-burning products providing the conditions where ESN = 10 erg is the supernova explosion en- ergy. Note that in the very dense ambient medium (n > to form a polluted 2G. Note that wind-driven bubbles do 5 −3 not stall in the outskirts of the cloud where they instead 10 cm ), strong radiative cooling sets in before the Sedov accelerate into a sharp density gradient and then blow out phase starts and the initial energy E0 could be even smaller into the surrounding ISM. This restricts the size of the pol- than this value (Terlevich et al. 1992). luted zone and results in a centrally concentrated second The SN remnant stalling radii are much larger than subpopulation. those of wind-driven bubbles expanding into an equally dense ambient medium. This allows one to obtain the half-mass radii required for SNR to stall before merging 2.4 Supernovae-driven bubbles with nearby wind-blown bubbles from the condition that Rstall,SN < 2X: In young stellar clusters with a normal IMF stellar winds 12γ 9(γ−1) represent a dominant negative feedback mechanism only 2 (15γ−11) 25G (15γ−11) R < × for a short while, unless the most massive stars do not hm N 1/3 7β explode as supernovae, but directly form black holes (see     Decressin et al. 2010; Krause et al. 2012, and references 6(γ−1) 4 9.1(9γ − 4)(1 − ǫ)N (15γ−11) (1 − ǫ)GM (15γ−11) therein). It is usually assumed that in massive star clusters tot × 125(γ − 1)L β(γ − 1)E the first supernova explodes at an age of about 3.5Myr and  ⋆   0  − since then supernova explosions become the most energetic − 5(γ 1) r2 (15γ−11) negative feedback events responsible for the gas expulsion 1+ M . (15) a2 tot from the recently formed clusters. In this section we discuss   the impact that SNe provide on the gas which a cluster wind The condition that r = 0 in equation (15) determines the did not expel from the star forming region and the condi- critical half-mass radius and the critical central density in tions required to form a second subpopulation enriched with the star forming cloud: the iron group elements. 12γ 9(γ−1) If the gas density inside a star forming cloud exceeds the 2 (15γ−11) 25G (15γ−11) R = × critical value determined by equation (11), supernova rem- hmSN,crit N 1/3 7β nants (SNRs) may stall either before or after merging with     6(γ−1) nearby wind-blown bubbles. As we show in the next sec- − 9.1(9γ − 4)(1 − ǫ)N (15γ 11) tion, the critical densities calculated by means of equation × 125(γ − 1)L (11) are large. In such a case the Sedov phase terminates  ⋆  very rapidly (Shull 1980; Wheeler et al. 1980) and super- 4 (1 − ǫ)GM (15γ−11) remnants evolve in the snow-plow regime. The thermal tot M , (16) β(γ − 1)E tot pressure inside the supernova remnant then drops as it grows  0  (Pasko & Silich 1986): 3M 3(1 − ǫ)M ρ = g = tot . (17) − SN,crit 3 3 3(γ 1) 3(γ−1) −3γ 4πacrit 4π(RhmSN,crit/1.3) P = E0R0 r , (12) 4π SN-driven bubbles sweep up the gas left over from star where R0 is the radius at which the transition to the snow- formation and if they stall at the distance larger than the plow regime has occurred and E0 is the thermal energy of mean separation between nearby massive stars, the stellar the remnant at this moment. The remnant stops after this winds formerly separated by the dense matter left over from pressure drops to that in the turbulent ambient medium star formation merge and add their energy to the thermal

MNRAS 000, 1–9 (2016) Gas expulsion vs gas retention in YSCs 5 energy of the hot bubble formed after a supernova explo- development of RT instabilities and the SN shell destruction. sion. A coherent shell that sweeps up the gas left over from It is likely that RT clumps left over from the disrupted shells star formation is then formed inside the cluster. The energy remain bound to the cluster (Krause et al. 2012) while the input rate in such a bubble grows rapidly as it overtakes iron enriched gas escapes into the ambient ISM through the neighboring wind sources. One can show (see Appendix A) broken shells and does not pollute the gas available for a that in this case the expanding shell accelerates even if it 2G. The fundamental difference between these two regimes moves into a constant density ambient medium. Such an ac- is that individual SN explosions are not synchronized in celerating shell becomes destroyed via RT instabilities. The time and space. They do not act coherently and have little hot gas then finally escapes from the cloud carrying away all effect on the leftover gas distribution (Tenorio-Tagle et al. supernova products whereas fragments from the broken shell 2015). On the other hand, if wind-driven bubbles merge, are likely to remain bound within a gravitational well of the they do merge simultaneously in the whole star forming vol- cluster (Krause et al. 2012). Massive stars not affected by ume and eventually form a powerful, mass loaded star clus- the SN blast wave then continue to contaminate the collec- ter wind. The coherence of energy sources is a fundamental tion of RT clumps formed after blowout of previous SNRs ingredient as was already stressed by Calura et al. (2015); as well as the rest of the gas left over after the formation of Sharma et al. (2014). a first stellar generation. The most massive and clusters located be- In the central zones of even more compact and dense low the dashed lines in the top panels and above the dashed star forming clouds SNRs stall before merging with nearby lines in the bottom panels, retain the pristine gas and some wind-driven bubbles. Only in this case and with the help supernovae products. In this case the 2G should present an of the global turbulent pressure SNe are able to eventually enhanced iron abundance. enrich the pristine gas left over from the 1G formation with The comparison of the left-hand and right-hand panels iron group elements. The radius of this zone, Renrich, is de- in Figure 1 shows that the regime of star formation strongly termined by the critical half-mass radius RhmSN,crit and the depends on the SFE of the 1G. The critical lines go down in actual half-mass radius of the proto-stellar cloud. the top panels and up in the bottom panels as the consid- ered SFE for the 1G becomes larger. This implies that natal clouds with a large SFE must be more compact and denser 3 GAS EXPULSION VS GAS RETENTION IN compared to their counterparts with a low SFE in order to YOUNG STELLAR CLUSTERS form the 2G subpopulation. Note also that the critical lines mark clouds which retain primordial gas only in a small cen- As mentioned above, some models of multiple populations tral zone. However, the size of this zone and the amount of in globular clusters require the star cluster volume to be the retained gas available for the 2G formation grow rapidly cleaned up whereas other require the primordial gas to be in more compact clusters with an Rhm < Rhm,crit. retained. In this section the critical lines which allow one to The critical gas central densities are presented on the distinguish between clusters which expel or retain the gas bottom panels. At first glance they look too large if one left over from star formation are presented. Also, how these takes as a reference value the interstellar gas density in the critical lines are affected by the proto-stellar gas metallicity or another nearby galaxy. However, the stellar and by the SFE are here discussed. mass density in globular clusters with multiple stellar pop- 5 −3 ulations reaches 10 M⊙ pc which corresponds to a num- ber density of atoms about n ≈ 107cm−3 (Renzini 2013). 3.1 Young clusters in present day galaxies This value is comparable and in many cases even larger Figure 1 presents the critical radii, Rhmw,crit and RhmSN,crit than the critical densities shown in Figure 1. Thus, it is (upper panels), and the critical gas central densities (lower likely that many young, massive and compact clusters may panels) as a function of the star forming cloud mass, all as- retain the gas left over from star formation in their central suming a solar gas metallicity. The left-hand and right-hand zones. This seems to be in conflict with the recent results panels in Figure 1 correspond to different star formation ef- by Bastian et al. (2013, 2014) who did not detect a signifi- ficiencies: ǫ = 0.3 and ǫ = 0.7, respectively. The critical radii cant amount of gas in many young (ages less than 20 Myr) 6 and critical gas central densities for the wind-dominated and massive (masses about 10 M⊙ or more) clusters located in SN-dominated regimes are displayed by solid and dashed different nearby galaxies and claimed that even high-mass lines, respectively. clusters expel their natal gas within a few Myr after for- There are three zones in these diagrams. In clusters lo- mation. Note, however, that not all nearby young massive cated above the solid line on the top panels and below the clusters are free of gas. Probably the best example of an solid lines on the bottom panels, individual wind-driven bub- equally young and massive star cluster that still remains 6 bles merge with neighboring sources and form a collection embedded within its natal cloud is a ∼ 10 M⊙, ∼ 4Myr of dense clumps in the whole star forming volume. Nearby old cluster, in the dwarf galaxy NGC 5253 (see Turner et al. stellar winds then begin to collide heating up the injected 2000, 2003, 2015; Beck 2015). In order to understand what matter and forming a mass loaded star cluster wind which determines such a profound difference between this cluster eventually cleans up the star forming cloud. Stellar winds are and those studied by Bastian and collaborators, we selected not able to merge in more compact and denser star form- from the list of Bastian et al. (2014) two clusters whose ages ing clouds located in between the solid and dashed lines in and masses are similar to the massive cluster in NGC 5253. these diagrams. However SNRs formed in such clusters en- Following Krause et al. (2016) we transformed the half-light gulf neighboring wind sources. They gain then more energy radii of the selected clusters into half-mass radii (see Ta- which results in their acceleration and eventually leads to the ble 1). Note that the half-mass radii may be even larger if

MNRAS 000, 1–9 (2016) 6 S. Silich and G. Tenorio-Tagle one accounts for a possible mass segregation in the observed cantly extends the window of opportunity to form a second clusters. As the cluster in NGC 5253 is still embedded into stellar population in a low-metallicity clouds. a dense and is not visible at optical or UV Clouds located below the solid lines on the top pan- wavelengths, we adopted the size of the radio supernebula els and above the solid lines on the bottom panels in Fig- detected around the cluster (see Turner et al. 2000; Beck ures 1 and 2 are able to form multiple stellar populations. 2015) as the maximum value for the star cluster half-mass The mass ratio M1G/MG2 depends on the star formation radius and then accommodated all clusters in our diagram. efficiency (see Tenorio-Tagle et al. 2016) and on the clus- In the case of a low star formation efficiency (see the ter compactness. Proto-globular clouds form more massive left-hand upper panel in Figure 1) T353/W38220 and NGC 2G population when located further away from the wind- 5253 clusters are located in the parameter space were clus- driven bubble critical line in the gas retention parameter ters should retain gas left over from star formation even space. However only very massive and compact clusters lo- in the supernovae-dominated regime. Even the older and cated below the dashed lines on the top panels and above less concentrated cluster Knot S is located in between the the dashed lines in the bottom panes in Figures 1 and 2 are wind and the SN critical lines and thus should expel SN able to retain SN products and form stellar populations with products, but retain the primordial gas and the dense RT different metallicities regarding the iron group elements. clumps caused by blowout events. However in the case of a larger star formation efficiency (the right-hand upper panel in Figure 1), only NGC 5253 cluster remains in the total gas 4 CONCLUDING REMARKS retention parameter space, while the Antennae clusters are to disperse their primordial gas during the wind-dominated The common belief that stellar winds and SNRs in compact, stage. Therefore we suggest that the star formation efficiency young stellar clusters rapidly merge due to a small separa- in the Antennae clusters must have been large. The major tion between nearby massive stars and expel the gas left over difference between these three clusters, very similar in their from star formation into the ambient ISM should be taken mass and age, is then their compactness: the most compact with care. Four major parameters (total mass, size, star for- cluster in the NGC 5253 galaxy remains embedded into its mation efficiency and the natal gas metallicity) determine natal cloud whereas the less compact clusters in the Anten- which one of the two processes - gas expulsion or gas reten- nae are not. The stalling bubble model may also explain the tion - dominates in each star forming cloud and select one lack of non-thermal radio emission from the NGC 5253 clus- of the three possible star formation regimes: ter (Beck et al. 1996; Mart´ın-Hern´andez et al. 2005) because - if the central gas density in the star forming cloud is in this case shock waves vanish rapidly, the Fermi acceler- small (ρg <ρw,crit), the collection of individual wind-driven ation mechanism does not work and therefore high energy bubbles merge and fragment. Neighboring stellar winds then particles cannot survive for a long time after supernova ex- collide, heat up the injected matter and form a powerful plosions. mass-loaded star cluster wind which eventually cleans out the cluster. In this case 2G stars are not formed unless the cluster accretes a sufficient amount of gas during the late 3.2 Young clusters in ancient low metallicity stages of its evolution, as suggested in the AGB scenario galaxies (D’Ercole et al. 2008, 2010) for formation; There is a common belief that stellar winds driven by stars - if the central gas density falls into the range ρw,crit < with a low metal abundances are less energetic than those ρg < ρSN,crit, stellar winds do not merge. However shells driven by their metal rich counterparts. Taking into ac- formed after individual supernova explosions engulf neigh- count this difference, one can calculate the critical half-mass boring massive stars, gain their stellar wind energy, accel- radii and the critical central densities for young proto-stellar erate and eventually are disrupted via RT instabilities. As clouds with a low gas metallicity as was done in the previ- individual SN explosions are not synchronized in time and ous section for present day young massive clusters. Critical space, they have little effect on the gas distribution in the lines calculated with a stellar wind mechanical rest of the star forming cloud. RT clumps are likely to remain as derived by D’Ercole et al. (2008) for stars with extremely bound within the gravitational well of the cluster. This col- 39 6 −1 low metal abundances, L⋆ = 3 × 10 (M⋆/10 M⊙) ers s , lection of clumps and the gas unused for the 1G formation are shown in Figure 2. are continuously contaminated by the 1G stars and likely An inspection of Figure 2 and its comparison with Fig- form finally a second subpopulation. The hot, iron enriched ure 1 lead one to conclude that the requirements for proto- gas however escapes from the cluster into the ambient ISM globular clusters to retain pristine gas left over from star thorough the broken shells and does not pollute the 2G with formation are less stringent than those derived for present products of SN explosions. All stellar subpopulations in such day massive clusters. Much less dense (about an order of clusters should present the same iron group metallicity; magnitude) and less compact proto-globular clusters may - if the central gas density in the proto-stellar cloud retain gas left over after the 1G stars have formed. This is is large enough, ρg > ρSN,crit, SN remnants in the central because stars with a low metal abundances have less en- zones of the star forming cloud stall before merging with ergetic winds and their wind-driven bubbles stall without nearby stellar winds. Such clusters retain metals injected merging with their neighbors in lower density star forming into the intra-cluster medium by SNe. Only in this case clus- clouds (compare the bottom panels in Figures 2 and 1). In ters with an [Fe/H] spread could be formed. this case SNRs also stall before merging with nearby stel- Clusters with the same mass, SFE and the natal gas lar winds in less dense star forming clouds as they explode metallicity but different mass concentrations evolve in dif- inside wind-driven bubbles with a smaller size. This signifi- ferent hydrodynamic regimes. More compact clusters retain

MNRAS 000, 1–9 (2016) Gas expulsion vs gas retention in YSCs 7


Table 1. Observational parameters of the selected clusters

Galaxy Cluster Age Mass Half-mass Radius 6 (Myr) (10 M⊙) (pc)

The Antennae . +2.5 T352/W38220 4 0.92 4 1−1.7 . +2.5 Knot S 5 1.6 13 6−2.5 NGC 5253 . +0.15 in cloud D 4.4 1.1 0 5−0.15

Figure 1. Critical radii and gas central densities for proto-stellar clouds with solar gas metallicity. The left-hand panels show how the critical half-mass radius and the central gas density change with the total mass of the proto-stellar cloud if the star formation efficiency ǫ = 0.3. The right-hand column shows the same in the case of the larger star formation efficiency ǫ = 0.7. In all panels solid and dashed lines display the critical cloud parameters derived for a stellar wind-dominated regime and after the onset of the supernovae explosions, respectively. Labeled symbols in the upper panels give the location of three selected clusters (see Table 1) with their corresponding error bars.

MNRAS 000, 1–9 (2016) 8 S. Silich and G. Tenorio-Tagle

Figure 2. The same as in Figure 1, but calculated for proto-stellar clouds with a primordial gas metallicity Z = 0.001Z⊙. the natal gas left over from the 1G formation whereas less our anonymous referee for a detailed report full of valuable compact ones expel it into the ambient . comments which helped to clarify our model and improve This justifies the presence of natal gas in the most compact the paper significantly. This study has been supported by super star cluster in NGC 5253 and the non-detection of gas CONACYT - M´exico, research grant 167169. left over from star formation in the less compact clusters T352/W38220 and Knot S, in the Antennae galaxies.


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MNRAS 000, 1–9 (2016) Gas expulsion vs gas retention in YSCs 9

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