Multiple stellar populations in Globular Clusters - A progress report

1. Multiple stellar populations in Globular Clusters - A progress report Raffaele Gratton INAF – OsservatorioAstronomico di Padova ITALY

2. Variations in the strength of CH and CN bands Noticed since early seventies (Osborn 1971) from DDO photometry and spectroscopy Bimodal distribution along the RGB (Norris & Smith 1980s) Variations among MS stars in 47 Tuc (Briley et al. 1994) NGC6752

3. Most GCs are not homogeneous for other elements  O-Na and Mg-Al anti-correlations Cohen, 1978, ApJ, 223, 487: Scatter in Na abundances in M13 and M3 red giants

4. Signature of the action of high temperature proton capture fusion Ivans et al. (2001) Survey of bright giants by the Lick-Texas group (Sneden, Kraft, Langer, Ivans et al) CNO: T > 20 MK NeNa: T~ 35 MK MgAl: T> 50 MK 24Mg depletion: T~70 MK The NeNa cycle which enhances Na, is expected to operate in the same fusion zones in which the ON part of the CNO cycle is fully operative (Denisenkov & Denisenkova 1990; Langer et al. 1993).

5. NGC 6397 NGC 6752 47 Tuc Li, C, N, O, Na, Al, Mg in unevolved cluster stars • negligible convective envelopes • the Al-Mg anticorrelation in TO stars in NGC 6752 requires too high core temperatures • dominant H-burning cycle is p-p not CNO Gratton et al. (2001), Bonifacio et al. (2003), Carretta et al. (2004) + Ramirez & Cohen (2002) M71 Internal mixing isruled out at least TWO stellar generations!!

6. The whole star, not only the outer convective envelope, is polluted

7. First part conclusions (2007) • Globular clusters host different generation of stars • Stars of the later generations formed from material expelled by a fraction of the 1st gen stars (the polluters) likely diluted by gas with pristine composition (Prantzos & Charbonnel 2006) • In most cases, no detectable production of Fe and heavy elements (some exceptions, mainly massive clusters) • The polluters underwent H-burning at high temperature ( Na-O and Mg-Al anti-correlations) • Various candidates: • Massive AGB stars (M>4 Mo): Ventura et al. 2001 • Fast rotating massive stars (during their main sequence phase): Decressin et al. 2007 • Massive binaries: De Mink et al. 2009 • The combination of these + interacting stars in dense environments (Elmegreen 2017) • Novae: Smith & Kraft 1996, Maccarone et al. 2012

8. Carretta et al. extensive survey (2004-15) NGC4833 NGC6093 NGC362

9. Apogee (Meszaros et al. 2015)

10. Other GCs with spectroscopic evidences of multiple populations NGC1261: Filler et al. 2012 NGC2419: Cohen & Kirby 2012 NGC5053: Boberg et al. 2015 NGC5634: Carretta et al. 2017 NGC5694: Mucciarelli et al. 2013 NGC5824: Da Costa et al. 2014 NGC5897: Koch & McWilliam 2014 NGC6266: Yong et al. 2014; Milone 2015 (phot) NGC6362: Dalessandro et al. 2014 (phot) NGC6366: Johnson et al. 2013 NGC6712: Yong et al. 2008 NGC6723: Gratton et al. 2015

11. Have all GCs multiple populations? Red: Na-O anticorrelation Crosses: no Na-O anticorrelation Empty: not yet studied Different symbols are for GCs in the MW, LMC or DSph’s New definition of GC! Rup106 Pal 3 NGC6366 Pal 14

12. GCs without evidences of multiple populations • Pal 12: Cohen et al. 2004 • Ter 7: Sbordone et al. 2004 • Pal 3: Koch et al. 2009 • Pal 14: Caliskan et al. 2012 • Rup 106: Villanova et al. 2013 • Ter 8: Carretta et al. 2014  They are all small objects

13. [Fe/H] spread and Mv There seems to be a correlation between [Fe/H] spread and mass: however the correlation is possibly not one-to-one: Ter 5 (Massari et al. 2014) and NGC5824 (Da Costa et al. 2014) are not very luminous GCs ω Cen NGC5824 Terzan 5 Garching, October 11, 2012

14. The extent of the O-Na anticorrelation depends on Mv and concentration

15. O-Na anticorrelation and HB(Recio-Blanco et al. 2006 , Carretta et al. 2010) ````

16. Na-O anticorrelation  He  HB • D’Antona et al. 2005: Na-rich stars should be richer in He • He-rich stars evolve faster • if same mass loss  evolved He-rich stars have lower mass  they are bluer when on the HB • Ex. NGC2808

17. Multiple populations in GCs: NGC2808 (MV=-9.4): broad band MS colours  ΔHe Piotto et al. 2002 Piotto et al. 2007, ApJL 661, L53

18. Multiple populations in GCs: NGC2808 (MV=-9.4): broad band MS colours  ΔHe E: Y~0.38 P: Y~0.24 I: Y~0.28 Piotto et al. 2002 Piotto et al. 2007, ApJL 661, L53 Broad band colours (as V-I) are sensitive to He  Na-O anticorrelation

19. Conclusion (2012) The presence of multiple populations is the rule for massive GCs They are not present in low mass GCs Mv is not the mass at the epoch of formation of the GCs: this was likely larger than the present mass The limiting mass for the onset of the phenomenon is not well clear, but it is at least ~2x105 Mo Other parameters might play a role (metallicity, concentration, epoch of formation) He abundance variations may be present There is a strong correlation with the second parameter effect on the HB

20. Multiple populations from HST photometry (Milone, Piotto et al.) Near UV bands are sensitive to N  Na-O anticorrelation From Dotter et al. 2014

21. Multiple populations from HST photometry (Milone, Piotto et al.) • Possibility to derive simultaneously information on Na-O anticorrelation and ΔHe if accurate (HST) photometry is available over a wide spectral range • Milone et al. (2012)

22. HST legacy program: 58 GCs (Piotto et al. 2015)

23. Definition of MPs (Milone et al. 2017)

24. General trends (Milone et al. 2017)

25. Results (2017) Multiple populations (variations of N, likely correlated with Na) are present in all GCs They can often be separated in a few discrete populations The separation between MP sequences are function of metallicity and Mv The fraction of FG stars decreases with Mv Large variations in He are rare: in most GCs the most He-rich stars have Y~0.3 or less

26. Concerns: Fornax and WLM Stellar debris is only a small fraction of the total stellar mass for a normal IMF The missing 90-95% of the FG stars had to escape These escaping stars presumably comprise the halo of the MW (Prantzos & Charbonnel 2006) BUT: no evidence of these stars in dwarf galaxies Fornax and WLM (Larsen et al. 2012, 2014; Elmegreen et al. 2012)

27. Concerns: YMC • Idea: observe very massive clusters when multiple generations are occurring • Various studies by Bastian and co-workers • No evidence of on-going star formation in YMO (Bastian et al. 2013; Cabrera-Ziri et al. 2014) • A number of exposed local YMC exist (M>=106 M⊙, age<=15 Myr)  even high-mass clusters are able to clear any natal gas within them within a few Myr after formation (Longmore et al. 2014) • the clusters in M83 are all exposed (no longer embedded) by <4 Myr (Hollyhead et al. 2015) • Difficulty to reproduce He, Na and O production with a single polluter (Bastian et al. 2015) • Conclusion: no satisfactory mechanism known (see also Renzini et al. 2015)

28. The EMSTO quarrel Many populous clusters in the LMS/SMC with age ~1 Gyr show an extended MS TO that can be interpreted as a spread in age of several hundreds Myr (Bertelli et al. 2003)  possible evidence of MPs (Mackey et al. 2008) In some cases dual red clump (Girardi et al. 2009) Bertelli et al. 2003 Keller et al. 2011

29. Controversy • Other effects may mimic an age spread: • Binaries (Bertelli et al. 2003) • Spread in stellar rotation (Bastian & De Mink 2009; D’Antona et al. 2015) • Interacting binaries (Yang et al. 2011) • Variability (Salinas et al. 2016) • Age spread still controversial; see e.g: • Pros’: Milone et al. (2017) • Cons’: Bastian et al. (2017)

30. Polluters • The polluters for Intermediate stars: • Is active in all GCs with M>2x105 Mo; the fraction of stars is 50-75% • Produce moderately extended Na-O anticorrelation (Na enhancement by ~0.6-0.8 dex, O depletion by ~0.4-0.6 dex) • In some clusters produce a moderate Mg-Al anticorrelation • Produce a small He enhancement (ΔY≤0.04) • Need to be a Li producer (log n(Li)~2.5)  this pattern well matches massive AGB stars undergoing HBB (lifetime 30-100 Myr)

31. Polluters • The polluters for Extreme stars: • Is active only in most massive GCs (M>106 Mo); the fraction of stars is at most ~25% of total • Produce stars with Na/Al enhanced by ~0.4-1.0 dexand O/Mg depleted by ~0.5-1.0 dex • In extreme cases even an Ar-K cycle (Cohen & Kirby 2012) • Produce a large He enhancement (ΔY~0.10) • Do not need to be a Li producer • He production (but not Mg-Al and Ar-K cycle) matches fast rotating massive stars (Decressin et al. lifetime a few Myr) • Alternatively, massive AGB stars may also be considered (D’Antona et al. 2016)

32. D’Antona et al. 2016 scenario (NGC2808)

33. D’Antona et al. 2016 scenario

34. The Elmegreen (2017) scenario Interactions between massive stars and massive star binaries can shred stellar envelopes Massive stars should also scatter low-mass stars out of the cluster Agreement with observations for a cloud core mass of ~4x106 Mo and density of 2x106 cm-3 Mechanism efficient at early epochs when pressure was higher Ratio of gas consumption over lifetime of massive stars should be low (~10%) Efficiency of scattering low-mass stars per unit dynamical time should be large (a few per cent) Velocity dispersion of embedded GC comparable to gas dispersions of galaxies  stellar ejection of multi-star interactions from dwarf galaxies (e.g. Fornax, WLM)