Multiple stellar populations and the horizontal branch of globular clusters

1. Multiple stellar populations and the horizontal branch of globular clusters Raffaele Gratton INAF – Osservatorio Astronomico di Padova

2. Angela Bragaglia Eugenio Carretta Valentina D’Orazi Sara Lucatello Yazan Momany Chris Sneden Antonio Sollima Franca D’Antona Paolo Ventura Santi Cassisi Giampaolo Piotto Anna Fabiola Marino Antonino Milone Alessandro Villanova Collaborators

3. Single stellar population (SSP) • A set of stars having: • Same age • Same chemical composition (both Y and Z) • Different mass (distributed according to an IMF) • Possibly, binaries included • It is described by a single isochrone in the CMD • Tool widely used in stellar and galactic evolution context • Stellar populations in galaxies are usually assumed to be reproduced by suitable weighted sums of SSPs • Stellar clusters are usually considered good examples of SSP

4. Evidences for multiple populations in GCs • Spectroscopy • From ’70s. But for a long time attributed to peculiar evolution • From 2001: Na-O anticorrelation on the MS • Photometry • From ’60s (HB second parameter). However, for a long time not understood • Still from ’60s: ω Cen  considered peculiar • From ’70s: NGC2808 and NGC1851  still considered as peculiar • From 2004: multiple MSs and SGBs

5. ESO Large Program 165-L0263 • The O-Na anticorrelation is present among TO-stars and subgiants in NGC6752. For the same stars, also a Mg-Al anticorrelation is observed • This clearly rules out deep mixing as explanation for the O-Na anticorrelation • The sum of C+N abundances is not constant: a substantial fraction of O is transformed into N in some NGC6752 stars  A fraction of the stars in GCs (second generation, SG) formed from the ejecta of an earlier population (first generation or primordial population)

6. Carretta et al. extensive survey (2009)(Flames@VLT2)

7. All GCs have multiple populations Red: have Na-O anticorrelation Green: do not have Na-O anticorrelation Empty: not yet studied NGC6791 Different symbols are for GCs in the MW, LMC or DSph’s

8. It is modulated by a combination of metallicity and cluster mass

9. O-Na anticorrelation and HB

10. Median mass of HB stars determined mainly by [Fe/H] and age Median masses on the HB can be derived by comparison with models If ages are known from main sequence photometry (e.g. Marin-French et al.) mass loss by stars along the RGB can be derived This mass loss result to be roughly a simple linear function of [Fe/H] Gratton et al. 2010

11. Multiple populations in GCs: NGC2808 (MV=-9.4) Piotto et al. 2002 Piotto et al. 2007, ApJL 661, L53

12. 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

13. ω Cen is the largest GC in the MW: Multiple RGB sequences • The distribution of stars with metallicity is not continuous: various episodes of star formation • There is a metal-rich population, with [Fe/H]~-0.6 (Pancino et al.): RGB-a Ferraro et al. 2004, ApJ, 603, L81 Bellini et al. 2009

14. ω Cen: Main sequence splitting Giraffe@VLT2 HST-ACS • There are two MSs; the blue one has ¼ of the stars • The bluest MS is more metal-rich [Fe/H]~-1.2) than the redder one ([Fe/H]~-1.6) • This agrees with the redder one be more populous • But this implies a higher He-content (Y~0.4 rather than 0.25)! • Populations suggest that the He-rich MS is connected to the extreme BHB Bedin et al. 2004, ApJ 605, L125 Piotto et al. 2005, ApJ 621, 777

15. He in HB-stars: expectations • Stars distribute along the HB of a GC according to their mass • TO masses of stars in a GC should depend on their He-content • Assuming similar mass loss, stars of different He should distribute along the HB • Redder HB/He-poor/O-rich/Na-poor • Bluer HB/He-rich/O-poor/Na-rich • On HB, possibility to derive He-abundances

16. Once [Fe/H] and ages are known, He can be derived from colours The spread in He derived from the spread in colours  masses of stars along the HB is correlated with the amplitude of the O-Na and Mg-Al anticorrelations. This is expected if He is produced with Na and Al

17. Villanova et al. 2009: NGC6752 (UVES@VLT2) evolved Diff+Rad lev.

18. Marino et al. 2010: M4(Flames@ VLT2)

19. Gratton et al. 2011: NGC2808(Flames@VLT2)

20. Gratton et al. 2012: 47 Tuc(Flames@VLT2)

21. The main parameter driving the multiple populations is the cluster mass

22. Conclusions • (All) GCs host multiple stellar populations • These populations differ in their abundances of He, C, N, O, Na, Mg, Al abundances (signature of H-burning at high temperature)  Formation mechanism • The location of the stars on the HB is determined by a number of factors • A mass loss proportional to metallicity • The ages of GCs • The spread in He (extension of the second population)  related to their mass • This explains most, perhaps all the second parameter issue

23. Perspectives • This scenario explains many observables! • If true, it connects the formation of GCs to that of the halo (which is mainly made of FG lost by GCs) • Main uncertainty is the nature of the stars responsible for the second generation  timescale of the phenomenon • The cosmological implications need still to be fully understood (e.g. mechanisms of star formations in disk and spheroids, missing satellite issue, reionization, etc.)