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

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

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

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

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)

Carretta et al. extensive survey (2009)

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

It is modulated by a combination of metallicity and cluster mass

O-Na anticorrelation and HB

Median mass of HB stars determined mainly by [Fe/H] and age Gratton et al 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]

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

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

ω Cen is the largest GC in the MW: Multiple RGB sequences Ferraro et al. 2004, ApJ, 603, L81 Bellini et al 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

ω Cen: Main sequence splitting Bedin et al. 2004, ApJ 605, L125Piotto et al. 2005, ApJ 621, 777 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 HST-ACS

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

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

Villanova et al. 2009: NGC6752 Diff+Rad lev. evolved

Marino et al. 2010: M4 VLT2)

Gratton et al. 2011: NGC2808

Gratton et al. 2012: 47 Tuc

The main parameter driving the multiple populations is the cluster mass

Conclusions (All) GCs host multiple stellar populations (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) 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 The location of the stars on the HB is determined by a number of factors A mass loss proportional to metallicity A mass loss proportional to metallicity The ages of GCs The ages of GCs The spread in He (extension of the second population)  related to their mass The spread in He (extension of the second population)  related to their mass This explains most, perhaps all the second parameter issue This explains most, perhaps all the second parameter issue

Perspectives This scenario explains many observables! 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) 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 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.) The cosmological implications need still to be fully understood (e.g. mechanisms of star formations in disk and spheroids, missing satellite issue, reionization, etc.)