Multiple stellar Multiple stellar populations and the populations and the horizontal branch of horizontal branch of
globular clustersglobular clustersRaffaele GrattonRaffaele Gratton
INAF – Osservatorio INAF – Osservatorio Astronomico di PadovaAstronomico di Padova
CollaboratorsCollaborators
Angela BragagliaAngela Bragaglia Eugenio CarrettaEugenio Carretta Valentina D’OraziValentina D’Orazi Sara LucatelloSara Lucatello Yazan MomanyYazan Momany Chris SnedenChris Sneden Antonio SollimaAntonio Sollima
Franca D’AntonaFranca D’Antona Paolo VenturaPaolo Ventura Santi CassisiSanti Cassisi Giampaolo PiottoGiampaolo Piotto Anna Fabiola MarinoAnna Fabiola Marino Antonino MiloneAntonino Milone Alessandro VillanovaAlessandro Villanova
Single stellar population Single stellar population (SSP)(SSP)
A set of stars having:A set of stars having: Same ageSame 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 includedPossibly, binaries included It is described by a single isochrone in the It is described by a single isochrone in the
CMDCMD Tool widely used in stellar and galactic Tool widely used in stellar and galactic
evolution contextevolution context Stellar populations in galaxies are usually assumed Stellar populations in galaxies are usually assumed
to be reproduced by suitable weighted sums of to be reproduced by suitable weighted sums of SSPsSSPs
Stellar clusters are usually considered Stellar clusters are usually considered good examples of SSPgood examples of SSP
Evidences for multiple Evidences for multiple populations in GCspopulations in GCs
SpectroscopySpectroscopy From ’70s. But for a long time attributed to From ’70s. But for a long time attributed to
peculiar evolutionpeculiar evolution From 2001: Na-O anticorrelation on the From 2001: Na-O anticorrelation on the
MSMS PhotometryPhotometry
From ’60s (HB second parameter). However, From ’60s (HB second parameter). However, for a long time not understoodfor a long time not understood
Still from ’60s: Still from ’60s: ωω Cen Cen considered peculiar considered peculiar From ’70s: NGC2808 and NGC1851 From ’70s: NGC2808 and NGC1851 still still
considered as peculiarconsidered as peculiar From 2004: multiple MSs and SGBsFrom 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 Carretta et al. extensive survey (2009)survey (2009)
(Flames@VLT2)(Flames@VLT2)
All GCs have multiple All GCs have multiple populationspopulations
Red: have Na-O anticorrelationGreen: do not have Na-O anticorrelationEmpty: not yet studied
Different symbols are for GCs in the MW, LMC or DSph’s
NGC6791
It is modulated by a It is modulated by a combination of combination of
metallicity and cluster metallicity and cluster massmass
O-Na anticorrelation and O-Na anticorrelation and HBHB
Median mass of HB stars Median mass of HB stars determined mainly by determined mainly by
[Fe/H] and age[Fe/H] and age
Gratton et al. 2010
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: Multiple populations in GCs: NGC2808 (MNGC2808 (MVV=-9.4)=-9.4)
Piotto et al. 2007, ApJL 661, L53Piotto et al. 2002
D’Antona et al. 2005: D’Antona et al. 2005: Na-rich stars should be Na-rich stars should be richer in Hericher in He
He-rich stars evolve He-rich stars evolve fasterfaster
if same mass loss if same mass loss evolved He-rich stars evolved He-rich stars have lower mass have lower mass they they are bluer when on the are bluer when on the HBHB
Na-O Na-O anticorrelation anticorrelation
He He HB HB
ωω Cen is the largest GC in Cen is the largest GC in the MW: Multiple RGB the MW: Multiple RGB
sequencessequences
Ferraro et al. 2004, ApJ, 603, L81 Bellini et al. 2009• 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
ωω Cen: Main sequence Cen: Main sequence splittingsplitting
Bedin et al. 2004, ApJ 605, L125 Piotto 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
Giraffe@VLT2HST-ACS
He in HB-stars: He in HB-stars: expectationsexpectations
Stars distribute along the HB of a GC Stars distribute along the HB of a GC according to their massaccording to their mass
TO masses of stars in a GC should depend TO masses of stars in a GC should depend on their He-contenton their He-content
Assuming similar mass loss, stars of Assuming similar mass loss, stars of different He should distribute along the HBdifferent He should distribute along the HB
Redder HB/He-poor/O-rich/Na-poorRedder HB/He-poor/O-rich/Na-poor Bluer HB/He-rich/O-poor/Na-richBluer HB/He-rich/O-poor/Na-rich
On HB, possibility to derive He-On HB, possibility to derive He-abundancesabundances
Once [Fe/H] and ages are Once [Fe/H] and ages are known, He can be derived known, He can be derived
from coloursfrom coloursThe 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: Villanova et al. 2009: NGC6752 (UVES@VLT2)NGC6752 (UVES@VLT2)
Diff+Rad lev.
evolved
Marino et Marino et al. 2010: al. 2010:
M4M4(Flames@ (Flames@
VLT2)VLT2)
Gratton et al. 2011: Gratton et al. 2011: NGC2808NGC2808
(Flames@VLT2)(Flames@VLT2)
Gratton et al. 2012: 47 TucGratton et al. 2012: 47 Tuc(Flames@VLT2)(Flames@VLT2)
-0.50
-0.30
-0.10
0.10
0.30
0.50
0.70
0.90
1.10
1.30
0.600 0.700 0.800 0.900 1.000
B-V
[Na/O]
Faint
Bright
The main parameter driving The main parameter driving the multiple populations is the multiple populations is
the cluster massthe cluster mass
ConclusionsConclusions (All) GCs host multiple stellar populations(All) GCs host multiple stellar populations These populations differ in their abundances of These populations differ in their abundances of
He, C, N, O, Na, Mg, Al abundances (signature of He, C, N, O, Na, Mg, Al abundances (signature of H-burning at high temperature)H-burning at high temperature)
Formation mechanismFormation mechanism The location of the stars on the HB is determined The location of the stars on the HB is determined
by a number of factorsby a number of factors A mass loss proportional to metallicityA mass loss proportional to metallicity The ages of GCsThe ages of GCs The spread in He (extension of the second population) The spread in He (extension of the second population)
related to their mass related to their mass This explains most, perhaps all the second This explains most, perhaps all the second
parameter issueparameter issue
PerspectivesPerspectives
This scenario explains many observables!This scenario explains many observables! If true, it connects the formation of GCs to If true, it connects the formation of GCs to
that of the halo (which is mainly made of FG that of the halo (which is mainly made of FG lost by GCs)lost by GCs)
Main uncertainty is the nature of the stars Main uncertainty is the nature of the stars responsible for the second generation responsible for the second generation timescale of the phenomenontimescale of the phenomenon
The cosmological implications need still to be The cosmological implications need still to be fully understood (e.g. mechanisms of star fully understood (e.g. mechanisms of star formations in disk and spheroids, missing formations in disk and spheroids, missing satellite issue, reionization, etc.)satellite issue, reionization, etc.)