Transcript Understanding the horizontal branch of globular clusters

Multiple stellar populations
and the horizontal branch of
globular clusters
Raffaele Gratton
INAF – Osservatorio Astronomico di
Padova
Collaborators
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Angela Bragaglia
Eugenio Carretta
Valentina D’Orazi
Sara Lucatello
Yazan Momany
Chris Sneden
Antonio Sollima
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Franca D’Antona
Paolo Ventura
Santi Cassisi
Giampaolo Piotto
Anna Fabiola Marino
Antonino Milone
Alessandro Villanova
Single stellar population (SSP)
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A set of stars having:
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Tool widely used in stellar and galactic evolution
context
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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
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
Evidences for multiple populations
in GCs
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Spectroscopy
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From ’70s. But for a long time attributed to peculiar
evolution
From 2001: Na-O anticorrelation on the MS
Photometry
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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
ESO Large Program 165-L0263
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The O-Na anticorrelation is present among TO-stars and
subgiants in NGC6752. For the same stars, also a Mg-Al
anticorrelation is observed
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This clearly rules out deep mixing as explanation for the ONa anticorrelation
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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)
(Flames@VLT2)
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
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
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
Multiple populations in GCs: NGC2808
(MV=-9.4)
Piotto et al. 2002
Piotto et al. 2007, ApJL 661, L53
Na-O anticorrelation
 He  HB
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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
ω Cen is the largest GC in the MW:
Multiple RGB sequences
• The distribution of stars with metallicity is not
Ferraro et al. 2004, ApJ, 603, L81continuous: various episodes
of et
star
Bellini
al. formation
2009
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There is a metal-rich population, with [Fe/H]~-0.6
(Pancino et al.): RGB-a
ω Cen: Main sequence splitting
HST-ACS
Giraffe@VLT2
•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)!
Bedin et al. 2004, ApJ 605, L125
Piotto et al. 2005, ApJ 621, 777
• Populations suggest that the He-rich MS is connected to
the extreme BHB
He in HB-stars: expectations
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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
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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
(UVES@VLT2)
evolved
Diff+Rad
lev.
Marino et al.
2010: M4
(Flames@
VLT2)
Gratton et al. 2011: NGC2808
(Flames@VLT2)
Gratton et al. 2012: 47 Tuc
(Flames@VLT2)
1.30
1.10
0.90
[Na/O]
0.70
0.50
Faint
0.30
Bright
0.10
-0.10
-0.30
-0.50
0.600
0.700
0.800
B-V
0.900
1.000
The main parameter driving the
multiple populations is the cluster
mass
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
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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
Perspectives
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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.)