Globular clusters role in
formation of the halo
Raffaele Gratton
INAF – Osservatorio Astronomico di
Padova
Chemical evolution in the Universe
Collaborators
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Angela Bragaglia
Eugenio Carretta
Sara Lucatello
Valentina D’Orazi
Yazan Momany
Chris Sneden
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Franca D’Antona
Paolo Ventura
Santi Cassisi
Giampaolo Piotto
Anna Fabiola Marino
Antonino Milone
Alessandro Villanova
Chemical evolution in the Universe
GCs and Open Clusters
GCs are more massive and
older than OCs
GCs host multiple stellar
populations; OC do not
GC formation scenario
(Carretta et al. 2010)
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Interaction between a still gaseous proto-dSph and the MW (or
between proto-dSph’s) (Bekki)
Formation of a precursor population, with a raise in [Fe/H],
sometimes fast
Triggering formation of a large primordial population (first
generation)
Winds from massive stars and core collapse SNe stop further
star formation and clean the region from primordial ISM
Low velocity wind from massive AGB stars generates a cooling
flow (Ventura et al., D’Ercole et al.)
Second generation stars form in this cooling flow
Core collapse SNe for this second generation stops further star
formation
In the meantime, decoupling between DM and gas
Chemical evolution in the Universe
Possible variations
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Other polluters have been proposed:
Fast Rotating Massive Stars (Decressin et al.)
 Novae (Smith & Kraft; Maccarone et al.)
 Massive interacting binaries (De Mink et al.)
 Super Massive Stars (Denissenkov & Hartwick)
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However, most features of the scenario remain
valid
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Globular clusters should have been much more
massive at their origin than now (from a few to ~15)
Chemical evolution in the Universe
GC and dSph luminosity
functions overlap
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We constructed a
luminosity function for
GCs (Harris catalogue)
and DSph (likely
incomplete at faint end)
Total integrated Mv for
MW:
GCs: -13.3 (initially: from
-15 up to -16)
 dSph: -14.4
Chemical evolution in the Universe
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Globular clusters and dwarf
galaxies: mass-to-light ratio
There is scarce
evidence of dark matter
in both GCs and UCD
galaxies, in agreement
with expectations for
small dark matter halo.
GCs have lower massto-light ratio likely
because of dynamical
evolution (since they
are relaxed objects)
From Dabringhausen et al. 2008 MNRAS 386 864
Chemical evolution in the Universe
The luminosity-metallicity relation
Kirby et al. 2008, arXiv:0807.1925
Chemical evolution in the Universe
The luminosity-metallicity relation
• dSph
(mean) metallicity depends on luminosity (present
baryonic mass?). This agrees with the concept that dSph make
their own metals
• GCs metallicity is fairly independent of luminosity (mass?).
This agrees with the concept that they inherited the metallicity
of the medium where they formed
• GCs have a bimodal metallicity distribution (Disk/Halo)
with a minimum at [Fe/H]~-0.9
• At a given metallicity, the dSph luminosity is a lower envelope
of GCs metallicities  Halo GCs formed in objects that were
more massive than they currently are!
Mv of the GC progenitors
Using the mass-metallicity relation for DSph we may assign an Mv to the progenitor of each GC
For the halo the median ratio progenitor/GCs is 27 (depends on the exact form of the relation)
Equal
Mv
Median
progenitor
Mv~-11.5
See also Leaman et al. 2013, arXiv1309.0822
GCs are tidally limited, dSph’s are not
According to this scenario, GCs
and dSph’s originated from similar
systems but:
Evolved in isolation
 GCs progenitors interacted with
the MW while still mainly gaseous
 dSph’s evolved in isolation (
luminosity-metallicity relation)
NFW
halo
Green: dSph’s
Red: Bulge/thick disk GCs
Blue: Inner Halo GCs
Black: Outer Halo GCs
Tidally limited region
Chemical evolution in the Universe
GC and dwarf galaxies as survivors
(tidal tails)
dSph: Sagittarius
Belokurov et al. 2006, ApJ 642, L137
GC: Pal 5
Odenkirchen et al. 2001, ApJ 548, L165
Chemical evolution in the Universe
Halo and GCs
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Current cosmological models (White & Rees 1978; Moore et al.
1999): the Milky Way’s stellar halo was assembled from many
smaller systems
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The metal-poor ([Fe/H]<-2) part of the halo may be made of
objects like the smallest DSph’s
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The bulk of the halo (-2<[Fe/H]<-1) is clearly different from
DSph’s stars of similar metallicity (largest DSPh’s) (e.g. the run
of [α/Fe] is different)
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The primordial (FG) population of GCs is a plausible
candidate (see also Vesperini et al., Martell & Grebel,
Conroy)
Chemical evolution in the Universe
Stars lost from the GCs
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Violent relaxation following gas expulsion and mass loss from
the most massive stars (see e.g. Baumgardt, Kroupa &
Parmentier 2008)
On a longer timescale: evaporation due to two-body
encounters and other mechanims (e.g. disk shocking: see e.g.
Aguilar, Hut & Ostriker 1988)
Due to this second effect, some per cent of the stars should be
removed within a relaxation time (~108-109 yr at half mass,
longer for more massive GCs, with a median value of about
5x108 yr: Harris 1996)
A substantial fraction of the original GC mass should have
been lost, this loss being more efficient among smaller GCs
Chemical evolution in the Universe
Evidences
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Tidal tails around some GCs (see e.g. Odenkirchen
et al. 2003)
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Deficiency of small mass stars preferentially lost if
energy equipartition holds (Henon 1969; Richer et al.
1991; De Marchi et al. 2007; De Marchi & Pulone
2007)
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Current GCs are the survivors of a potentially
larger initial population
Chemical evolution in the Universe
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A major issue is whether a part (even large) of
the Galactic field stars were formed in GCs and
later ended up as a main component of the halo
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Infant cluster mortality might be the major
source of halo stars (see e.g. Baumgardt et al.
2008)
Chemical evolution in the Universe
Multiple stellar generations
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The SG population is typical of GCs (Gratton
et al. 2001)
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The fraction (~2.5%) of Na-rich and/or CNrich stars in the field halo (Carretta et al. 2010a;
Martell & Grebel 2010)
 stars evaporated from GCs
Chemical evolution in the Universe
GC stellar populations
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FG with the typical composition of the ejecta of core-collapse
SNe (e.g. Truran & Arnett 1971) are only 1/3 of the current total
population
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To reproduce n(FG)/n(SG), a large fraction of stars of the FG
must have been lost from GCs
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The same from theoretical studies and numerical simulations of
dynamical evolution (Decressin et al. 2008, D’Ercole et al. 2008
and references in Carretta et al. 2010a)
Chemical evolution in the Universe
The present mass of the GCs
compared to the mass of the halo
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Mass of the inner halo GCs vs field population from in situ
star counts (Juric et al. 2008)
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To avoid contamination by the galactic thick disk, polar caps
defined (Z>5 kpc, and rgalactocentric<18 kpc).
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We summed up the luminosity of all GCs in the Harris (1996)
catalogue that are within this volume + M/LV = 2 (Mandushev
et al. 1991; Pryor & Meylan 1993)
 mass(GCs)=4.1x106 Mo
Chemical evolution in the Universe
Mass of the halo
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Mass of halo stars:
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Density law for the halo by Juric et al. (2008)
Their normalisation of halo/thin disk density in the solar neighbourhood
Local stellar density of 0.038 Mo/pc3 (Jahreiss & Wielen 1997)
 mass(Halo)=3.3x108 Mo
 mass(GCs)~1.2% of mass(Halo)
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Similar value for the whole halo outside 4 kpc
Total stellar mass within halo GCs ~1.4x107 Mo, in the field
~1.2x109 Mo
Chemical evolution in the Universe
The original GC mass
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Carretta et al. (2009): SG=2/3 of GC stars
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Carretta et al. (2010) and Martell and Grebel (2010):
stars with a composition similar to SG in GCs (i.e. Na-rich and/or CN-rich):
~2.5% of current halo stars
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If:
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They are SG stars lost by GC
SG stars are lost after GC formation at the same rate as FG ones
 stars lost by GCs after the formation of SG stars ~3.7% of the
current halo stars
Chemical evolution in the Universe
The original GC mass
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Currently 1.2% of the halo mass is still in GCs
 they should have lost some 75% of their
original mass
 They were originally ~4 times more massive
than they are now
 ~5% of the halo were in GCs (after the
formation of SG stars)
Chemical evolution in the Universe
The mass of the FG
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However, mass involved in the process of
formation of GCs >> GCs mass after formation
of the SG
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Bulk of the SG stars formed from the ejecta of
only a fraction of the FG stars
Chemical evolution in the Universe
The mass of the FG
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With simple, realistic assumptions on:
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IMFs of FG and SG stars
initial-final mass relation
on suitable mass ranges for the most favourite candidate polluters
(Carretta et al. 2010)
 m(FG)/m(after formation of SG stars) ~5-10
 m(FG) ~25-50% of the halo mass
Even more if part of the mass lost by the polluters was not used to
form SG stars
 The galactic halo might be made of the FG
Chemical evolution in the Universe
Tests
Metallicity
distribution
Filled circles = field
stars (Ivezic et al.)
Red solid lines =
generalised
histograms for GCs
Chemistry
Black dots = field
stars (Venn et al.)
Red solid lines =
range for GCs
Density
Distribution
BHB LFs
Solid line = best fit
through data
Dotted line = slope
of the inner halo
density (Juric et al.
2008).
GC (Piotto et al.
2003) :solid
histogram
Field (Brown et al.
2008: red dashed
line).
(Also binaries: Lucatello et al. 2013, in preparation)
Chemical evolution in the Universe
An upper limit from Fornax GCs
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We may set an upper limit to the original mass
of GC by counting stars in GCs and the field in
a dSph with a high specific frequency of GCs
(Carretta et al. 2010)
Fornax DSph has 5 GCs
They show a Na-O anticorrelation (signature of
multiple generations: Latarte et al. 2006)
Our original estimate: an upper limit of ~15
Chemical evolution in the Universe
Larsen et al. 2012, A&A, 544, L14
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Cluster [Fe/H]
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Field star [Fe/H]
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Larsen et al. (2012 A&A, 546, 53)
Letarte et al. (2006, A&A, 453, 547)
Two sets consistent within 0.1 dex
Battaglia et al. (2006), based on Rutledge et al. (1997)
calibration of the Ca triplet on the Carretta & Gratton (1997)
scale
Corrections for distribution within the galaxy
Conclusion: there are only ~4-5 metal-poor Fornax
field stars for every GC star
Chemical evolution in the Universe
Larsen et al. 2013
Chemical evolution in the Universe
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How many GCs should be considered?
F4 is more metal-rich than the four other clusters
 F1 (smaller cluster ) is consistent with pure FG
(D’Antona et al. 2013, MNRAS, 434, 1138)
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Are Fornax GCs identical to those of our
galaxy? Possibly they keep more FG stars…
D’Antona et al. (2013): F2, F3 and F5 contain
substantial fractions of SG stars (0.54 – 0.65)
 However, only 2 out of 9 stars observed by Latarte
et al. are Na-rich stars  SG: probability that this
happens by chance is only 2.5% if the fraction of SG
stars is 0.6
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Chemical evolution in the Universe
Are the metallicity scales used for
field and GCs consistent?
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M15:
Latarte et al.: [Fe/H]= -2.40+/-0.03
 Rutledge et al.: [Fe/H]=-2.02+/-0.04
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According to our last scale (Carretta et al. 2009,
A&A, 508, 695): [Fe/H]=-2.33+/-0.02, close to
(Latarte et al. 2006)
However, offset of 0.3-0.4 dex  In Battaglia et
al sample, there are ~35 stars with [Fe/H]<-2
and 47 more with -2<[Fe/H]<-1.7
Chemical evolution in the Universe
A new upper limit
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The value of 4-5 given by Larsen et al. should be
corrected for:
The fraction of SG stars might be lower in Fornax
GCs than in typical halo GCs. Let us assume ½
rather than 2/3 (~1.3)
 The metallicity scales for GCs and field stars are
inconsistent (~2.3)
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The product of these factors is ~3
A ratio of ~15 is consistent with GC formation
scenario derived from multiple populations
Chemical evolution in the Universe
The importance of Larsen et al. paper
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Usually cluster mass function
assumed to be represented by
a Press-Schechter relation
See e.g. M82 clusters from
Bastian et al. (2013, MNRAS,
419, 2606)
Is this appropriate for the
case of the Fornax dSph? 
Clear excess of massive
clusters!
Chemical evolution in the Universe
Conclusions
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GCs are complex objects hosting multiple
generation of stars
They are the end result of the evolution of
originally much more massive objects, possibly
the inner halo counterpart of dSph
Currently, ~1.2% of halo stars are in GCs, but
likely ~5% of halo stars formed within GCs
Much more might have formed in the episodes
that ultimately formed GCs
Chemical evolution in the Universe
The case of Sagittarius
Filled circles: Sgr GC’s
Open circles: Sgr field (McWilliam & S,ecker-Hane)
Open squares: Sgr field (Sbordone et al.)
Filled squares: Fornax GC’s (Letarte et al.)
Mottini & Wallerstein, 2008, AJ 136, 731
Heavy s-elements
Filled circles: Sgr GC’s
Open circles: Sgr field (McWilliam & S,ecker-Hane)
Open squares: Sgr field (Sbordone et al.)
Filled squares: Fornax GC’s (Letarte et al.)
Mottini & Wallerstein, 2008, AJ 136, 731
Binaries
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Wide binaries may be destroyed in very dense fields
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Within the galactic disk, binaries are more common in
low density environments (like Taurus) rather than in
higher density ones (like Orion) (Lada & Lada 2003)
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The binary frequency and the maximum separation
may then be used to estimate the density of the birth
environment (Goodwin, 2010)
Chemical evolution in the Universe
Field
Frequency of spectroscopic binaries with periods less than
6000 days (Carney et al. 2003):
 Field metal poor red giants: 16±4%
 Field metal-poor dwarfs: 17±2%
 These values are similar to those obtained for
population I stars, if we restrict ourselves to the same
period range, since only a third of the binaries in the
solar neighbourhood have a period shorter than 6000 d
(Duquennoy and Mayor 1991)
Chemical evolution in the Universe
Globular Clusters
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Binary fraction in GCs is much lower (Romani & Weinberg
1991)
From c-m diagrams (Milone et al. 2008): only a few per cent of
stars in GCs are binaries (here, the total fraction of binaries,
irrespective of their periods)
The binary fraction is a strong function of the cluster mass,
from 2% for the most massive GCs, up to 20% for the less
massive ones
The mass weighted average value is at most 1/3 than for field
halo stars
Not surprising considering higher density in GCs
The fraction of wide binaries among halo stars indicates that
they did not originate in GCs which subsequently disintegrated
(Ryan 1992)
Chemical evolution in the Universe
Binaries in FG and SG stars
The incidence of binaries seems to be very different
among FG and SG stars (see D’Orazi et al. 2010 and
Lucatello et al. 2010):
 SG: 1%
 FG: ~15% ~ field metal-poor stars
Consistent with that obtained by Milone et al. (2008), if:
 No long period binaries in GCs (reasonable)
 FG made up only 1/3 of the GC population.
Chemical evolution in the Universe
Consequences
The difference between the frequency of binaries in FG and SG is
dramatic:
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most of the destruction of binaries (or lack of formation) among
SG stars occurred before relaxation of the system, because later
we do not expect large differences in the destruction rates
between FG and SG
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only the most compact binaries have a significant probability to
form or survive in the very dense regions where SG stars of GC
formed
Chemical evolution in the Universe
GCs vs field: light and -elements
Chemical Evolution of Dwarf Galaxies
and Stellar Clusters - Garching, July 2125, 2008
GCs vs field: Fe-peak elements
GCs vs field: n-capture elements
• GCs have a composition similar to that of the galactic
halo, save for the O-Na anticorrelation (see last part of the
talk)
• The P-population in GCs have a composition virtually
identical to that of field stars