Transcript Observational evidence of multiple stellar populations in

Observational evidence of multiple
Stellar Populations in
Galactic Globular Clusters
Giampaolo Piotto
Dipartimento di Astronomia
Universita’ di Padova
Collaborators: L.R. Bedin, I.R. King, J. Anderson,
S. Cassisi, S. Villanova, A. Milone,
A. Bellini, Y. Momany, A Renzini, and
A. Sarajedini
+ the HST GC Tresaury Project team
Globular clusters
are the ideal laboratory
for the study of stellar
population and stellar
evolution
Indeed, normal hydrogen
burning stars, in the stellar
core or in a shell typically
behave as canonical stellar
evolution models predict.
And we have CMDs
which are a clear evidence
that globular clusters are
typically populated by stars
with homogeneous
composition and born at the
same time.
However, we do have a number of problems which have been
there, unsolved, for too many years. For example, we never really
understood the general behaviour of He core burning sequences.
The classical second parameter problem, i.e. the fact that GCs
with the same metallicity have horizontal branches with quite
different morphologies still lacks a comprehensive explanation.
Ferraro et al. 1997, ApJ, 484, L145
…and some HBs are surely more complicate to understand than others.
GAPS
JUMPS
NGC 2808
EXTENDED
HOT BLUE
TAILS
Momany et al. (2004)
May be they are telling us that GC stellar population is not as
simple as we thought.
NGC2808
We do have another long standing
problem, i.e. the large spread
in abundances for some elements, like
C,N,O, Na, Mg, Al, s-process elements
inside the same cluster (see
Gratton et al. 2003, ARAA, for a
comprehensive discussion), even
in clusters which do not have
any dispersion in [Fe/H] and Fe peak
elements
Some of these abundance spreads
are present also at the level of
main sequence and subgiant branch
stars, which gives strong support to
the idea that they could be primordial.
(from Carretta et al. 2006, A&A, 450, 523)
• RGB stars
• unevolved stars
• NGC 2808
Some of these
anomalies have
a well defined pattern
like the
NaO anticorrelation, or
the MgAl
anticorrelation.
Both anticorrelations
indicate the presence of
proton capture processes,
which transform Ne into Na,
and Mg into Al.
These processes are possible
only at temperatures of a
few 10 million degrees, in the
complete CNO cycle (which
implies also an O depletion)
(from Carretta et al. 2006)
not reached in present day
Are the HB anomalies and the chemical globular cluster main
sequence and red giant stars.
anomalies related with each other?
Let’s start with
my favourite
“special” case:
Omega Centauri
Omega Centauri
Multiple RGBs
Lee et al. 1999
Pancino et al. 2000
Extended HB
Multiple MSs
Bedin et al. (2004)
Most massive
Galactic
“globular cluster”
(present day mass:
~4 million solar
masses).
Well known
(since the ’70s)
spread in
metallicity
among RGB
stars.
The main sequence
of Omega Centauri
is splitted into two
“main” main sequences
(Anderson, 1997,
PhD thesis, Bedin et
al. 2004, ApJ, 605,
L125).
This is the first
direct evidence
ever found
of multiple stellar
populations in
globular clusters.
Indeed, also
a third main
sequence is
clearly visible
Villanova, Piotto, Anderson et al. (2006), in prep.
The most
surprising
discovery (Piotto et
al. 2005, ApJ,
621,777) is that
the bluest main
sequence is less
metal poor than
the redder one:
Apparently, only
an overabundance
of helium (Y~0.40)
can reproduce the
observed blue
main sequence, as
anticipated by
Norris (2004), and
Bedin et al. (2004)
The strong He overabundance is
really puzzling, but confirmed by
other observational evidence.
E.g., Castellani et al (2007, ApJ,
663, 1021) provide further support
to the He enhancement scenario
from the comparison of the star
counts on the MS, RGB, and HB,
and theoretical models
Castellani et al. found that only a
mix of 70% of canonical He
content (Y=0.23) stars plus a 30%
of He enhanced (Y=0.33, 0.42)
stars can reproduce the observed
ratio of RGB/MS stars.
The same mixture of canonical
and He enhanced stars reduces
the discrepancy between the
predicted and observed ratio of
HB/MS stars, though the observed
ratio is still 15-25% higher than
expected.
The radial distribution of MS stars:
We performed a careful analysis of the radial distribution
of the bMS and rMS stars, complementing the work by
Sollima et al. (2006, ApJ, 654, 915), using HST and
FORS/VLT data (Bellini et al., in preparation).
We find that he ratio of bMS/rMS stars is
constant in the inner 6-7 arcmin (~1.5 half
mass radii), then it constantly decreases.
Castellani et al. (2007) found that
the ratio of extremely hot HB (EHB)
stars/hot HB stars is constant in the
inner 7-8 arcmin, then it decreases.
By itself, this may be another observational evidence
that bMS stars are related to the EHB stars in ωCen.
Is this radial distribution primordial or due to
dynamical relaxation? Note that log(trh)=10 Gyr.
The multiple population
scenario in Omega Centauri
is even more complex than
what expected from the already
puzzling multiple MS.
There are at least 4 distinct
populations, plus other more
spreaded stars (Villanova et
al. 2007, ApJ, 663, 296)
Stars at a given
metallicity have a large
magnitude spread at the
level of the SGB
(>0.1 magnitudes).
This is a clear
indication of an
age spread.
The size of age
dispersion depends on
the assumption on the
metal and He content of
the different SGBs.
A detailed analysis of
the metallicity pattern
along the SGB is
ongoing.
ω Cen is becoming a
really challenging
object…
But is it a unique
case?
The triple main sequence in NGC 2808
Data from GO10922, PI Piotto
TO
Piotto et al. 2007, ApJ, 661, L35
Accurate HST’s ACS
photometry shows that
the MS of NGC 2808
splits in three
separate branches
Overabundances of
helium (Y~0.30, Y~0.40)
can reproduce the two
bluest main sequences.
We tentatively attribute
the three branches to
successive round of star
formation with different
helium content.
The TO-SGB regions are
so narrow that any
difference in age between
the three groups must be
significantly smaller than
1 Gyr
A clear NaO anticorrelation has been
identified by Carretta et al. (2006, A&A, 450,
523) in NGC 2808.
Besides a bulk of O-normal stars with the
typical composition of field halo stars,
NGC2808 seems to host two other groups of
O-poor and super O-poor stars
NGC2808 has a very
complex and very
extended HB (as ω Cen).
The distribution of stars
along the HB is
multimodal, with at least
three significant gaps
and four HB groups
(Sosin et al 1997, Bedin et
al 2000)
A MS broadening in
NGC2808 was
already seen by
D’Antona et al.
(2005).
D’Antona et al.
(2005) linked the
MS broadening to
the HB morphology,
and proposed that
three stellar
populations, with
three different He
enhancements,
could reproduce
the complicate HB.
D’Antona et al. 2005, ApJ, 631, 868
We found them in
the form of three
main sequences!!!
In summary, in NGC 2808,
it is tempting to link together:
the multiple MS,
the multiple HB,
and the three oxygen groups,
as indicated in the table below.
NGC 2808 represents the
second, direct evidence of
multiple stellar populations
in a globular cluster.
And Three! The Double Subgiant Branch
of NGC 1851
Milone et al. 2008, ApJ, in press, arXiv:0709.3762M
Accurate HST’s ACS
photometry reveals that
the SGB of NGC 1851
splits into two well
defined branches
The split may be due to a
large age spread (1 Gyr) or to
a combination of abundance
anomalies and a much
smaller age spread
45%
63%
37%
45% of the stars are
in the lower SGB;
37% in the blue HB.
NGC1851 is known as a prototype of bimodal HB globular
clusters.
Are faint (older) SGB stars related to the blue HB?
Is NGC 1851 case related to the cases of NGC 2808 and ωCen?
Apparently there
is no large He
spread among
the MS stars.
A first quick
reduction of new
HST data
(GO11233,
collected at the
end of
November) sets
an upper limit to
the He spread in
NGC 1851 of
Delta Y ~ 0.03
(work in progress)
Very recently, Cassisi et al. (2007,
ApJ, 672, 115) showed that the two
SGBs and the double HB can be
reproduced by assuming that the
fainter SGB is populated by a strongly
CNNa enhanced population, which
evolve into the blue HB, while the
brighter SGB contains normal
composition stars. The age difference
between the two groups may be very
small (107-108 years).
This idea is supported also by the
recent finding by Yong and Grundahl
2007, arXiv:0711.3823)) of two
groups of stars, one with normal
composition, and one strongly
CN, Na, s-process element enhanced,
O depleted.
In conclusion, the SGB split may be
mainly due to the presence of two
groups of stars, with two different
metal patterns, small age difference.
…continuing…
Multiple Stellar Populations in Globular Clusters. IV.
NGC 6388!
Piotto et al. (2007, in preparation): GC ACS/HST Tresaury data
NGC 6388 double SGB
Present in many,
independent databases,
and visible in different
photometric bands.
NGC 6388, as its twin NGC 6441, are
two, very peculiar globular clusters.
[Fe/H]=-0.6
Since Rich et al. (1997, ApJ, 484,
L25) it is known that NGC 6388
and NGC 6441 have an anomalous
HB. The HB of these two clusters
is very different from the HB of
NGC 1851, but similar to the HB
of ωCen and NGC 2808
[Fe/H]=-0.5
The HB is anomalous because of:
1) The blueward extension, with
the presence of an EHB;
2) The presence of a tilt
Also, the RR Lyr (in NGC6441)
tells that the HB is anomalously
bright
NGC 6441
In order to reproduce the
anomalous HB, Caloi and
D’Antona (2007) propose an
even more complicate
scenario with 3 distinct
populations:
1. a normal population
(Y~0.25);
2. a polluted pop.
(0.27<Y<0.33);
3. A strongly He
enhanced pop.
(Y~0.4)
Caloi and D’Antona, 2007, A&A, 463, 949
Three He populations
in NGC 6388 and
NGC 6441, as in
NGC 2808 and
perhaps ωCen?
NGC 6441
Gratton et al. (2007), A&A, 464, 953
Also NGC 6388 shows a NaO and a MgAl
anticorrelation.
(Carretta et al. 2007, A&A, 464, 967)
Interestingly enough,
NGC6441 shows clear
evidence of the NaO
anticorrelation,
and the [O/Na]
distribution roughly
resembles the HB
distribution.
So far, we have identified four massive globular clusters for which we have a direct
evidence of multiple stellar populations, and they are all quite different:
1) In Omega Centauri (~4x106 solar masses), the different populations manifest
themselves both in a MS split (interpreted as a split in He and metallicity
abundances) and in a SGB split (interpreted in terms of He, metallicity, and age
variations > 1Gyr) which implies at least four different stellar groups within the
same cluster. Omega Centauri has also a very extended HB (EHB), as NGC 2808.
2) In NGC 2808 (~1.6x106 solar masses), the multiple generation of stars is inferred
from the presence of three MSs (also in this case interpreted in terms of three
groups of stars with different He content), possibly linked to three stellar groups
with different oxygen abundances, and possibly to the multiple HB. Age
difference between the 3 groups must be significantly <1 Gyr. It has an EHB.
3) In NGC 6388 (~1.6x106 solar masses) we have evidence of two stellar groups
from a SGB split (age difference ~1Gyr?). An EHB as in NGC 2808 suggests He
enhancement. No information on the MS, yet. NGC6441 may be an analogous case.
4) ) In the case of NGC1851 (~1.0x106 solar masses), we have evidence of two stellar
groups from the SGB split, which apparently imply two star formation episodes
separated by ~1Gyr. No evidence of MS split, yet. Bimodal HB, but no EHB.
The investigation continues.
38 HST orbits allocated in Cycle 16 (GO 11233, PI Piotto) for the
Search of multiple MSs
Stay tuned……
Relevant exception
to the presence of
a double MS in
massive clusters:
47Tuc.
It is at least as
massive as
NGC6388 and
NGC2808 but it
does have neither
an evident double
main sequence nor
an anomalously hot
HB.
(data from HST GO10775)
Proposed scenario (1)
Ejecta (10-20 km/s) from
intermediate mass AGB
stars (4-6 solar masses)
could produce the observed
abundance spread (D’Antona et al
(2002, A&A, 395, 69). These
ejecta must also be He, Na, CN,
Mg) rich, and could explain the
NaO and MgAl anticorrelations,
the CN anomalies, and the He
enhancement.
Globular cluster stars with He
enhancement could help
explaining the anomalous
multiple MSs, and the extended
horizontal branches.
Alternative explanation (2)
Pollution from fast rotating
massive stars (Decressin et al.
2007, A&A, 475, 859).
The material ejected in the disk has two
important properties:
1) It is rich in CNO cycle products,
transported to the surface by the
rotational mixing, and therefore it can
explain the abundance anomalies;
2) It is released into the circumstellar
environment with a very low velocity,
and therefore it can be easily retained
by the shallow potential well of the
globular clusters.
Open problems
Both proposed scenarios have a number of problems.
Among them:
1) Both scenarios need either an anomalously flat (topheavy) IMF or to assume that a large fraction of the
original cluster population has left the cluster (e.g.,
because of the evaporation).
2) There are serious dynamical problems: how is the gas
retained? How is the gas re-accumulated? How is the
second epoch star formation event triggered?
3) Is (part of) the ejected material He-rich enough to
explain the strongly He-enhancement populations?
The case of M54
Multiple RGB
Multiple SGB
Multiple MS?
But…
Who is who?
Data from the ACS Tresaury and from GO10922,
Piotto et al. (in preparation)
M54 coincides with the nucleus
of the Sagittarius dwarf galaxy
It might be born in the nucleus
or it might be ended into the
nucleus via dynamical friction
(see, e.g., Monaco et al. 2005),
but the important fact is that,
now, M54 is part of the nucleus
of a disaggregating galaxy.
M54
The CMDs of M54
and Omega Centauri are
astonishingly similar!
Omega Centauri
It is very likely that M54 and the
Sagittarius nucleus show us what
Omega Centauri was a few billion
years ago: the center of a dwarf
galaxy, now disrupted by the Galactic
tidal field.
Is this true for all globular clusters?
NaO anticorrelation present also il low mass
globular clusters (M71: 3x104 solar masses)
The interesting case of M4
Mass: 8x104 solar masses
Strong NaO anticorrelation
Two distinct groups of stars
Marino et al., in prep.
CN strong
CN weak
Na rich, O poor
stars are
CN strong
Na poor, O-rich
stars are CN
weak
The two stellar groups are well distinguishable also in the
color-magnitude and two color diagrams:
M4
Marino et al., in prep.
Noticeable: a double generation of stars in a globular
cluster with a present day mass ~2% of the mass of
Omega Centauri!!!
saturation
M4
Note: this CMD is proper motion
cleaned and corrected for diff. redd.
Apparently no
main sequence
split. However, it
needs further
investigation with
the refurbished
ACS camera or
WF3.
Mackey et al. (2007, MNRAS, 379,151)
suggested the presence of two populations
with an age difference of ~300Myr in
the 2Gyr old LMC cluster NGC 1846.
The presence of two populations is
inferred by the presence of two TOs in
the color magnitude diagram of the
cluster.
Are these two populations the consequence
of tidal capture of two clusters, or are
they showing something related to the
multiple MSs identified in Galactic
Globular clusters?
Multiple generations of stars in LMC
clusters was already proposed in the past
(see the case of NGC 1850, Vallenari et al.
1994, A&A, 244, 487)
Conclusions
These new results on the multiple populations would have never
been possible without HST. Thanks to them,we are now looking at
globular cluster (and cluster in general) stellar populations with
new eyes.
De facto, a new era on globular cluster research is started:
1) Many serious problems remain unsolved, and we still have a
rather incoherent picture. The new HST cameras that will be
available after SM4 will play a main role in composing the puzzle.
2) For the first time, we might have the key to solve a number of
problems, like the abundance anomalies and possiby the second
parameter problem (which have been there as a nightmare for
decades), as well as the newly discovered multiple sequences in
the CMD.
3) Finally, we should not forget that what we will learn on the origin
and on the properties of multiple populations in star clusters has a
deep impact on our understanding of the early phases of the
photometric and chemical evolution of galaxies (see Meynet et al. 2007).