Evidenze osservative di popolazioni stellari multiple in ammassi globulari (galattici)

Giampaolo PiottoDipartimento 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 clustersare the ideal laboratoryfor the study of stellar population and stellar evolution

Indeed, normal hydrogen burning stars, in the stellar core or in a shell typicallybehave as canonical stellar evolution models predict.And we have CMDswhich are a clear evidence that globular clusters aretypically populated by stars with homogeneouscomposition and born at thesame 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

We do have another long standingproblem, i.e. the large spreadin 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), evenin clusters which do not have any dispersion in [Fe/H] and Fe peak elements

Some of these abundance spreadsare present also at the level ofmain sequence and subgiant branchstars, which gives strong support tothe idea that they could be primordial.

NGC2808

(from Carretta et al. 2006, A&A, 450, 523)

• RGB stars

• unevolved stars

• NGC 2808

(from Carretta et al. 2006)

Some of theseanomalies havea well defined patternlike the NaO anticorrelation, orthe 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 thecomplete CNO cycle (whichimplies also an O depletion) not reached in present dayglobular cluster mainsequence and red giant stars.

Are the HB anomalies and the chemical Are the HB anomalies and the chemical anomalies related with each other?anomalies related with each other?

Let’s start withmy favourite“special” case:Omega Centauri

Most massive Galactic“globular cluster”(present day mass: ~4 million solar masses).

Well known(since the ’70s)spread in metallicity among RGB stars.

Multiple RGBsLee et al. 1999Pancino et al. 2000

Multiple MSsBedin et al. (2004)

Extended HB

Omega Centauri

The main sequenceof Omega Centauriis 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 stellarpopulations in globular clusters.

Indeed, alsoa third main sequence isclearly visible

Villanova, Piotto, Anderson et al. (2007, ApJ, 663, 296).

The double main sequence in Omega Centauri

Piotto et al. (2005, ApJ, 621,777)

17x12=204 hours i.t.

RedMS:Rad. Vel.: 235+-11km/s[Fe/H]=-1.56

BlueMS: Rad. Vel.: 232+-6km/s[Fe/H]=-1.27It is more metal rich!

The most surprising discovery (Piotto et al. 2005) 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, asanticipated byNorris (2004), andBedin et al. (2004)

The multiple main sequence of Omega Centauri.

Only cluster stars (proper motion selection)

Bedin et al., in prep

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 modelsCastellani 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 multiple populationscenario in Omega Centauriis even more complex than what expected from the alreadypuzzling multiple MS.

There are at least 4 distinct populations, plus other morespreaded stars (Villanova etal. 2007)

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 anage 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?

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

The triple main sequence in NGC 2808

Piotto et al. 2007, ApJ, 661, L35

TO

A clear NaO anticorrelation has beenA clear NaO anticorrelation has beenidentified by Carretta et al. (2006, A&A, 450, identified by Carretta et al. (2006, A&A, 450, 523) in NGC 2808.523) in NGC 2808.Besides Besides a bulk of O-normal starsa bulk of O-normal stars with the with the typical composition of field halo stars, typical composition of field halo stars, NGC2808 seems to host two other groups of NGC2808 seems to host two other groups of O-poor and super O-poor stars 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)

D’Antona et al. 2005, ApJ, 631, 868

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.

We found them inthe form of threemain sequences!!!

The Double Subgiant Branch of NGC 1851

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

Milone et al. 2008, ApJ, 673, 241

45% of the stars are in the lower SGB; 37% in the blue HB.

63%

37%From Yong and Grundahl (2007) we know that 40% of the stars are CN-strong and s-process element enhanced.

Are the SGB stars related to the blue HB,And to the CN-strong, s-process element enhanced subpopulation??

45%

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. In such a case, the age difference between the two groups may be very small (107-108 years).

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.

Is NGC 1851 case related Is NGC 1851 case related to the cases of NGC 2808 to the cases of NGC 2808 and and ωωCen?Cen?

Apparently there is no large He spread among the MS stars.

A first quick reduction of new HST data from ongoing GO11233 program sets an upper limit to the He spread in NGC 1851 of

Delta Y ~ 0.03

(work in progress)

…continuing…Multiple Stellar Populations in Globular Clusters. IV.

NGC 6388!

Piotto (2008, arXiv0801.3177): GC ACS/HST Tresaury data

NGC 6388 double SGB present in many, independent databases, and visible in different photometric bands. Including IR (see recent results with MAD in the poster by Moretti et al.)

NGC 6388, as its twin NGC 6441, are two, very peculiar globular clusters.

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]=-0.6

[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 tiltAlso, the RR Lyr (in NGC6441)tells that the HB is anomalously bright

Canonical HB models (dashed red)cannot reproducethe observed HB (Raimondo, Castellani,V. et al. 2002, A&A, 569, 975)

Only a HB modelwith strong heliumenhancement (Y=0.40) can reproduce the the observed HB,at all wavelengthsAbout 13% of thestars have such anextreme He enhancement

(Busso et al., 2007, A&A, 474, 105)

NGC 6441

Caloi and D’Antona, 2007, A&A, 463, 949

In order to reproduce the anomalous HB, Caloi and D’Antona (2007) propose an even more complicatescenario with 3 distinctpopulations:

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)

Three He populations in NGC 6388 and NGC 6441, as inNGC 2808 and perhaps ωCen?

So far, we have identified four massive globular clustersfour massive globular clusters for which we have a direct evidence of multiple stellar populationsmultiple 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. 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. No evidence of MS split. Bimodal HB, but no EHB.

Relevant exceptionto the presence ofa double MS in massive clusters: 47Tuc.

It is at least as massive asNGC6388 and NGC2808 but itdoes have neitheran evident double main sequence nor an anomalously hot HB.

(data from HST GO10775)

The investigation continues.

38 HST orbits allocated in Cycle 16 (GO 11233, PI Piotto) for the Search of multiple MSs

So far, no evidence of large MS splits in NGC1851, M13, and M80

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.

Proposed scenario (1)

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 (top-heavy) 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? Y>0.32-0.33 might be a serious problem.

The case of M54

Data from the ACS Tresaury and from GO10922,Piotto et al. (in preparation)

Multiple RGBMultiple SGBMultiple MS?

But…

Who is who?

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, today, M54 is part of the nucleus of a disaggregating galaxy.

M54

Omega Centauri

The CMDs of M54and Omega Centauri areastonishingly similar!

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 massglobular clusters (M71: 3x104 solar masses)

The interesting case of M4

Mass: 8x104 solar massesStrong NaO anticorrelationTwo distinct groups of stars

CN strong

CN weak

Marino et al., in prep.

Na rich, O poorstars areCN strong

Na poor, O-rich stars are CN weak

M4

The two stellar groups are well distinguishable also in the color-magnitude and two color diagrams:

Marino et al., in prep.

A double generation of stars in a globular cluster with a present day mass ~2% of the mass of Omega Centauri?

The distribution of the Na-rich, CN strong stars along the RGB does not depend on the evolutionary state: the split is well visible also below the RGB bump.It must be primordial!

Apparently no main sequence split. However, it needs further investigation with the refurbished ACS camera or WF3.

Note: this CMD is proper motion cleaned and corrected for diff. redd.

M4saturation

Mackey et al. (2007, MNRAS, 379,151)suggested the presence of two populationswith an age difference of ~300Myr inthe 2Gyr old LMC cluster NGC 1846.The presence of two populations is inferred by the presence of two TOs inthe color magnitude diagram of the cluster.Three additional LMC candidates proposed by Mackey et al. (2008, astro-ph 0804.3475).Are these two populations the consequenceof tidal capture of two clusters, or arethey showing something related to themultiple 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)

Work in progressbut…

…there are more!About 15% of the clusters in the sample

LMC cluster

field cluster

200 Myr age difference

See also Mackey et al. (2008)

The LMC clusters are one or more orders of magnitude less massive than Omega Centauri and NGC 2808.

Does the same scenarios proposed for the multiple population in Galactic clusters work also for these objects?

What is the role of the capture of field star scenario proposed by Kroupa (1997)? It might work, at least in the case of LMC clusters.

LMC cluster

ConclusionsThanks to the new results on the multiple populations we are we are now looking at globular cluster (and cluster in general) stellarnow looking at globular cluster (and cluster in general) stellar populations with new eyes. 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.

Despite the fact that the NaO anticorrelation is much better defined than the MgAl relation, the spread in [Mg/Al] increases at increasing the spread of [O/Na]: this is a clear cut evidence that theNeNa and MgAl cycles involved come from the very same source, which cannot be active in present day MS or RGB stars.

In a very interesting paper, Carretta (2006, AJ, 131, 1766) provides a few additional hints for our discussion. He calculated the interquartile range of the various NaO and MgAl anticorrelations, and used it as a quantitative estimate of the extension of the chemical inhomogeneities within a cluster.

The level of chemical inhomogeneitesseems higher for higher mass clusters. Does this indicate a better ability of more massive clusters to retain the polluting ejects?

The level of chemical inhomogeneites is clearly higher for more extended HBs: further evidence that the two phenomenamay be related.

Carretta (2006) found also that the larger the orbit sizes and revolution periods, the larger the amount of inhomogeneities. Larger spreads in [O/Na] and [Mg/Al] distributions are also clearly found for clusters withlarge maximum heights above the Galactic plane and/or with larger inclination angles of the orbit with respect to the plane.

To be further investigated!!!!

NGC 6791 WD cooling sequence has a double peaked LF.

Bedin et al. 2008a, ApJ, in press.

Two peaks are not expected by standard cooling theoryBrighter peak significantly brighter than expected from TO age.Fainter peak still too bright, but some modification to classical cooling theory can account for it)

NGC 6791 has a large fraction of binary.About 35 of the measured stars are in photometric binaries.Total binary fraction >50% (as in M67)

Binaries may be the solution of the puzzle.

34% of WD+WD binaries (implying a total binary fraction of 54% can solve the problem