Observational evidence of multiple Stellar Populations in Galactic

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, Ap. J, 484, L 145

…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.

NGC 2808 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 (from Carretta et al. 2006) Are the HB anomalies and the chemical anomalies related with each other? Some of these anomalies have a well defined pattern like the Na. O anticorrelation, or the Mg. Al 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) not reached in present day globular cluster main sequence and red giant stars.

Let’s start with my favourite “special” case: 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 ’ 70 s) spread in metallicity among RGB stars.

The main sequence of Omega Centauri is splitted into two “main” main sequences (Anderson, 1997, Ph. D thesis, Bedin et al. 2004, Ap. J, 605, L 125). 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, Ap. J, 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, Ap. J, 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 b. MS and r. MS stars, complementing the work by Sollima et al. (2006, Ap. J, 654, 915), using HST and FORS/VLT data (Bellini et al. , in preparation).

We find that he ratio of b. MS/r. MS 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 b. MS 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 at least 4 distinct populations, plus other more spreaded stars (Villanova et al. 2007, Ap. J, 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 GO 10922, PI Piotto TO Piotto et al. 2007, Ap. J, 661, L 35 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 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 Na. O 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, NGC 2808 seems to host two other groups of O-poor and super O-poor stars NGC 2808 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 NGC 2808 was already seen by 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, Ap. J, 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, Ap. J, in press, ar. Xiv: 0709. 3762 M 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. NGC 1851 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 (GO 11233, 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, Ap. J, 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, ar. Xiv: 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, Ap. J, 484, L 25) 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 NGC 6441) 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 Na. O and a Mg. Al anticorrelation. (Carretta et al. 2007, A&A, 464, 967) Interestingly enough, NGC 6441 shows clear evidence of the Na. O 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, populations and they are all quite different: 1) In Omega Centauri (~4 x 106 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 > 1 Gyr) 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. 6 x 106 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. 6 x 106 solar masses) we have evidence of two stellar groups from a SGB split (age difference ~1 Gyr? ). An EHB as in NGC 2808 suggests He enhancement. No information on the MS, yet. NGC 6441 may be an analogous case. 4) ) In the case of NGC 1851 (~1. 0 x 106 solar masses), we have evidence of two stellar groups from the SGB split, which apparently imply two star formation episodes separated by ~1 Gyr. 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: 47 Tuc. It is at least as massive as NGC 6388 and NGC 2808 but it does have neither an evident double main sequence nor an anomalously hot HB. (data from HST GO 10775)

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 Na. O and Mg. Al 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 M 54

Multiple RGB Multiple SGB Multiple MS? But… Who is who? Data from the ACS Tresaury and from GO 10922, Piotto et al. (in preparation)

M 54 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, M 54 is part of the nucleus of a disaggregating galaxy.

M 54 The CMDs of M 54 and Omega Centauri are astonishingly similar! Omega Centauri It is very likely that M 54 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?

Na. O anticorrelation present also il low mass globular clusters (M 71: 3 x 104 solar masses)

The interesting case of M 4 Mass: 8 x 104 solar masses Strong Na. O 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: M 4 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 M 4 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 WF 3.

Mackey et al. (2007, MNRAS, 379, 151) suggested the presence of two populations with an age difference of ~300 Myr in the 2 Gyr 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 SM 4 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 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).