Multiple Stellar Populations in Globular Clusters Giampaolo Piotto

Transcription

1 Multiple Stellar Populations in 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. Sarajedini + the HST GC Tresaury Project team

2 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.

3 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. Ferraro et al. 1997, ApJ, 484, L145 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.

4 and some HBs are surely more complicate to understand than others. GAPS JUMPS Momany et al. (2004) NGC 2808 EXTENDED HOT BLUE TAILS May be they are telling us that GC stellar population is not as simple as we thought.

5 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)

6 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 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) not reached in present day globular cluster main sequence and red giant stars.

7 Omega Centauri Let s start with my favourite special case: Omega Centauri Extended HB Multiple RGBs Lee et al Pancino et al Most massive Galactic globular cluster (present day mass ~4 million solar masses). Multiple MSs Bedin et al. (2004) Well known (since the 70s) spread in metallicity among RGB stars.

8 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.

9 Indeed, also a third main sequence is clearly visible Villanova, Piotto, Anderson et al. (2006), in prep.

10 Piotto et al. (2005, ApJ, 621,777) The double main sequence in Omega Centauri RedMS: Rad. Vel.: km/s [Fe/H]= x12=204 hours i.t. BlueMS: Rad. Vel.: km/s [Fe/H]=-1.27 It is more metal rich!

11 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)

12 More recently, Castellani et al (2007) provided 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. (2007, ApJ, 663, 1021) 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.

13 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).

14 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(t rh )=10 Gyr.

15 Are the results on the distribution of bms/rms and hot HB stars contradicting the results on the BSS distribution? Or is the bms/rms radial distribution telling us something about the distribution of the material from which the two stellar populations formed? Ferraro et al (2006, ApJ, 638, 433) found no evidence of mass segregation among the blue stragglers in Omega Centauri, at variance with what found in other globular clusters. Is this an evidence that also the center of Omega Centauri is not completely relaxed?

16 There are at least 4 distinct populations, plus other more spreaded stars (Villanova et al. 2007, ApJ, 663, 296) The multiple population scenario in Omega Centauri is even more complex than what expected from the already puzzling multiple MS.

17 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 dispersions depends on the assumption on the metal content of the different SGBs. A detailed analysis of the metallicity pattern along the SGB is ongoing.

18 And indeed we find a large age spread, of a few Gyr, among the five stellar components, but assuming only two He populations. Sollima et al. (2005, Apj, 634, 332) find a smaller age spread (~2Gyr), but assuming for each population a different He content. ω Cen is becoming a really challenging object But is it a unique case? We need a better knowledge of the metallicity and metallicity distribution of the different SGBs, to properly quantify the age spread.

19 The triple main sequence in NGC 2808 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

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

21 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!!!

22 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.

23 And Three! The Double Subgiant Branch of NGC 1851 Milone et al. 2007, ApJ, in press, arxiv: 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

24 45% 63% 45% of the stars are in the lower SGB; 37% in the blue HB. 37% 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?

25 Apparently there is no large He spread among the MS stars. A first quick reduction of new HST data (observed at the end of November) sets an upper limit to the He spread in NGC 1851 of Delta Y = 0.03

26 Very recently, Cassisi et al. (2007, arxiv: ) 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 ( years). This idea is supported also by the recent finding by Yong and Grundahl 2007, arxiv: )) 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.

27 continuing Multiple Stellar Populations in Globular Clusters. IV. NGC 6388! Piotto et al. (2007, in preparation): GC ACS/HST Tresaury data

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

29 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 [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

30 (Busso et al., 2007, A&A, in press, arxiv: ) Canonical HB models (dashed red) cannot reproduce the observed HB (Raimondo, Castellani, V. et al. 2002, A&A, 569, 975) Only a HB model with strong helium enhancement (Y=0.40) can reproduce the the observed HB, at all wavelengths About 13% of the stars have such an extreme He enhancement

31 In this case, only 8% of the stars have an extreme He (Y=0.38) enhancement (Busso et al., 2007, A&A, in press, arxiv: ) Note that this cluster has half the mass of his twin NGC 6388 in which 13% of the stars show extreme He enhancement, And about 1/3 the Mass of Omega Centauri. In the latter, the extreme He populations was ~20% of the MS stars

32 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 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) Three He populations in NGC 6388 and NGC 6441, as in NGC 2808 and perhaps ωcen?

33 Also NGC 6388 shows a NaO and a MgAl anticorrelation. (Carretta et al. 2007, A&A, 464, 967) NGC 6441 Interestingly enough, NGC6441 shows clear evidence of the NaO anticorrelation, and the [O/Na] distribution roughly resembles the HB distribution. Gratton et al. (2007), A&A, 464, 953

34 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 (~4x10 6 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 ) In NGC 2808 (~1.6x10 6 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 (~1.6x10 6 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 (~1.0x10 6 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.

35 Proposed scenarios 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. Globular cluster stars with He enhancement could help explaining the anomalous multiple MSs, and the extended horizontal branches. 2) Alternative explanation: the polluting material come from the wind from fast rotating massive stars (Decressin et al. 2007, A&A, 475, 859)

36 (data from HST GO10775) 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.

37 The investigation continues. 38 HST orbits allocated in Cycle 16 (GO 11233, PI Piotto) for the Search of multiple MSs M54 observed with the WFPC2 (GO 10922, PI Piotto) in June 2007 for the search of multiple MSs. Stay tuned

38 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 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)

39 Marino et al. 2008, in prep. M4 Note also that the NaO anti-correlation is found also in M4, which has only a mass of only 6.3x10 4 solar masses Is the multipopulation phenomenon confined to massive clusters only?

40 Open questions Is the intermediate mass AGB pollution scenario proposed to explain both the chemical inhomogeneities and useful to explain the anomalously hot HBs and the multiple MSs working to explain real clusters? I am still concerned by three facts: 1) This scenario implies a rather peculiar (strongly top-heavy) IMF according to Bekki and Norris (2006) for. normal GCs 2) The gas ejected by intermediate mass AGB stars is hot gas traveling at a speed of a couple of tens of km/s, that needs to quickly (1-2x10 5 yr) concentrate into the cluster core. How could a star formation burst start inside this gas? Is the entire self pollution from AGB star picture plausible from the dynamical point of view? 3) How can we have a stellar population with a He enhancement as high as Y=0.40, implying an astonishing DeltaY/DeltaZ~70? Are all present day massive GCs originated as dwarf galaxies?

41 Additional questions: 1. Why Omega Centauri, whose present mass is of the order of 2-4 times the mass of NGC 1851, NGC 2808, NGC 6388, and NGC 6441 has such a more complicate stellar population? 2. Is the multi-population phenomenon present only in massive GCs? What is the limiting mass for the onset of multipopulations? 3. What is the role of the ejecta from massive rotating stars (Maeder and Meynet 2006, Norris and Chiba 2007, Decressin et al. 2007), and of SnII (Norris 2004, Piotto et al. 2005), proposed as possible polluters? 4. What is the role of field star capture, may be within a now dissolved dwarf galaxy (see the discussion with Pavel, yesterday)? Finally: Are we going in the right direction in the interpretation of the multipopulations?

42 In a very interesting paper, Carretta et al (2006, AJ, 131, 1776) 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. 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 the NeNa and MgAl cycles involved come from the very same source, which cannot be active in present day MS or RGB stars.

43 The level of chemical inhomogeneites is clearly higher for more extended HBs: further evidence that the two phenomena may be related. The level of chemical inhomogeneites seems higher for higher mass clusters. Does this indicate a better ability of more massive clusters to retain the polluting ejects?

44 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 with large maximum heights above the Galactic plane and/or with larger inclination angles of the orbit with respect to the plane. No obvious interpretation (though Eugenio has one!) To be further investigated!!!!

45 Interestingly enough, also the maximum extension of the HB correlates with the cluster mass. Recio-Blanco et al. 2006, A&A, 452, 875 Following the line of thought of a gas cloud polluted by matter processes in a previous generation of stars (which must be He enriched), we expect more massive clusters to be able to retain more ejecta, and therefore create He enriched stars, responsibile of present day anomalously extend HBs.

46

47 NGC6791 is an old, massive, super metal rich [Fe/H]=+0.4 open cluster MS age: 9Gyr WD age < 4Gyr? Bedin et al. 2005, ApJ, 624, L45

48 Bedin et al. 2007, in preparation. NGC 6791 WD cooling sequence has a double peaked LF. Also the second peak is brighter than expected for a WD cooling sequence of a 9 Gyr cluster, but the difference is less dramatic than for the case of the first peak.

49 There is an apparent broadening of the RGB. Maurizio idea in order to explain the brighter WD LF peak: fast rotation --> broadening alla base dell'rgb ---> higher He core mass ---> brighter luminosity of the RGB tip ---> enhanced mass loss ---> loss of the envelope ---> He WD ---> brighter WD LF peak. Fainter peak due to normal CO WDs.