Evidenze osservative di popolazioni stellari multiple in ammassi globulari (galattici)
Giampaolo PiottoDipartimento di AstronomiaUniversita 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 evolutionIndeed, 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
and some HBs are surely more complicate to understand than others.NGC 2808Momany et al. (2004)GAPSJUMPSEXTENDEDHOT BLUETAILSMay be they are telling us that GC stellar population is not as simple as we thought.
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 anomalies related with each other?
Lets start withmy favouritespecial case:Omega Centauri
Most massive Galacticglobular 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. 2000Multiple MSsBedin et al. (2004)Extended HBOmega Centauri
The main sequenceof Omega Centauriis splitted into twomain 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 CentauriPiotto 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 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., 2008, 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 Cen. Is this radial distribution primordial or due to dynamical relaxation? Note that log(trh)=10 Gyr.
Ferraro et al (2006, ApJ, 638, 433) found no evidence of mass segregation amongthe 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 notcompletely relaxed?Are the results on the distribution ofbMS/rMS and hot HB stars contradicting the results on the BSS distribution?Or is the bMS/rMS radial distributiontelling us something about the distribution of the material from which the two stellar populations formed?
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)
Agedifference among the different SGB populations strongly depends on their abundance and abundance ratiosMAD+HST
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. Cen is becoming a really challenging objectBut is it a unique case?
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 smallerage spread (~2Gyr) , but assuming for each population a different He content.
We need a better knowledge of the metallicity and metallicity distribution of the different SGBs, to properlyquantify the age spread. Cen is becoming a really challenging objectBut is it a unique case?
Accurate HSTs 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 2808Piotto et al. 2007, ApJ, 661, L35 TO
A clear NaO anticorrelation has beenidentified 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 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)
DAntona et al. 2005, ApJ, 631, 868A MS broadening in NGC2808 was already seen by DAntona et al. (2005).
DAntona 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!!!
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 ofmultiple stellar populationsin a globular cluster.
The Double Subgiant Branch of NGC 1851
Accurate HSTs 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 spreadMilone 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 t