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Register a new userThe influence of the C+N+O abundances on the determination of the relative ages of Globular Clusters: the case of NGC 1851 and NGC 6121 (M4)
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The color magnitude diagram (CMD) of NGC 1851 presents two subgiant branches (SGB), probably due the presence of two populations differing in total CNO content. We test the idea that a difference in total CNO may simulate an age difference when comparing the CMD of clusters to derive relative ages. We compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find that, with a suitable shift of the CMDs that brings the two red horizontal branches at the same magnitude level, the unevolved main sequence and red giant branch match, but the SGB of NGC 6121 and its red giant branch bump are fainter than in NGC 1851. In particular, the SGB of NGC 6121 is even slightly fainter than the the faint SGB in NGC 1851. Both these features can be explained if the total CNO in NGC 6121 is larger than that in NGC 1851, even if the two clusters are coeval. We conclude by warning that different initial C+N+O abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference.
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Among the newly discovered features of multiple stellar populations in Globular Clusters, the cluster NGC 1851 harbours a double subgiant branch, that can be explained in terms of two stellar generations, only slightly differing in age, the younger one having an increased total C+N+O abundance. Thanks to this difference in the chemistry, a fit can be made to the subgiant branches, roughly consistent with the C+N+O abundance variations already discovered two decades ago, and confirmed by recent spectroscopic data. We compute theoretical isochrones for the main sequence turnoff, by adopting four chemical mixtures for the opacities and nuclear reaction rates. The standard mixture has Z=10$^{-3}$ and [$alpha$/Fe]=0.4, the others have C+N+O respectively equal to 2, 3 and 5 times the standard mixture, according to the element abundance distribution described in the text. We compare tracks and isochrones, and show how the results depend on the total CNO abundance. We notice that different initial CNO abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference. We simulate the main sequence and subgiant branch data for NGC 1851 and show that an increase of C+N+O by a factor $sim$3 best reproduces the shift between the subgiant branches. We compare the main sequence width in the color m$_{F336W}$-m$_{F814W}$ with models, and find that the maximum helium abundance compatible with the data is Y$simeq$0.29. We consider the result in the framework of the formation of the second stellar generation in globular clusters, for the bulk of which we estimate a helium abundance of Y$simlt 0.26$.
We present chemical abundance analysis of a sample of 15 red giant branch (RGB) stars of the Globular Cluster NGC~1851 distributed along the two RGBs of the (v, v-y) CMD. We determined abundances for C+N+O, Na, $alpha$, iron-peak, and s-elements. We found that the two RGB populations significantly differ in their light (N,O,Na) and s-element content. On the other hand, they do not show any significant difference in their $alpha$ and iron-peak element content. More importantly, the two RGB populations do not show any significant difference in their total C+N+O content. Our results do not support previous hypotheses suggesting that the origin of the two RGBs and the two subgiant branches of the cluster is related to a different content of either $alpha$ (including Ca) or iron-peak elements, or C+N+O abundance, due to a second generation polluted by SNeII.
Globular clusters display significant variations in their light-element content, pointing to the existence of a second stellar generation formed from the ejecta of an earlier generation. The nature of these internal polluters is still a matter of debate: the two most popular scenarios indicate intermediate-mass AGB stars and fast rotating massive stars. Abundances determination for some key elements can help distinguish between these competitor candidates. We present in this paper Y abundances for a sample of 103 red giant branch stars in NGC 6121. Within measurement errors, we find that the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a subsample of six stars we also find [Rb/Fe] to be constant, consistent with previous studies showing no variation in other s-process elements. We also present a new set of stellar yields for intermediate-mass AGB stellar models of 5 and 6 solar masses, including heavy element s-process abundances. The uncertainties on the mass-loss rate, the mixing-length parameter, and the nuclear reaction rates have a major impact on the stellar abundances. Within the IM-AGB pollution scenario, the constant abundance of heavy elements inside the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a constrain could still be satisfied by the present models assuming a stronger mass-loss rate. The uncertainties mentioned above are limiting the predictive power of intermediate-mass AGB models. For these reasons, at the moment we are not able to clearly rule out their role as main polluters of the second population stars in globular clusters.
Using archival HST/WFPC2 imaging of 7 LMC globular clusters, and following the methods outlined in our previous study, we have reached the tightest constrain so far on their age dispersion, which cannot be greater than ~ 0.5 Gyr. We also confirm earlier results that their average age is comparable to that of the metal-poor Galactic globulars. Evidence is also provided that NGC 1841 is younger than the rest of LMC globulars.
We present the first chemical analysis of stars on the double subgiant branch (SGB) of the globular cluster NGC 1851. We obtained 48 Magellan IMACS spectra of subgiants and fainter stars covering the spectral region between 3650-6750AA, to derive C and N abundances from the spectral features at 4300AA (G-band) and at ~ 3883AA (CN). We added to our sample ~ 45 unvevolved stars previously observed with FORS2 at the VLT. These two datasets were homogeneously reduced and analyzed. We derived abundances of C and N for a total of 64 stars and found considerable star-to-star variations in both [C/H] and [N/H] at all luminosities extending to the red giant branch (RGB) base (V~18.9). These abundances appear to be strongly anticorrelated, as would be expected from the CN-cycle enrichment, but we did not detect any bimodality in the C or N content. We used HST and ground-based photometry to select two groups of faint- and bright-SGB stars from the visual and Stromgren color-magnitude diagrams. Significant variations in the carbon and nitrogen abundances are present among stars of each group, which indicates that each SGB hosts multiple subgenerations of stars. Bright- and faint-SGB stars differ in the total C+N content, where the fainter SGB have about 2.5 times the C+N content of the brighter ones. Coupling our results with literature photometric data and abundance determinations from high-resolution studies, we identify the fainter SGB with the red-RGB population, which also should be richer on average in Ba and other s-process elements, as well as in Na and N, when compared to brighter SGB and the blue-RGB population.
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