Abundances in a Large Sample of Stars in M3 and M13

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Abstract

We have carried out a detailed abundance analysis for 21 elements in a sample of 25 stars with a wide range in luminosity from luminous giants to stars near the main-sequence turnoff in the globular cluster M13 ([Fe/H] = -1.50 dex) and in a sample of 13 stars distributed from the tip to the base of the red giant branch (RGB) in the globular cluster M3 ([Fe/H] = -1.39 dex). The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R = λ/Δλ = 35,000). Most elements, including Fe, show no trend with T_(eff) and low scatter around the mean between the top of the RGB and near the main-sequence turnoff, suggesting that at this metallicity, non-LTE effects and gravitationally induced heavy-element diffusion are not important for this set of elements over the range of stellar parameters spanned by our sample. We have detected an anticorrelation between O and Na abundances, observed previously among the most luminous RGB stars in both of these clusters, in both M3 and in M13 over the full range of luminosity of our samples, i.e., in the case of M13 to near the main-sequence turnoff. M13 shows a larger range in both O and Na abundance than does M3 at all luminosities, in particular having a few stars at its RGB tip with unusually strongly depleted O. We detect a correlation between Mg abundance and O abundance among the stars in the M13 sample. We also find a decrease in the mean Mg abundance as one moves toward lower luminosity, which we tentatively suggest is due to our ignoring non-LTE effects in Mg. Although CN burning must be occurring in both M3 and in M13, and ON burning is required for M13, we combine our new O abundances with published C and N abundances to confirm with quite high precision that the sum of C+N+O is constant near the tip of the giant branch, and we extend this down to the bump in the luminosity function. The same holds true for a smaller sample in M3, with somewhat larger variance. Star I-5 in M13 has large excesses of Y and of Ba, with no strong enhancement of Eu, suggesting that an s-process event contributed to its heavy-element abundances. The mean abundance ratios for M3 and for M13 are identical to within the errors. They show the typical pattern for metal-poor globular clusters of scatter among the light elements, with the odd atomic number elements appearing in the mean enhanced. The Fe-peak elements, where the odd atomic number elements are excessively depleted, do not show any detectable star-to-star variations in either cluster. The abundance ratios for 13 Galactic globular clusters with recent detailed abundance analyses, obtained by combining our samples with published data, are compared with those of published large surveys of metal-poor halo field stars. For most elements, the agreement is very good, suggesting a common chemical history for the halo field and cluster stars.

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