Abundances in Stars from the Red Giant Branch Tip to Near the Main-Sequence Turnoff in M71. III. Abundance Ratios

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Abstract

We present abundance ratios for 23 elements with respect to Fe in a sample of stars with a wide range in luminosity, from luminous giants to stars near the turnoff in a globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more luminous than the red horizontal branch (RHB), three HB stars, nine giant stars less luminous than the RHB, and three stars near the turnoff. The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R = λ/Δλ = 35,000). We find that the neutron capture, the iron peak, and the α-element abundance ratios show no trend with T_(eff) and low scatter around the mean between the top of the RGB and near the main-sequence turnoff. The α-elements Mg, Ca, Si, and Ti are overabundant relative to Fe. The anticorrelation between O and Na abundances observed in other metal-poor globular clusters is detected in our sample and extends to the main sequence. A statistically significant correlation between Al and Na abundances is observed among the M71 stars in our sample, extending to M_V = +1.8, fainter than the luminosity of the RGB bump in M5. Lithium is varying, as expected, and Zr may be varying from star to star as well. M71 appears to have abundance ratios very similar to M5, whose bright giants were studied by Ivans et al., but seems to have a smaller amplitude of star-to-star variations at a given luminosity, as might be expected from its higher metallicity. Neither extremely O-poor, Na-rich stars nor extremely O-rich, Na-poor, stars such as are observed in M5 and in M13, are present in our sample of M71 stars. The results of our abundance analysis of 25 stars in M71 provide sufficient evidence of abundance variations at unexpectedly low luminosities to rule out the mixing scenario. Either alone or, even more powerfully, combined with other recent studies of C and N abundances in M71 stars, the existence of such abundance variations cannot be reproduced within the context of our current understanding of stellar evolution.

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