Abundances in Stars from the Red Giant Branch Tip to near the Main-Sequence Turnoff in M5

Abstract

We present abundance ratios for 23 elements with respect to Fe in asample of stars with a wide range in luminosity, from luminous giants tostars near the turnoff in a globular cluster. Our sample of 25 stars inM71 includes 10 giant stars more luminous than the red horizontal branch(RHB), three HB stars, nine giant stars less luminous than the RHB, andthree stars near the turnoff. The analyzed spectra, obtained with HIRESat the Keck Observatory, are of high dispersion(R={λ}/{Δ}{λ}=35,000). We find that the neutron capture,the iron peak, and the {α}-element abundance ratios show no trendwith T_{eff} and low scatter around the mean between the top ofthe RGB and near the main-sequence turnoff. The {α}-elements Mg, Ca,Si, and Ti are overabundant relative to Fe. The anticorrelation betweenO and Na abundances observed in other metal-poor globular clusters isdetected in our sample and extends to the main sequence. A statisticallysignificant correlation between Al and Na abundances is observed amongthe M71 stars in our sample, extending to M_{V}=+1.8, fainterthan the luminosity of the RGB bump in M5. Lithium is varying, asexpected, and Zr may be varying from star to star as well. M71 appearsto have abundance ratios very similar to M5, whose bright giants werestudied by Ivans et al., but seems to have a smaller amplitude ofstar-to-star variations at a given luminosity, as might be expected fromits higher metallicity. Neither extremely O-poor, Na-rich stars norextremely 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 abundanceanalysis of 25 stars in M71 provide sufficient evidence of abundancevariations at unexpectedly low luminosities to rule out the mixingscenario. Either alone or, even more powerfully, combined with otherrecent studies of C and N abundances in M71 stars, the existence of suchabundance variations cannot be reproduced within the context of ourcurrent understanding of stellar evolution. Some of the data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California, and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation.