Chemical Composition in the Globular Cluster M71 from Keck HIRES Spectra of Turnoff Stars

We have made observations with the Keck I telescope and HIRES at a resolutionof $\sim$45,000 of five nearly identical stars at the turn-off of themetal-rich globular cluster M 71. Our mean Fe abundance, [Fe/H]=-0.80 +-0.02,is in excellent agreement with previous cluster determinations from both giantsand near-turnoff stars. There is no clear evidence for any star-to-starabundance differences or correlations in our sample. Abundance ratios of theFe-peak elements (Cr, Ni) are similar to Fe. The turn-off stars in M71 haveremarkably consistent enhancements of 0.2 - 0.3 dex in [Si/Fe], [Ca/Fe] and[Ti/Fe] -- like the red giants. Our [Mg/Fe] ratio is somewhat lower than thatsuggested by other studies. We compare our mean abundances for the five M 71stars with field stars of similar [Fe/H] -- 8 with halo kinematics and 17 withdisk kinematics. The abundances of the alpha-fusion products (Mg, Si, Ca, Ti)agree with both samples, but seem a closer match to the disk stars. The Mgabundance in M71 is at the lower edge of the disk and halo samples. Theneutron-capture elements, Y and Ba, are enhanced relative to solar in the M71turn-off stars. Our ratio [Ba/Fe] is similar to that of the halo field starsbut a factor of two above that for the disk field stars. The important [Ba/Y]ratio is significantly lower than M71 giant values. The Na content of the M71turn-off stars is remarkably similar to that in the disk field stars, but morethan a factor of two higher than the halo field star sample. We find [Na/Fe] =+0.14 $\pm$0.04 with a spread less than half of that found in the red giants inM71. Excluding Mg, the lack of intracluster $\alpha$-element variations(turn-off vs giants) suggests the polluting material arose in a moretraditional $s$-process environment such as AGB stars.Comment: Accepted for Ap.J. August 20, 2005 issue 25 pages, 9 figures, 9 table