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

Abstract

We have made observations with the Keck I telescope and HIRES at a resolution of ~45,000 of five nearly identical stars at the turnoff of the metal-rich globular cluster M71. We derive stellar parameters and abundances of several elements. Our mean Fe abundance, [Fe/H] = -0.80 ± 0.02, is in excellent agreement with previous cluster determinations from both giants and near-turnoff stars. There is no clear evidence for any star-to-star abundance differences or correlations in our sample. Abundance ratios of the Fe peak elements (Cr, Ni) are similar to Fe. The turnoff stars in M71 have remarkably 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 that suggested by other studies. We compare our mean abundances for the five M71 stars with field stars of similar metallicity [Fe/H]: eight with halo kinematics and 17 with disk kinematics. The abundances of the α-fusion products (Mg, Si, Ca, Ti) agree with both samples but seem a closer match to the disk stars. The Mg abundance in M71 is at the lower edge of the disk and halo samples. The neutron-capture elements, Y and Ba, are enhanced relative to solar in the M71 turnoff stars. Our ratio [Ba/Fe] is similar to that of the halo field stars but a factor of 2 above that for the disk field stars. The important [Ba/Y] ratio is significantly lower than M71 giant values; the precluster material may have been exposed to a higher neutron flux than the disk stars or self-enrichment has occurred subsequent to cluster star formation. The Na content of the M71 turnoff stars is remarkably similar to that in the disk field stars but more than a factor of 2 higher than the halo field star sample. We find [Na/Fe] = +0.14 ± 0.04 with a spread less than half of that found in the red giants in M71. Excluding Mg, the lack of intracluster α-element variations (turnoff vis-a-vis giants) suggests that the polluting material needed to explain the abundance patterns in M71 did not arise from explosive nucleosynthesis but in a more traditional s-process environment such as AGB stars. The determination of light s-peak abundances should reveal whether this pollution occurred before or after cluster formation.