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Chemical Composition in the Globular Cluster M71 from Keck/HIRES Spectra of Turn-Off Stars | | Ann Merchant Boesgaard
; Jeremy R. King
; Ann Marie Cody
; Alex Stephens
; Constantine P. Deliyannis
; | | Date: |
7 May 2005 | | Journal: | Astrophys.J. 629 (2005) 832-848 | | Subject: | astro-ph | | Abstract: | We have made observations with the Keck I telescope and HIRES at a resolution of $sim$45,000 of five nearly identical stars at the turn-off of the metal-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 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 turn-off 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 M 71 stars with field stars of similar [Fe/H] -- 8 with halo kinematics and 17 with disk 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 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 turn-off stars. Our ratio [Ba/Fe] is similar to that of the halo field stars but 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 M71 turn-off stars is remarkably similar to that in the disk field stars, but more than 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 in M71. Excluding Mg, the lack of intracluster $alpha$-element variations (turn-off vs giants) suggests the polluting material arose in a more traditional $s$-process environment such as AGB stars. | | Source: | arXiv, astro-ph/0505139 | | Services: | Forum | Review | PDF | Favorites | |
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