Barium and Sodium Abundances in the Very Metal-Poor Globular Clusters M15 and M92

Details Barium and Sodium Abundances in the Very Metal-Poor Globular Clusters M15 and M92 Barium and Sodium Abundances in the Very Metal-Poor Globular Clusters M15 and M92

Dec 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...193.6704p&link_type=abstract

American Astronomical Society, 193rd AAS Meeting, #67.04; Bulletin of the American Astronomical Society, Vol. 30, p.1345

Astronomy and Astrophysics

Astronomy

Scientific paper

We have surveyed upper giant branch members of Galactic globular clusters M15 and M92 with the WIYN 3.5m telescope and Hydra multi-object fiber-fed spectrometer. Medium-resolution spectra (Requiv lambda /delta lambda ~ 11000) in the wavelength range 5740--5980 Angstroms were obtained for 31 stars in M15 and 34 stars in M92. The stellar samples range over the top three magnitudes of the cluster color-magnitude diagrams; the faintest of the stars are at or below the luminosity of the horizontal branch. We have derived Ca, Ba, and Na abundances for all stars from synthetic spectrum analyses of the Ca I 5857.4 Angstroms, the Ba II 5853.7 Angstroms, and the Na I D 5889.9, 5895.9 Angstroms lines. The large negative radial velocities of these clusters shift the stellar Na D lines completely away from contaminating, possibly variable ISM Na D lines. Taking Ca as a surrogate element for the Fe-metallicity, we find <[Ca/H]> in the M15 sample to be about 0.15 dex larger than in M92, consistent with earlier suggestions (e.g. Sneden et al. 1991, AJ, 102, 2001) of a small overall metallicity difference between these two clusters. The <[Ba/Ca]> ratios are --0.1 and --0.4 for M15 and M92, also in agreement with an earlier suggestion of a neutron-capture element difference that was based on data for very few stars in these clusters (Armosky et al. 1994, AJ, 108, 1364). The mean ratios <[Na/Ca]> ~ +0.1 in both clusters, but exhibit large star-to-star scatter (sigma ~ 0.4) at all giant branch positions in both clusters. This large scatter confirms the existence of large proton-capture abundance variations down to stellar luminosities comparable to the horizontal branch.

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