Sodium and Oxygen Abundances in the Open Cluster NGC 6791 from APOGEE H-band Spectroscopy

Cunha, K. and Smith, V. V. and Johnson, J. A. and Bergemann, M. and Mészáros, Szabolcs (2015) Sodium and Oxygen Abundances in the Open Cluster NGC 6791 from APOGEE H-band Spectroscopy. ASTROPHYSICAL JOURNAL LETTERS, 798 (2). ISSN 2041-8205

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Abstract

The open cluster NGC 6791 is among the oldest, most massive and metal-richopen clusters in the Galaxy. High-resolution H-band spectra from the ApachePoint Observatory Galactic Evolution Experiment (APOGEE) of 11 red giantsin NGC 6791 are analyzed for their chemical abundances of iron, oxygen, andsodium. The abundances of these three elements are found to be homogeneous(with abundance dispersions at the level of ∼ 0.05 - 0.07 dex) in these cluster redgiants, which span much of the red-giant branch (Teff ∼ 3500K - 4600K), and in-clude two red-clump giants. From the infrared spectra, this cluster is confirmedto be among the most metal-rich clusters in the Galaxy (<[Fe/H]> = 0.34 ±0.06), and is found to have a roughly solar value of [O/Fe] and slightly enhanced[Na/Fe]. Non-LTE calculations for the studied Na I lines in the APOGEE spec-tral region (λ16373.86˚A and λ16388.85˚A) indicate only small departures fromLTE (≤ 0.04 dex) for the parameter range and metallicity of the studied stars.The previously reported double population of cluster members with different Naabundances is not found among the studied sample.

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