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

TL;DR

This study uses APOGEE H-band spectra to analyze chemical abundances of iron, oxygen, and sodium in red giants of the old, metal-rich open cluster NGC 6791, confirming its high metallicity and homogeneous element distribution.

Contribution

First detailed high-resolution infrared spectroscopic analysis of NGC 6791's red giants, confirming homogeneity and refining sodium abundance measurements with non-LTE calculations.

Findings

NGC 6791 is among the most metal-rich clusters in the Galaxy.

Elemental abundances are homogeneous among the studied giants.

No evidence of multiple populations with different Na abundances.

Abstract

The open cluster NGC 6791 is among the oldest, most massive and metal-rich open clusters in the Galaxy. High-resolution $H$-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) of 11 red giants in NGC 6791 are analyzed for their chemical abundances of iron, oxygen, and sodium. The abundances of these three elements are found to be homogeneous (with abundance dispersions at the level of $\sim$ 0.05 - 0.07 dex) in these cluster red giants, which span much of the red-giant branch (T$_{\rm eff}$ $\sim$ 3500K - 4600K), and include two red-clump giants. From the infrared spectra, this cluster is confirmed to be among the most metal-rich clusters in the Galaxy ($<$[Fe/H]$>$ = 0.34 $\pm$ 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 spectral region ($\lambda$16373.86\AA\ and $\lambda$16388.85\AA) indicate only small departures from LTE ($\leq$ 0.04 dex) for the parameter range and metallicity of the studied stars. The previously reported double population of cluster members with different Na abundances is not found among the studied sample.