Chemical abundances of the young inner-disk open cluster NGC 6705 observed by APOGEE: sodium-rich and not $\alpha$-enhanced

Abstract

Previous results in the literature have found the young inner-disk open cluster NGC 6705 to be mildly $\alpha$-enhanced. We examined this possibility via an independent chemical abundance analysis for 11 red-giant members of NGC 6705. The analysis is based on near-infrared APOGEE spectra and relies on LTE calculations using spherical model atmospheres and radiative transfer. We find a mean cluster metallicity of $[Fe/H]=+0.13\pm 0.04$, indicating that NGC 6705 is metal-rich, as may be expected for a young inner-disk cluster. The mean $\alpha$-element abundance relative to iron is $⟨[\alpha /Fe]⟩=-0.03\pm 0.05$, which is not at odds with expectations from general Galactic abundance trends. NGC 6705 also provides important probes for studying stellar mixing, given its turn-off mass of M$\sim$3.3 M${}_{\odot }$. Its red giants have low ${}^{12}$C abundances ([${}^{12}$C/Fe]=$-$0.16) and enhanced ${}^{14}$N abundances ([${}^{14}$N/Fe]=+0.51), which are key signatures of the first dredge-up on the red giant branch. An additional signature of dredge-up was found in the Na abundances, which are enhanced by [Na/Fe]=+0.29, with a very small non-LTE correction. The ${}^{16}$O and Al abundances are found to be near-solar. All of the derived mixing-sensitive abundances are in agreement with stellar models of approximately 3.3 M${}_{\odot }$ evolving along the red giant branch and onto the red clump. As found in young open clusters with similar metallicities, NGC 6705 exhibits a mild excess in the s-process element cerium, with $[Ce/Fe]=+0.13\pm 0.07$.

17 pages, 10 figures, 6 tables

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