Chemical abundances of the young inner-disc open cluster NGC 6705 observed by APOGEE: sodium-rich and not α-enhanced
Holtzman, J. A.; Allende Prieto, C.; Bizyaev, D.; Chiappini, C.; Jönsson, H.; Cunha, K.; Majewski, S. R.; Horta, D.; Mészáros, S.; Nidever, D. L.; Beaton, R.; Smith, V. V.; Daflon, S.; Pinsonneault, M.; Loaiza-Tacuri, V.; Souto, D.; Guerço, R.; Sales-Silva, J. V.; Frinchaboy, P.; Hasselquist, S.; Hayes, C. R.; Zasowski, G.
Brazil, United States, France, Germany, Spain, Russia, Canada, Sweden, Hungary
Abstract
Previous results in the literature have found the young inner-disc open cluster NGC 6705 to be mildly α-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 $\rm [Fe/H] = +0.13 \pm 0.04$, indicating that NGC 6705 is metal-rich, as may be expected for a young inner-disc cluster. The mean α-element abundance relative to iron is $\rm \langle [\alpha /Fe]\rangle =-0.03 \pm 0.05$, 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 ~ 3.3 M⊙. Its red giants have low 12C abundances ([12C/Fe] = -0.16) and enhanced 14N abundances ([14N/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 16O 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⊙ 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 $\rm [Ce/Fe] = +0.13\pm 0.07$.