The open cluster NGC 2509: Stellar rotation and main sequence turnoff extension from FLAMES spectroscopy

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Abstract
NGC 2509 is a distant (≈2.5 kpc) and little-studied open cluster located in the third Galactic quadrant. It is a moderately old cluster, whose age has not yet been precisely determined. The main-sequence stars in NGC 2509 follow a narrow distribution in the colormagnitude diagram, unlike other clusters of similar age. In addition, its chemical composition has never been investigated. To address these issues and characterize the cluster we performed moderate- and high-resolution spectroscopy with FLAMES@VLT of 132 stars, both dwarfs and giants, which represents a significant fraction (≈73%) of likely members. We provide atmospheric stellar parameters and, for the first time, chemical abundances for 21 species with atomic numbers up to 60. In our analysis we followed two different methodologies, both of which will be used for the incoming WEAVE stellar surveys. We find an average radial velocity for NGC 2509 of 58.6±1.3km s −1 and a mild supersolar metallicity ([Fe/H]≈0.1 dex). This value is slightly higher than expected according to its galactocentric distance, but still compatible with the Galactic gradient. From the lithium content of the dwarfs and the isochrone-fitting method we obtain an age for NGC 2509 of 1.26±0.3Gyr. The reddening across the cluster field is negligible ( A V = 0.25 ± 0.02 mag). The cluster peculiar main sequence turnoff is due to a narrow distribution of the rotational velocities peaking at v sin i ≈ 80 km s −1 , with little dispersion. The chemical pattern of NGC 2509 follows the Galactic trends shown by other open clusters in the Galactic thin disk.

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