The high-altitude, inner-disc, and chemically peculiar open cluster UBC 1052

Abstract

Out of all the discovered open clusters (OCs) that are located in the inner part of the Galaxy, only a small fraction has been observed with high-resolution spectroscopy. An intriguing population of inner-disc OCs at relatively high altitudes ($Z$) from the Galactic plane remains poorly studied. There are few reliable detections of such OCs, and their occurrence rate, dynamical origin and survival mechanism remain uncertain. We perform a detailed spectroscopic analysis of UBC 1052, located at a cylindrical galactocentric radius $R_{GC}=6.1$ kpc and $Z=340$ pc, which stands out as the oldest and highest-|$Z$| inner-disc OC studied at high resolution to date. We used FLAMES/VLT to acquire high signal-to-noise ratio UVES spectra of four red clump (RC) members ($G\sim 14$ mag). From them we derived high-precision radial velocities ($v_r$) and local thermodynamic equilibrium chemical abundances for 23 elements through a strict line-by-line differential analysis, achieving a median precision in [X/H] of $\simeq 0.06$ dex for each star. The four RC stars have fully compatible chemical abundances, with [X/H] dispersions among them <$0.03$ dex for 20 elements. We also acquired GIRAFFE spectra of other candidate members and derived their $v_r$. We find that UBC 1052 has an age of $2.25\pm 0.25$ Gyr, a distance of $3.11\pm 0.07$ kpc, an extinction $A_V=1.23$ mag, a mean radial velocity of $34.0\pm 0.6$ $km$ $s^{-1}$, and a slightly super-solar [Fe/H] = $0.05\pm 0.01$ dex. Such relatively low [Fe/H] at its $R_{GC}$ suggests that UBC 1052 is a rare candidate for an inward-migrated OC in the inner disc. Its detailed abundance pattern (e.g. [Ba/Zr] and [Nd/Y]) shows some interesting features that appear to be unique in the current census of OCs studied at high resolution, making it an interesting object for potential strong chemical-tagging searches for already dispersed member stars. [Abridged]

23 pages (15 pages main text, 8 pages appendix) and 15 figures. Accepted for publication in Astronomy & Astrophysics