2019MNRAS.485.4375D

Stars in kinematic groups are believed to share the same age, metallicity, and velocity vectors, and are disrupted by events such as encounters with massive objects, leaving a residue of stars sharing the same kinematic properties, thus bridging an important evolutionary gap between clusters and field stars. We determine the most likely metallicity for the unevolved stars of the group, collectively, by assuming that these stars all conform to a single zero-age main sequence line and requiring that residual T_eff and luminosity values be minimized: the [Fe/H] value that best describes the observed HR diagram of the group lies in the range +0.05 < [ Fe/H] < +0.10. In this work, we report the detailed spectroscopic analysis of 23 stars in the Ursa Major Group (UMaG), together with seven young field stars that function as a control sample, based on high S/N, R ∼ 60 000 optical spectra. We present abundances for C, Na, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Fe, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Gd, and Eu. We also found that there is a clear kinematic nucleus in the UVW space, but from the spectroscopic analysis only a fraction of these stars could be considered chemically and chronologically homogeneous. The results suggest that the UMaG has physical existence in a chronochemokinematic sense, but is not chemically peculiar: high Ba abundance seems to be a common feature of young stars. From the initial sample, only 14 (61 per cent) stars were finally considered members of the UMaG, resulting in an average metallicity and standard deviation of [ Fe/H] = 0.01 and 0.07, respectively, consistent with previous works. We finally conclude that even though the UMaG stars share a common kinematics, age, and metallicity, they do not have the same chemical abundance pattern, pointing to a dynamical origin of the Group.