Two groups within the thick disc of the Milky Way?


E-mail: (LMS); (DLL)


Compositions of F and G dwarf stars in two groups of thick disc stars are presented. The groups identified by Schuster et al. have mean characteristics ([Fe/H], V(rot), Age, math formula) of (−0.7 dex, 120 km s−1, 12.5 Gyr, 62 km s−1) and (−0.4, 160, 10.0, 45.8). Abundances for 23 elements obtained from high-resolution spectra are presented for 59 stars in the metal-rich group and 27 stars in the metal-poor group. The run of abundance ratios [X/Fe] versus [Fe/H] for the two groups define a single relation for each element (designated as X) with no intrinsic scatter and without a measurable discontinuity at −0.6 < [Fe/H] < −0.5, the metallicity at which the two groups overlap. The relations [X/Fe] versus [Fe/H] are those determined previously for thick disc stars. It is suggested that these two groups and the thick disc as a whole have a common origin in terms of prior chemical evolution.