Behavior of Abundances in Chemically Peculiar Dwarf and Subgiant A-Type Stars: HD 23193 and HD 170920

Abstract

To understand the origin of the abundance peculiarities of non-magnetic A-type stars, we present the first detailed chemical abundance analysis of a metallic line star HD 23193 (A2m) and an A-type subgiant HD 170920 (A5) which could have been a HgMn star on the main sequence. Our analysis is based on medium (R$\sim$14000) and high (R$\sim$40000) resolution spectroscopic data of the stars. The abundance of 18 elements are derived: C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Sr, Y, and Ba. The masses of HD 23193 and HD 170920 are estimated from evolutionary tracks, as $2.3\pm0.1$ $M_{\odot}$ and $2.9\pm0.1$ $M_{\odot}$. The ages are found $635\pm33$ Myr for HD 23193 and $480\pm50$ Myr for HD 170920 using isochrones. The abundance pattern of HD 23193 shows deviations from solar values in the iron-peak elements and indicates remarkable overabundances of Sr (1.16), Y (1.03), and Ba (1.24) with respect to the solar abundances. We compare the derived abundances of this moderately rotating ($v$sin$i = 37.5 $ km s$^{-1}$) Am star to the theoretical chemical evolution models including rotational mixing. The theoretically predicted abundances resemble our derived abundance pattern, except for a few elements (Si and Cr). For HD 170920, we find nearly solar abundances, except for C ($-0.43$), S (0.16), Ti (0.15), Ni (0.16), Zn (0.41), Y (0.57), and Ba (0.97). Its low rotational velocity ($v$sin$i = 14.5 $ km s$^{-1}$), reduced carbon abundance, and enhanced heavy element abundances suggest that the star is most-likely an evolved HgMn star.