Elusive helium stars in the gap between subdwarfs and Wolf-Rayet stars III. Formation of the Galactic population of stripped helium stars

Abstract

Population synthesis based on the grid of pre-computed evolutionary sequences for close binaries is applied to the problem of formation of the Galactic population of stripped helium stars, the remnants of $5⪅M/M_{\odot }⪅24$ primary components of close binaries after RLOF. Stellar rotation is taken into account. For the current star formation rate in the Galaxy $2M_{\odot }y{r}^{-1}$, the estimate of the number of 1 to $7M_{\odot }$ objects with $T_{eff}⪆$25000K ranges from 14700 to 28500, while for the number of objects with masses from $2M_{\odot }$ to $7M_{\odot }$ it is 3200 to 5500, depending on the assumptions about formation of common envelopes and their outcomes. Other factors influencing the estimate are adopted star formation rate, distributions of close binaries over initial masses of primary components, mass ratios of components, orbital periods. Observational selection may severely limit the number of detectable objects. Angular velocities of rotation of companions to stripped helium stars are 60 to 80 per cent of the critical one. Using the models of initially homogeneous helium stars as approximations for the remnants after mass loss results in the underestimate of pre-supernovae masses.

38 pages, 17+2 figures, 2 tables. Accepted for publication in Astronomy Letters