Elusive hot stripped helium stars in the Galaxy

I. Evolutionary stellar models in the gap between subdwarfs and Wolf-Rayet stars

¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
e-mail: yungelson@astro.physik.uni-potsdam.de
² Institute of Astronomy, Russian Academy of Sciences, 48 Pyatnitskaya Str., Moscow 119017, Russia
³ Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, 14 Universitetsky Pr., Moscow 119992, Russia
⁴ Faculty of Computational Mathematics and Cybernetics, M.V. Lomonosov Moscow State University, 1-52 Leninskiye Gory, Moscow 119991, Russia

Received: 25 August 2023
Accepted: 29 November 2023

Abstract

Context. Stellar evolution theory predicts the existence of helium-core remnants of the primary components of intermediate-mass close binaries that lost most of their hydrogen-helium envelopes due to the mass exchange. These remnants are expected to be observed as hot helium-rich stars with masses of (1−7) M_⊙, located in the area of the Hertzsprung-Russell diagram between OB subdwarfs and Wolf-Rayet stars. While several thousands of such stripped helium stars are expected to exist in the Galaxy, none of them have been identified so far.

Aims. We aim to provide comprehensive predictions of the numbers and fundamental properties of stripped helium stars and their binary companions in the Galaxy. This is a necessary first step to guide observations, to enable a comparison between binary evolution models and realistic stellar populations, and to determine the feedback parameters of stripped helium stars in the Galaxy.

Methods. We expanded the previously considered space of parameters describing close binary systems producing stripped helium stars and applied a population synthesis based on a grid of evolutionary models computed by the code MESA, using a spin-dependent algorithm for the treatment of mass and angular momentum loss from the system.

Results. We show that the number of Galactic binaries hosting (1−7) M_⊙ He-stars is ≃20 000 and that it steeply declines with an increase in the He-star mass (≃3000 with mass ≳2 M_⊙). The decisive factor that defines the low number of stripped He-stars is runaway mass-loss after Roche lobe overflow (RLOF) by primary components of the binaries, which leads to the formation of common envelopes and the further merger of components. This effect strongly restricts the initial ranges of masses of components of the progenitors of stripped stars and orbital periods. In addition, stripped helium stars are much less numerous than expected, since a significant fraction of binaries in which the primaries have masses less than (5−7) M_⊙ produce subdwarfs with masses ≲1 M_⊙. Our calculations show that the overwhelming majority of helium stars reside in binaries with an early-type companion star and can be identified neither by the UV excess nor by emission features. The large periods of a significant fraction of binaries hosting stripped stars (≳several hundred days) also hamper their discovery.