Population Synthesis of Galactic Be-star Binaries with A Helium-star Companion

LB-1 was originally suggested to harbour a very massive ($ \sim 70 M_\odot $) black hole, but was recently suggested to be a post-mass transfer binary containing a Be star and a helium (He) star. In this paper, we use the binary population synthesis method to simulate the potential population of the Be$ - $He binaries in the Milky Way. Mass transfer process during the progenitor binary evolution plays a vital role in determining the possible properties of the Be$ - $He binary population. By constructing a range of physical models with significantly different mass-transfer efficiencies, we obtain the predicted distributions at the current epoch of the component masses and the orbital periods for the Be$ - $He binaries. In particular, we show that, LB-1 very likely has evolved through non-conservative mass transfer if it is indeed a Be$ - $He system. We estimate that there are more than $ 10^{3} $ Be$ - $He binaries with V-band apparent magnitudes brighter than LB-1.