Population Synthesis of Black Hole Binaries with Normal-star Companions. I. Detached Systems

Abstract

Optical observations of normal stars in binary systems with massive unseen objects have been proposed to search for candidate black holes (BHs) and provide a direct measurement of their dynamical masses. In this paper, we have performed binary population synthesis calculations to simulate the potential population of detached binaries containing BHs and normal-star companions in the Galaxy. We focus on the influence of the BH progenitors. In the traditional model, BHs in binaries evolve from stars more massive than ∼25 M ⊙ . However, it is difficult for this model to produce BH low-mass X-ray binaries. Recent investigations of massive star evolution have suggested that the BH progenitors have masses as low as ∼15 M ⊙ . Based on this model, we provide the expected distributions of various parameters for detached BH binaries with normal-star companions, including the component masses, the orbital parameters of the binary systems, the radial velocity semi-amplitudes, and the astrometric signatures of the optical companions. Our calculations show that there are thousands of such detached binaries in the Galaxy, and hundreds of them are potentially observable systems with luminous companions brighter than 20 mag. In addition, detached BH binaries are dominated by those with main-sequence companions and only a small percent of them are expected to have giant companions.