Evolution of Black Holes in the Galaxy

In this article we attempt a consistent calculation of the evolution (and formation rate) of black holes in the Galaxy, especially those in binaries which can be observed by the X-rays from material falling onto them from companion stars. Our chief results come from the bimodal distribution of progenitors of high-mass black holes, stars of ZAMS masses $\sim 20 -35 \msun$ and $\sim 80 - 100 \msun$. Recently Bethe & Brown (1998b) showed that very massive progenitors of ZAMS masses $\sim 80 -100 \msun$ were needed to evolve high-mass black holes in binaries such as Cyg X-1, with massive companion. These are important for LIGO and LISA which will detect gravitational waves from mergers. Although there are few of them, their mergers can be seen to higher distances because of their greater mass. The most copious high-mass black-holes of masses $\sim 6 -7 \msun$ have been found in the transient sources such as A0620. These have low-mass companions, predominantly of $\lsim 1 \msun$, such as K-- or M--stars. In order to force the helium core to be "clothed" while it burns, and to get the main sequence star close to it, we need a small companion mass. We believe this is why K-- and M--star companions are favored. Once exposed, following the loss of the hydrogen envelope, the helium stars will lose mass rapidly by wind. The higher mass black holes from the ZAMS interval $\sim 20 - 35\msun$ lose more mass by wind than the lower mass ones, so the trend is to a clustering about $\sim 6-7\msun$.