Formation of High Mass X-ray Black Hole Binaries

Abstract

The discrepancy in the past years of many more black-hole soft X-ray transients (SXTs), of which a dozen have now been identified, had challenged accepted wisdom in black hole evolution. The sheer number of inferred SXTs requires that many, if not most, stars of ZAMS masses $20-35\msun$ end up as black holes (Portegies Zwart et al. 1997; Ergma & van den Heuvel 1998). In this paper we show that this can be understood by challenging the accepted wisdom that the result of helium core burning in a massive star is independent of whether the core is covered by a hydrogen envelope, or "naked" while it burns. The latter case occurs in binaries when the envelope of the more massive star is transferred to the companion by Roche Lobe overflow while in either main sequence or red giant stage. For solar metallicity, whereas the helium cores which burn while naked essentially never go into high-mass black holes, those that burn while clothed do so, beginning at ZAMS mass $\sim 20\msun$, the precise mass depending on the $^{12}C(α,γ)^{16}$O rate as we outline. In this way the SXTs can be evolved, provided that the H envelope of the massive star is removed only following the He core burning. We also show that in order to evolve a black hole of mass $\gsim 10\msun$ such as observed in Cyg X-1, even employing extremely massive progenitors of ZAMS mass $\gsim 60\msun$ for the black hole, the core must be covered by hydrogen during a substantial fraction of the core burning. In other words, the progenitor must be a WNL star. We evolve Cyg X-1 in an analogous way to which the SXTs are evolved, the difference being that the companion in Cyg X-1 is more massive than those in the SXTs, so that Cyg X-1 shines continuously.