Formation of high mass X-ray black hole binaries

The discrepancy in the past years of many more black-hole soft X-ray transi ents (SXTs), of which a dozen have now been identified, had challenged acce pted wisdom in black hole evolution. Reconstruction in the literature of hi gh-mass X-ray binaries has required stars of up to similar to 40 M-. to evo lve into low-mass compact objects, setting this mass as the limit often use d for black hole formation in population syntheses. On the other hand, the sheer number of inferred SXTs requires that many, if not most, stars of ZAM S masses 20-35 M-. end up as black holes (Portegies Zwart et al., 1997; Erg ma and van den (Heuvel, 1998). In this paper we show that this can be understood by challenging the accept ed wisdom that the result of helium core burning in a massive star is indep endent of whether the core is covered by a hydrogen envelope, or 'naked' wh ile it burns. The latter case occurs in binaries when the envelope of the m ore massive star is transferred to the companion by Roche Lobe overflow whi le in either main sequence or red giant stage. For solar metallicity, where as the helium cores which burn while naked essentially never go into high-m ass black holes, those that burn while clothed do so, beginning at ZAMS mas s similar to 20 M-., the precise mass depending on the C-12(alpha, gamma) O -16 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 bu rning. Whereas this scenario was already outlined in 1998 by Brown et al. [NewA 4 (1999) 313], their work was based on evolutionary calculations of Woosley e t al. [ApJ 448 (1995) 315] which employed wind loss rates which were too hi gh. In this article we collect results for lower, more correct wind loss ra tes, finding that these change the results only little. We go into the details of carbon burning in order to reconstruct why the lo w Fe core masses from naked He stars are relatively insensitive to wind los s rate. The main reason is that without the helium produced by burning the hydrogen envelope, which is convected to the carbon in a clothed star, a ce ntral C-12 abundance of similar to1/3 remains unburned in a naked star foll owing He core burning. The later convective burning through C-12 + C-12 rea ctions occurs at a temperature T similar to 80 keV. Finally, we show that in order to evolve a black hole of mass greater than or similar to 10 M-. such as observed in Cyg X-1 even employing extremely m assive progenitors of ZAMS mass greater than or similar to 60 M-. for for t he black hole, the core must be covered by hydrogen during a substantial fr action of the core burning. In other words, the progenitor must be a WNL st ar. We evolve Cyg X-1 in an analogous way to which the SXTs are evolved, th e difference being that the companion in Cyg X-1 is more massive than those in the SXTs, so that Cyg X-1 shines continuously. (C) 2001 Published by El sevier Science B.V.