Astron. Astrophys. 321, 207-212 (1997)

6. Conclusions

Our computations, in contradiction to the results of Romani 1992, predict a formation rate for low mass X-ray binaries with a black hole which is much smaller than the value derived from the observed numbers and estimated X-ray lifetime. The striking difference between our and his results can be seen immediately by comparing his Fig. 1 with our Fig. 1. Romani's values for are much higher than ours: this is due to the fact that he uses instead of q in the equation for the Roche lobe (our Eq.  1). (This error has been silently corrected in Romani 1994.) This is true both for binaries in which mass transfer preceeds the supernova explosion, and for binaries in which the primary explodes after losing its envelope without ever having filled its Roche-lobe. The discrepancy between theoretical and observed formation rates cannot be solved by invoking different metallicities for the progenitor systems, nor by assuming different efficiencies for the envelope ejection during spiral-in.

Black-hole binaries can be produced in larger numbers only if it is assumed that stars with initial masses less than approximately 20  can collapse to black holes; or alternatively if it is assumed that the angular momentum loss caused by the stellar wind is so high that the binary orbit shrinks; or alternatively if the collapse of a helium core to a black hole is asymmetric, so that the post-supernova orbit can be smaller than the pre-supernova orbit (see Portegies Zwart & Verbunt 1996).

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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