Astron. Astrophys. 352, L87-L90 (1999)

4. Discussion and conclusions

In this Letter we show that both the high observed velocities of Cyg X-1 and Nova Sco, and the low velocities of the black hole X-ray binaries with low mass donors can be explained by ejection of 30% of the mass of the exploding helium star in the SN that formed the black hole. This removes the need to invoke a large kick for Nova Sco (and Cyg X-1) and at the same time a small or no kick for the remaining systems Brandt et al. 1995, which seems highly unlikely to us.

The radial velocity of Nova Sco can only be explained by large mass-loss fractions () and the assumption that the mass transfer has already started some time ago. If this mass transfer was non-conservative (consistent with observed jets), the velocity can be explained more easily. This may also be needed if Nova Sco also has a transverse velocity component.

Tutukov & Cherepahschuk Tutukov and Cherepashchuk (1997) state that the high velocity of Nova Sco could be obtained by having the pre-SN mass ratio above 0.24, i.e. (they use 2.3 and 4 for the current masses). However, this is not in agreement with the assumption in their equation, that the maximal amount of mass is lost. Using our modification (Eq. (1)) to their equation, we find indeed that for their masses it is impossible to obtain a velocity higher than 93 km s ^-1, in agreement with our findings that the post-SN orbit must be different from the current orbit.

The fact that black holes in X-ray binaries may have lost several tens of percents of their progenitor mass in the SN, makes it necessary that some of their progenitor (helium) stars must have had masses above 10 , which is in clear disagreement with the suggestion from some stellar evolution models that all Wolf-Rayet stars have a mass at the moment they explode in a supernova Woosley et al. 1995.

Finally, it should be noticed that the conclusion that black holes eject a substantial amount of material during their formation has consequences for the orbital period distribution of close black hole pairs, which are expected to be prime sources for ground based gravitational wave detectors. Mass ejection will widen the orbit, which happens twice during the formation of a black hole pair, possibly preventing black holes to form in a close orbit at all. Only kicks from an asymmetry in the SN could then form close pairs. But as shown above, there is not much evidence for kicks and the magnitude of any kicks is severely limited to 40 km s^-1 by the black hole X-ray binaries with low mass donors, unless the black holes in these system formed in a direct collapse.

© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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