Recently (Brown \& Bethe 1994) it was suggested that most stars with main
sequence mass in the range of about 18β30Mββ explode, returning
matter to the Galaxy, and then go into low-mass (β₯1.5Mββ) black
holes. Even more massive main-sequence stars would, presumably, chiefly g o
into high-mass (βΌ10Mββ) black holes. The Brown-Bethe estimates
gave approximately 5Γ108 low-mass black holes in the Galaxy. A
pressing question, which we attempt to answer here, is why, with the possible
exception of the compact objects in SN1987A and 4U\,1700--37, none of these
have been seen.
We address this question in three parts. Firstly, black holes are generally
``seen'' only in binaries, by the accretion of matter from a companion star.
High mass black holes are capable of accreting more matter than low-mass black
holes, so there is a selection effect favoring them. This, in itself, would not
be sufficient to show why low-mass black holes have not been seen, since
neutron stars (of nearly the same mass) are seen in abundance.
Secondly, and this is our main point, the primary star in a binary ---the
first star to evolve--- loses its hydrogen envelope by transfer of matter to
the secondary and loss into space, and the resulting ``naked'' helium star
evolves differently than a helium core, which is at least initially covered by
the hydrogen envelope in a massive main-sequence star. We show that primary
stars in binaries can end up as neutron stars even if their initial mass
substantially exceeds the mass limit for neutron star formation from single
stars (βΌ18Mββ). An example is 4U\,1223--62, in which we suggest
that the initial primary mass exceeded 35Mββ, yet X-ray pulsationsComment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm