A Scenario for a Large Number of Low-Mass Black Holes in the Galaxy

We point out that stars within a fairly large range of masses, roughly 18-30 Msun, can both explode as supernovae (and give off neutrinos as observed in SN 1987A) and then go into black holes. The masses of the black holes so formed are only slightly above 1.5 Msun, the maximum mass for compact cores, according to our arguments. These masses are substantially smaller than those of the best candidates for black holes observed thus far. We discuss the possibility that SN 1987A produced one of these small black holes. We review the requirements that observed nucleosynthesis puts on theoretical nucleosynthesis. Especially the empirical value of ΔY/ΔZ = 4±1.3 gives a sensitive determination of the mass at which nucleosynthesis must be cut off, by heavier stars collapsing into black holes without returning matter to the Galaxy, because otherwise heavy stars would produce chiefly metals and unduly lower the calculated value of A Y/AZ. We arrive at a value of Mcutoff = 25±5 Msun for the mass at which nucleosynthesis must be cut off. We show that the introduction of kaon condensation sufficiently softens the equation of state of dense matter, so that compact cores of mass greater than Mmax ≃Msun will not be stable. For the first ≳12 s, however, compact cores up to ∼1.84 Msun are stabilized; this gives the nucleosynthesis mass cutoff as ∼30 Msun, at the upper end of our limit from nucleosynthetic demands. Our soft equation of state extends black hole production to stars of lower mass than previously estimated, and, therefore, increases the estimated number of black holes by an order of magnitude or more, to ∼109 in the Galaxy.