2004NewA....9..225B

Earlier works, which we review, have shown that if the Fe core in a presupernova star is to be sufficiently massive to collapse into a black hole, earlier in the evolution of the star the He core must be covered (clothed) by a hydrogen envelope during He core burning and removed only following this, in, e.g., common envelope evolution. This is classified as Case C mass transfer. These previous arguments were based chiefly on stellar evolution, especially depending on the way in which 12C burned. In this work we argue for Case C mass transfer on the basis of binary evolution. The giant progenitor of the black hole will have a large radius ∼1000R at the end of its supergiant stage. Its lifetime at that point will be short, ∼1000 years, so it will not expand much further. Thus, the initial giant radius for Case C mass transfer will be constrained to a narrow band about ∼1000R. This has the consequence that the final separation af following common envelope evolution will depend nearly linearly on the mass of the companion md which becomes the donor after the He core of the giant has collapsed into the black hole. The separation at which this collapse takes place is essentially af, because of the rapid evolution of the giant. (In at least two binaries the black hole donor separation has been substantially increased because of mass loss in the black hole formation. These can be reconstructed from the amount of mass deposited on the donor in this mass loss.) We show that the reconstructed preexplosion separations of the black hole binaries fit well the linear relationship.