The Evolution of Massive Helium Stars, Including Mass Loss

The evolution of helium stars with initial masses in the range 1.6-120 M-circle dot. is studied, including the effects of mass loss by winds. These stars are assumed to form in binary systems when their expanding hydrogenic envelopes are promptly lost just after helium ignition. Significant differences are found with single-star evolution, chiefly because the helium core loses mass during helium burning rather than gaining it from hydrogen shell burning. Consequently, presupernova stars for a given initial mass function have considerably smaller mass when they die and will be easier to explode. Even accounting for this difference, the helium stars with mass loss develop more centrally condensed cores that should explode more easily than their single-star counterparts. The production of low-mass black holes may be diminished. Helium stars with initial masses below 3.2 M-circle dot. experience significant radius expansion after helium depletion, reaching blue supergiant proportions. This could trigger additional mass exchange or affect the light curve of the supernova. The most common black hole mass produced in binaries is estimated to be about 9 M-circle dot. A new maximum mass for black holes derived from pulsational pair-instability supernovae is derived, 46 M-circle dot, and a new potential gap at 10-12 M-circle dot. is noted. Models pertinent to SN 2014ft are presented, and a library of presupernova models is generated.