The Presupernova Evolution and Explosion of Helium Stars That Experience Mass Loss

Abstract

The evolution of helium stars with initial masses in the range 4-20 Msun is followed through all stages of hydrostatic nuclear burning. We identify these objects as Wolf-Rayet stars that have lost their hydrogen envelopes, either before or early in their helium-burning phase, probably because they were in a mass- exchanging binary system or, for the more massive stars, because they were subject to a strong stellar wind. Stripped of their envelopes, these stars are subject to efficient (mass-dependent) mass loss. As a result, the final masses converge to a narrow range of small values: 2.26-3.55 Msun for all stars considered. We identify these as progenitors of Type Ib and, perhaps, Type Ic supernovae and investigate the dependence of the presupernova structure on the initial mass of the helium star. For two models, the extra mass loss that could occur in a close binary, the product of common envelope evolution, is also considered. Five of our presupernova models are then exploded, using pistons, and their nucleosynthesis and bolometric light curves calculated. Peak luminosities are in the range (1.5-4) × 1042 ergs s-1. The (unmodified) mass of 56Ni for 10 explosions (variable parameterizations of explosion in the five stars) lies in a narrow range, 0.07-0.15 Msun. Other abundances from carbon through nickel are coproduced in approximately solar proportions along with interesting amounts of the long-lived radioactivities, 26Al and 60Fe. The light curves agree reasonably well with observations of Type Ib and Ic supernovae, including Type Ib SN 1983N and the recent Type Ic SN 19941. A 56Ni mass of 0.05+0.02-0.01 Msun is derived for the latter (for a distance of 7 Mpc), and speculations are presented regarding SN 1991bg. Ultimately, spectroscopic diagnostics of these models should aid in testing them.