THE PRESUPERNOVA EVOLUTION AND EXPLOSION OF HELIUM STARS THAT EXPERIENCE MASS-LOSS

The evolution of helium stars with initial masses in the range 4-20 M. 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 M. 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) x 10(42) ergs s(-1). The (unmodified) mass of Ni-56 for 10 explosions (variable parameterizations of explosion in the five stars) lies in a narrow range, 0.07-0.15 M. Other abundances. from carbon through nickel are coproduced in approximately solar proportions along with interesting amounts of the long-lived radioactivities, Al-26 and Fe-60. 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 1994I. A Ni-56 mass of 0.05(-0.01)(+0.02) M. is derived for the latter (for a distance of 7 Mpc), and speculations are presented regarding SN 1998bg. Ultimately, spectroscopic diagnostics of these models should aid in testing them.