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 ide ntify these objects as Wolf-Rayet stars that have lost their hydrogen envelopes, either before or early in their helium-burning phase, proba bly because they were in a mass-exchanging binary system or, for the m ore massive stars, because they were subject to a strong stellar wind. Stripped of their envelopes, these stars are subject to efficient (ma ss-dependent) mass loss. As a result, the final masses converge to a n arrow 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 supern ovae and investigate the dependence of the presupernova structure on t he initial mass of the helium star. For two models, the extra mass los s that could occur in a close binary, the product of common envelope e volution, is also considered. Five of our presupernova models are then exploded, using pistons, and their nucleosynthesis and bolometric lig ht 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 (vari able parameterizations of explosion in the five stars) lies in a narro w 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 super novae, including Type Ib SN 1983N and the recent Type Ic SN 1994I. A N i-56 mass of 0.05(-0.01)(+0.02) M. is derived for the latter (for a di stance of 7 Mpc), and speculations are presented regarding SN 1998bg. Ultimately, spectroscopic diagnostics of these models should aid in te sting them.