The presupernova evolution and explosion and explosion of helium stars that experience mass loss

The evolution of helium stars with initial masses in the range 4 to 20 M _sun is followed through all stages of hydrostatic nuclear burning. We identify these objects as Wolf-Rayet stars that have lost their hydrogen envelope either before or early in their helium burning phase, probably because they were in a mass exchanging binary system or, for the most massive stars, by a strong stellar wind. Stripped of their envelope, 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 to 3.55 M _sun for all stars considered. We identify these as progenitors of Type Ib and Ic supernovae and investigate the dependence of the pre-supernova structure on the initial mass of the helium star. Five of these 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_(_4_2_) erg s_(_-_1_). The (unmodified) mass of _(_5_6_)Ni for 10 explosions (variable parameterizations of explosion in the 5 stars) lies in a narrow range, 0.07 to 0.15 M _sun . Other abundances from carbon through nickel are coproduced in approximately solar proportions along with interesting amounts of the long lived radioactivities, _(_2_6_)Al and _(_6_0_)Fe. The light curves agree reasonably well with observations of Type Ib and Ic supernova, including Type Ib SN 1983N and the recent type Ic SN 19941. A _(_5_6_)Ni mass of 0.05 (+0.02-0.01) M _sun 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. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman