Presupernova Evolution with Improved Rates for Weak Interactions

Abstract

Recent shell-model calculations of weak-interaction rates for nuclei in the mass range A = 45-65 have resulted in substantial revisions to the hitherto standard set of Fuller, Fowler, & Newman (FFN). In particular, key electron-capture rates, such as that for ⁶⁰Co, are much smaller. We consider here the effects of these revised rates on the presupernova (post-oxygen burning) evolution of massive stars in the mass range 11-40 M_☉. Moreover, we include, for the first time in models by our group, the effects of modern rates for beta decay in addition to electron capture and positron emission. Stars of 15, 25, and 40 M_☉ are examined in detail using both the full FFN rate set and the revised rates. An additional finely spaced (in mass) grid of 34 models is also calculated in order to give the systematics of iron core and silicon core masses. Values for the central electron mole number at the time of iron core collapse in the new models are typically larger, by ΔY_e = 0.005-0.015, than those of Woosley & Weaver, with a tendency for the more massive models to display larger differences. About half of this change is a consequence of including beta decay; the other half, the result of the smaller rates for electron capture. Unlike what might be expected solely on basis of the larger Y_e values, the new iron core masses are systematically smaller owing to a decrease in the entropy in the outer iron core. The changes in iron core mass range from zero to 0.1 M_☉ (larger changes for high-mass stars). It would be erroneous however to estimate the facility of exploding these new models based solely upon their iron core mass since the entire core structure is altered and the density change is not so different as the adjustments in composition might suggest. We also observe, as predicted by Aufderheide et al., a tendency toward beta equilibrium just prior to the collapse of the core, and the subsequent loss of that equilibrium as core collapse proceeds. This tendency is more pronounced in the 15 M_☉ model than in the heavier stars. We discuss the key weak reaction rates, both beta decay and electron capture, responsible for the evolution of Y_e and make suggestions for future measurements.