Evolutionary sequences for Nova V1974 Cygni using new nuclear reaction rates and opacities

The outburst of Nova V1974 Cyg 1992 is arguably the best observed of this century, with realistic estimates now available for the amount of mass ejected, the composition of the ejecta and the total energy budget. These data strongly support the conclusion that this was indeed a 'neon' nova that occurred on an oxygen, neon, magnesium white dwarf. In addition, X-ray studies of its outburst imply that the mass of the white dwarf is about 1.25 M_⊙. We, therefore, report on the results of new calculations of thermonuclear runaways on 1.25-M_⊙ oxygen, neon, magnesium white dwarfs, using our one-dimensional, fully implicit, hydrodynamic stellar evolution code that includes a large nuclear reaction network. We have updated the nuclear reaction network, with the inclusion of new and improved experimental and theoretical determinations of the nuclear reaction rates. We have also incorporated the OPAL carbon rich tables and have investigated the effects of changes in convective efficiency on the evolution. Our results show that the changes in the reaction rates and opacities that we have introduced produce important changes with respect to our previous studies. For example, relevant to nucleosynthesis considerations, a smaller amount of ²⁶Al is produced, while the abundances of ³¹P and ³²S increase by factors of more than two. This change is attributed to the increased proton-capture reaction rates for some of the intermediate mass nuclei near ²⁶Al and beyond, such that nuclear fusion to higher mass nuclei is enhanced. The characteristics of our models are then compared to observations of the outburst of V1974 Cyg 1992 and we find that the predicted amount of mass ejected is at least a factor of 10 less than observed. The low values for the amount of ejected mass are a consequence of the fact that the OPAL opacities are larger than those we previously used, which results in less mass being accreted on to the white dwarf. This is a general problem with respect to the comparison of observations and theory for ONeMg novae and we suggest a possible resolution of this discrepancy.