Non-conservative Evolution of Cataclysmic Variables

We suggest a new mechanism to account for the loss of angular momentum in binaries with non-conservative mass exchange. It is shown that in some cases the loss of matter can result in increase of the orbital angular momentum of a binary. If included into consideration in evolutionary calculations, this mechanism appreciably extends the range of mass ratios of components for which mass exchange in binaries is stable. It becomes possible to explain the existence of some observed cataclysmic binaries with high donor/accretor mass ratio, which was prohibited in conservative evolution models.Comment: LaTeX, 32 pages, to be published in Astron. Z

Supernova Ia: a Converging Delayed Detonation Wave

A model of a carbon-oxygen (C--O) presupernova core with an initial mass 1.33 M_\odot, an initial carbon mass fraction 0.27, and with an average mass growth-rate 5 x 10^{-7} M_\odot/yr due to accretion in a binary system was evolved from initial central density 10^9 g/cm^3, and temperature 2.05 x 10^8 K through convective core formation and its subsequent expansion to the carbon runaway at the center. The only thermonuclear reaction contained in the equations of evolution and runaway was the carbon burning reaction 12C + 12C with an energy release corresponding to the full transition of carbon and oxygen (with the same rate as carbon) into 56Ni. As a parameter we take \alpha_c - a ratio of a mixing length to the size of the convective zone. In spite of the crude assumptions, we obtained a pattern of the runaway acceptable for the supernova theory with the strong dependence of its duration on \alpha_c. In the variants with large enough values of \alpha_c=4.0 x 10^{-3} and 3.0 x 10^{-3} the fuel combustion occurred from the very beginning as a prompt detonation. In the range of 2.0 x 10^{-3} >= \alpha_c >= 3.0 x 10^{-4} the burning started as a deflagration with excitation of stellar pulsations with growing amplitude. Eventually, the detonation set in, which was activated near the surface layers of the presupernova (with m about 1.33 M_\odot) and penetrated into the star down to the deflagration front. Excitation of model pulsations and formation of a detonation front are described in detail for the variant with \alpha_c=1.0 x 10^{-3}.Comment: 13 pages, 11 figures, to appear in Astronomy Letter

Binary progenitors of Supernovae

Supernovae of both Type I (hydrogen-poor) and Type II (hydrogen-rich) can be expected to occur among binary stars. Among massive stars (>10 M•), the companion makes it more difficult for the primary to develop an unstable core of >1.4.M• while still retaining the extended, hydrogen-rich envelope needed to make a typical Type II light curve. Among 1-10 M• stars, on the other hand, a companion plays a vital role in currently popular models for Type I events, by transferring material to the primary after it has become a stable white dwarf, and so driving it to conditions where either core collapse or explosive nuclear burning will occur. Several difficulties (involving nucleosynthesis, numbers and lifetimes of progenitors, the mass-transfer mechanism, etc.) still exist in these models. Some of them are overcome by a recent, promising scenario in which the secondary also evolves to a degenerate configuration, and the two white dwarfs spiral together to produce a hydrogen-free explosion, long after single stars of the same initial masses have ceased to be capable of fireworks. © 1984 Indian Academy of Sciences