Thermal Runaway near the Surface of Accreting White Dwarfs

Abstract

Giannone and Weigert (1967) calculated models of white dwarfs on which a hydrogen rich envelope is assumed to increase with time. They have shown that a flash occurs for M=10−9M⊙/year in their sequence D. We have studied the same process in order to find out the conditions with which an accreting white dwarf can assimilate a nova phenomenon. When the rate of accretion M is large, the hydrogen-rich envelope is heated almost adiabatically. In this case the hydrogen is ignited near the surface and the flash is not strong enough to produce a nova. When M is too small, there is enough time for the excess energy to escape from the envelope and the ignition temperature is not reached. Strong flash occurs between these two cases. Mass of the degenerate core is also an important factor. Several sequences of models were calculated. A carbon-oxygen white dwarf of 1.3 solar mass with M=10−10 M⊙ y−1 makes a strong and rapid flash. We may expect a nova phenomenon for this set of parameters. For a helium white dwarf of 0.4 solar mass with M=10−8 and 10−13 M⊙ y−1 however, the flash process is very slow.