Thermonuclear Runaway Model

Abstract

The nova outburst requires an energy source that is energetic enough to eject material and is able to recur. The Thermonuclear Runaway (TNR) model, coupled with the binary nature of nova systems, satisfies these conditions. The white dwarf/red dwarf binary nature of novae was first recognized as a necessary condition by Kraft (1963,1964, and these conference proceedings). The small separation characteristic of novae systems allows the cool, red secondary to overflow its Roche lobe. In the absence of strong, funneling magnetic fields, the angular momentum of this material prevents it from falling directly onto the primary, and it first forms a disk around the white dwarf. This material is eventually accreted from the disk onto the white dwarf. As the thickness of this hydrogen-rich layer increases, the degenerate matter at the base reaches a temperature that is high enough to initiate thermonuclear fusion of hydrogen. Thermonuclear energy release increases the temperature which in turn increases the energy generation rate. Because the material is degenerate, the pressure does not increase with temperature, which normally allows a star to adjust itself to a steady nuclear burning rate. Thus the temperature and nuclear energy generation increase and a TNR results. When the temperature reaches the Fermi temperature, degeneracy is lifted and the rapid pressure increase causes material expansion. The hydrogen-rich material either is ejected or consumed by nuclear burning, and the white dwarf returns to its pre-outburst state. The external source of hydrogen fuel from the secondary allows the whole process to repeat.