What We Have Learned about White Dwarfs from Studies of Novae

Abstract

The nova outburst is one consequence of accretion of hydrogen-rich material onto a white dwarf. Although most of our interest has been in trying to understand the cause and evolution of the nova outburst, it is impossible to separate that from an interest in the structure and characteristics of white dwarfs. For example, it is the strong electron degeneracy of the white dwarf that forces the nuclear reactions to temperatures that exceed 10⁸K. Nova studies have also shown that the peak temperature in the nuclear burning region and the peak effective temperature depend on the mass of the white dwarf. It may be the case, in addition, that the lifetime of the nova outburst, the amount of accreted mass, and the ejected mass depend on the mass of the white dwarf. It is also important to realize that there have been numerous determinations of the elemental abundances in nova ejecta and they show, unequivocally, that there are two composition classes for the white dwarfs in nova systems. One class consists of carbon-oxygen (CO) white dwarfs and they must have originated as main sequences stars with masses of around 5M_⊙ or below. There is a second class of outbursts in which the elemental abundance pattern in the ejected gas can only be explained if it consists of material from the interior of an oxygen-neonmagnesium (ONeMg) white dwarf that has been processed through hot, hydrogen burning. This class must have originated from main sequence stars with masses from 8M_⊙ to about 12M_⊙. Finally, the evolutionary studies of the explosion suggest strongly that the white dwarf must be relatively cool which, in turn, implies that nova systems are relatively old objects.