On the masses of the white dwarfs in classical nova systems

Abstract

Significant progress in our understanding of the nature of the outbursts of the classical novae has occurred over the past two decades (see, e.g., reviews by Truran 1982; Starrfield 1986). Their outbursts are now understood to be driven by thermonuclear runaways proceeding in the accreted hydrogen-rich shells on the white dwarf components of close binary systems. Critical parameters which serve to dictate the varied characteristics of the observed outbursts include the intrinsic white dwarf luminosity, the rate of mass accretion, the composition of the envelope matter prior to runaway, and the white dwarf mass. Our concern in this paper is specifically with the question of the white dwarf mass. The expected average mass of isolated white dwarfs in the interstellar medium is 0.6 0.7 MO; this follows straightforwardly from the assumption of a Salpeter (1955) stellar mass function and standard stellar evolution theory, and is confirmed by observations (Koester, Schulz, and Weidemann 1979). The question arises as to whether white dwarf masses in this range should be typical of the white dwarfs in close binary systems. We seek to provide a partial answer to this question, for the specific case of classical nova systems, on the basis of observations of the dynamical evolution of novae in outburst (Section II) and of the abundances in nova nebular ejecta (Section III). We then provide an estimate of the average mass of the white dwarf component of observed classical nova systems (Section IV), on the basis of theoretical arguments concerning the critical envelope mass necessary to trigger runaway on a white dwarf of specified mass. Finally, we review existing mass estimates for novae and discuss these in the light of theoretical models.