“Bloated Dwarf” Model of Nova Light Curves

Abstract

A model is proposed to account for nova light curves, especially those which show regular or quasi-regular oscillations during decay. Various numerical calculations indicate that sudden nuclear burning of matter accreted onto a white dwarf in a binary star system can cause explosive ejection of matter. However, if some of the matter ejected from the white dwarf surface never escapes and tries to return to the underlying star, a distended white dwarf may temporarily result—a “Bloated Dwarf” (BD). The continuum light curve would then come from two sources: (1) the expanding ejected shell which becomes optically thin and weakly luminous after about one week (but later producing most of the line spectrum) and (2) radiation from the optically thick underlying BD. With a white dwarf of mass M_*⋍M_⊙, envelope mass M_e⋍10^-4-10^-5M_⊙,and radius R_e⋍10¹¹-10¹² cm, the BD can pulsate with periods of order 1-10 days. (By tran- sit time arguments, an expanding shell is unlikely to permit such variations.) Although the damping time of the oscillations is difficult to estimate, one finds that contraction of the envelope will occur on a time scale of months. The increase of temperature with decreasing visual luminosity observed in several novae arises from the shift in the peak emissivity toward the UV as the BD contracts. The model also accounts for the outburst size-decay time relation; the continuing activity, including mass loss, long after the initial event; and the existence of strong circular polarization in the continuum light.