Light-curve analysis of classical novae

Theoretical light curves for the decay phase of classical novae are presented, and the information to be derived from comparison between theory and observation (e.g., for Nova Cygni 1978 = Nova V1688 Cyg) is outlined. Using optically thick wind theory, we construct sequences of models consisting of a white dwarf of mass 1.2, 1.0, and 0.8 M⊙, and an envelope of uniform chemical composition [(X, Y, Z) = (0.45, 0.226, 0.324)] through which a steady wind flows. Hydrogen is burning at the base of the envelope. Decay time scales of much less than 1 yr, as found for many classical novae, are featured. Essential for achieving such short decay times is a large opacity near the surface. The required large opacity is achieved by adopting a large over-abundance of heavy elements, as observed in many classical novae, and by assuming an opacity enhancement of ∼30%-60% at temperatures T < 8 × 10⁵ K, as suggested by recent developments in opacity calculations. We present theoretical visual and UV light curves, as well as the time dependence of the photospheric velocity. We identify several observable characteristics which are particularly helpful for deriving useful information from a comparison with theoretical predictions and emphasize the importance of making new opacity calculations for conditions found in nova envelopes: R = (density in g cm^-3)/(temperature in 10⁶ K)³ ∼ 10^-7-10^-5, small X, large Z.