Optically thick winds in nova outbursts

Citations Over TimeTop 10% of 1994 papers

Abstract

Using the OPAL opacity, we have calculated a sequence of optically thick wind solutions, which mimics a time-dependent evolution of the decay phase of novae. Strong winds are driven by a large peak in the OPAL opacity and, as a result, theoretical timescale of nova duration is drastically shortened compared with the use of the old opacity. Thus, we are able to resolve a theoretical problem of nova duration: the theoretical timescale of nuclear burning for the initial envelope mass at ignition is too long to reconcile with the observational durations of novae. Good quality light curves are automatically obtained from the combination of the optically thick wind theory and the OPAL opacity. We have compared our theoretical light curves with the observations of a well-studied classical nova, Nova Cygni 1978, and found that our 1.0 solar mass white dwarf model shows an excellent agreement with the observations both for the visual light curves and for the ultraviolet light curves and also for the expansion velocities of the envelope. These results strongly suggest the validity of our steady state approach and indicate that the optically thick wind really occurs on the white dwarf at least in the decay phase of the nova. In other words, optically thick winds, in which the matter is accelerated deep inside the photosphere, are the main acceleration mechanism in the decay phase of novae. Comparison of our theoretical light curves with observational ones enables us to determine the mass of the white dwarf and the distance of the star. The distance to Nova Cygni 1978 is estimated to be 2.9 - 3.1 kpc with the white dwarf mass of 1.0 solar mass. Thus, quantitative studies of light curve fitting will be able to provide/add useful information of binary parameters that have been poorly known. The effects of the drag luminosity in the common envelope phase have also been estimated by a one-dimensional (spherical) model. It is found that the drag luminosity is as small as or smaller than 1% of the photospheric luminosity because the density of the envelope drops sharply near/outside the accelerating region of wind. Only for relatively low-mass white dwarfs, the decline rate of the light curves is much affected by the effects of a companion, for example, the decline rate of a 0.6 solar mass white dwarf and a 0.2 solar mass companion shows a similar rate of 0.7 solar mass without a companion. However, we may conclude that the accuracy of the mass determination of the white dwarf is still within 0.1 solar mass.

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