The nature of premaximum halts of classical nova outbursts: V723 Cassiopeiae and V463 Scuti

Abstract

We present a new interpretation of long premaximum halts of nova outbursts. For V723 Cas (Nova Cas 1995) and V463 Sct (Nova Sct 2000), we have reproduced light curves, excluding the brightness maxima, starting from the long premaximum halt through the late decay phase of the outbursts using a steady-state optically thick wind model. When the hydrogen-rich envelope of the white dwarf (WD) is massive enough, the star expands to \sim 100 R_\sun or over and its surface temperature decreases to below 7000 K. At this supergiant mimicry stage, the changes in both the photospheric radius and the temperature are small against the large increase in the envelope mass. These changes cause a saturation in visual magnitude that lasts a long time before it begins to decline. This saturation is known as the premaximum halt of a classical nova outburst. The visual magnitude during the saturation is close to the bolometric magnitude, which is an upper limit for a given WD mass. Since the WD masses are estimated to be 0.59 M_\sun for V723 Cas and 1.1 M_\sun for V463 Sct by fitting the decline rate of nova light curves, we can determine the absolute magnitude of premaximum halts. It is a refined Eddington luminosity. Thus, the premaximum halt of a nova works as a standard candle.