Analysis of archive UV observations of the symbiotic nova PU Vul during its nebular stage and brightness minimum in 1993–1994

Abstract

We present an analysis of the poorly studied UV spectra of the symbiotic Nova PU Vul taken in 1991–1996. The continuum spectral energy distribution during that period (the nebular phase) can be reproduced by the standard model, which includes radiation from the hot component and nebula, in contrast to spectra observed in the 1980s, when the hot component of PU Vul was in its “supergiant” stage. The hot component’s temperature gradually increased from74 000 K (1991) to 100 000 K (1996), and the evolution of this component reflected by the Hertzsprung-Russell diagram corresponds to theoretical models describing the evolution of a thermonuclear outburst in the envelope of a white dwarf. Based on our analysis of UV observations of the eclipse in 1993–1994, we estimate the size of the cool component to have been at least 285R⊙. This confirms earlier suggestions that the cool component of PU Vul belongs to luminosity class II (bright giants) or I (supergiants), rather than class III (normal giants). Our analysis of Rayleigh scattering of the hot component’s radiation on atomic hydrogen in the extended atmosphere of the cool component suggests that the mass-loss rate of the cool component of PU Vul is variable.