On the ultraviolet spectrum of AG Peg.

Abstract

AG Peg has displayed the slowest known nova outburst. Since the system contains an M giant, it can be classified as symbiotic star. Presented are ultraviolet spectra of AG Peg taken with the IUE satellite. A variety of regimes having characteristically different spectral features is found. These include a high-velocity wind in which high-ionization P Cygni lines are seen and a nebular emission zone of lower velocity but very high electron density. These data are interpreted in terms of a thermonuclear event on the surface of a white dwarf which has gradually accreted hydrogen-rich gas from the wind of the M giant. The high-velocity wind is a manifestation of mass loss from this star. The ionization of the hot component extends close to the surface of the M giant, and there gives rise to the lower-velocity emission. A tail of this emission forms behind the M star by ablation in the intense ultraviolet radiation and hot wind of the companion. This tail gives the illusion of a stream of gas flowing from the hot star to the cool. This ablation from the M giant is paralleled with the similar situation thought to exist in quasars and active galaxies.