EG And: FUSE and HST/STIS Monitoring of an Eclipsing Symbiotic Binary

TL;DR

This study uses ultraviolet and optical observations of the symbiotic binary EG And to analyze the circumstellar gas and wind processes of the giant star, revealing detailed ionization and wind characteristics.

Contribution

First detailed spatially-resolved ultraviolet analysis of the wind and circumstellar environment in an eclipsing symbiotic binary, providing insights into mass-loss processes.

Findings

High ionization features vary on hourly timescales.

Wind absorption lines indicate chromospheric and wind material.

Dwarf radiation does not dominate the wind acceleration region.

Abstract

We present highlights and an overview of 20 FUSE and HST/STIS observations of the bright symbiotic binary EG And. The main motivation behind this work is to obtain spatially-resolved information on an evolved giant star in order to understand the mass-loss processes at work in these objects. The system consists of a low-luminosity white dwarf and a mass-losing, non-dusty M2 giant. The ultraviolet observations follow the white dwarf continuum through periodic gradual occultations by the wind and chromosphere of the giant, providing a unique diagnosis of the circumstellar gas in absorption. Unocculted spectra display high ionization features, such as the OVI resonance doublet which is present as a variable (hourly time-scales), broad wind profile, which diagnose the hot gas close to the dwarf component. Spectra observed at stages of partial occultation display a host of low-ionization, narrow, absorption lines, with transitions observed from lower energy levels up to ~5eV above ground. This absorption is due to chromospheric/wind material, with most lines due to transitions of SiII, PII, NI, FeII and NiII, as well as heavily damped HI Lyman series features. No molecular features are observed in the wind acceleration region despite the sensitivity of FUSE to molecular hydrogen. From analysis of the ultraviolet dataset, as well as optical data, we find that the dwarf radiation does not dominate the wind acceleration region of the giant, and that observed thermal and dynamic wind properties are most likely representative of isolated red giants.