Synthetic Spectral Analysis of the Hot Component in the S-Type Symbiotic Variable EG Andromedae

Abstract

We have applied grids of non-LTE (NLTE) high-gravity model atmospheres and optically thick accretion disk models for the first time to archival IUE and FUSE spectra of the S-type symbiotic variable EG Andromedae taken at superior spectroscopic conjunction, when Rayleigh scattering should be minimal and the hot component is viewed in front of the red giant. For EG And's widely accepted, published hot-component mass, orbital inclination, and distance from the Hipparcos parallax, we find that hot, high-gravity, NLTE photosphere model fits to the IUE spectra yield distances from the best-fitting models that agree with the Hipparcos parallax distance, but at temperatures substantially lower than the modified Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same orbital phase as the IUE spectra yield the same temperature as the IUE temperature (50,000 K). However, for the same hot-component mass, inclination, and parallax-derived distance, accretion disk models at moderately high inclinations, ~60°–75°, with accretion rates from 1 × 10-8 to 1 × 10-9 M⊙ yr-1 for white dwarf masses Mwd = 0.4 M⊙ yield distances grossly smaller than the distance from the Hipparcos parallax. Therefore, we rule out an accretion disk as the dominant source of the far-UV flux. Our findings support a hot bare white dwarf as the dominant source of far-UV flux.