An investigation into the nature of the symbiotic stars

Abstract

A sample of twelve symbiotic variables were examined using both low and high resolution ground-based and satellite observations between 1977 and 1981. The majority of the stars were quiescent stellar, or S-type, systems; observations were also obtained for CH Cygni, AX Persei and AG Draconis during or following their respective 1977, 1978 and 1980 eruptions. High resolution photoelectric Zeeman spectra of AG Pegasi and EG Andromedae failed to find confirming evidence for coherent surface magnetic fields; similar data for CH Cyg following the 1977 eruption also indicated such fields are not present at outburst, thus weakening the single star magnetic model. Contemporaneous optical and ultraviolet spectra for AG Peg yielded revised estimates for the orbital parameters, confirming an orbital period of 830 [plus or minus sign] 12 days and establishing the photometric period of 759 [plus or minus sign] 15 days. The visual photometric variations arise from orbital modulation by the giant secondary of an off axis hot spot formed by heating from the hot primary and/or by the pulsation of the secondary. The ultraviolet variations directly reflect the orbital modulation of the luminosity of the primary. The slow decline from the 1860 outburst suggests that the hot primary is undergoing the early evolution into the central star of a planetary nebula. Composite continuum energy distributions were derived for ten stars from 1200 A to 3.4 microns. . . . The role of accretion in the symbiotics remains uncertain. Accretion models posit a main sequence primary in order to explain the lack of a significant soft X-ray flux. The models for AG Peg and AG Dra, however, indicate the primaries are subluminous evolved stars, similar to the central objects of planetary nebulae. These results suggest accretion in the symbiotics may be sporadic, occurring in "bursts," and that at quiescence the accretion disks may dissipate.