The nature of symbiotic stars.

Synthetic spectra from 0.1 to 3.5 μm are computed for various binary models of symbiotic stars. These models consist of three components: (1) a late-type giant or bright giant, (2) a hot companion, and (3) a surrounding nebula which is photoionized by the hot component. Hot stellar sources, disk accretion onto white dwarf stars, and disk accretion onto main sequence stars are considered as possible hot components. Available IUE and optical data for 19 systems are used to test these models over a wide range of accretion rates and blackbody luminosities. It is found that the properties of the continua of many symbiotic stars can be understood if the hot components in these systems are main sequence stars accreting material at ≡10^-5M_sun; yr^-1. Other systems seem to contain hot blackbodies (R ⪉ 2 R_sun;, T_eff ⪆ 25000K). No example of a symbiotic star containing an accreting white dwarf could be found. In a final section, thermonuclear and accretion models for the outbursts of symbiotic stars are discussed.