TEMPERATURE AND LUMINOSITY OF HOT COMPONENTS IN SYMBIOTIC STARS

The nature of the hot component in symbiotic binaries is still controversial. The existence of a hot radiative source is inferred from the presence of a photoionized nebula. The nebular spectrum points to typical radiation temperatures of T* almost-equal-to 100 000 K. The bulk of the ionizing radiation is therefore emitted below 1200 angstrom, the effective short end of IUE (International Ultraviolet Explorer). We set ourselves the task of deriving temperatures and luminosities for the hot component of a representative sample of symbiotic systems. For that purpose the Zanstra method is adapted to the special requirements of symbiotic stars. T* is basically derived by comparing the nebular recombination line He II lambda-1640 with the underlying continuum. We allow for the fact that at this wavelength the observed continuum originates in the hot stellar source as well as in the nebula. By comparing in addition the continuum flux at 1640 angstrom with the nebular Balmer continuum at the long end of the IUE wavelength range, we can distinguish particle bounded from radiation bounded nebulae. We have looked at all symbiotic stars observed by IUE. For the symbiotic stars emitting He II-lambda-1640 we find 0.3 L. less-than-or-similar-to L less-than-or-similar-to 37 000 L. and 55 000 K less-than-or-similar-to T* less-than-or-similar-to 210 000 K. The derived radii range approximately from 0.002 R. to 0.8 R.. In addition, we determine interstellar extinctions E(B-V). Where binary periods are known, we have estimated mass-loss rates for the red giants; typical values are M(c) almost-equal-to 10(-7) M.yr-1. As several symbiotic systems have been suggested to possess accretion disks, we have searched the spectra for such signatures, however, we found no evidence for permanent disks emitting a substantial fraction of the ionizing radiation. From our investigation it appears that the hot components in symbiotic systems lie on the post red giant track. In some cases this may be a white dwarf, in others it may be closer to the central star of a planetary nebula, and in other cases it is a white dwarf in a slow nova-like outburst, which may last for decades. The two stars in a symbiotic system are thus a red giant and a post red giant. The system as a whole is at a late state of stellar evolution.