THE HOT STARS IN SYMBIOTIC SYSTEMS

Abstract

ABSTRACT Symbiotic stars are interacting binaries, consisting of a late giant and a very hot companion, whose radiation ionizes the wind from the cool star. They strike the observers by their complex spectra and variability. A small subgroup, the symbiotic novae, undergo outbursts with an amplitude of several magnitudes and a duration of several decades. The ionizing binary component is usually too hot to be observed in the optical light. It emits mainly EUV photons, and in spectral regions better accessible to observers, nebula and cool star are much brighter than the hot star. Direct observations of hot components have therefore been restricted to special cases. Consequently, our knowledge about the hot components was poor. This thesis presents indirect methods allowing to extract crucial information on the ionizing star from the spectrum of the ionized nebula. Fundamental characteristics such as temperature, radius, and outburst energy are determined. They characterize the nature of the hot star, its outbursts, and the evolutionary status of the system. The IUE archive proved best suited as observational base. It contains thousands of far UV spectra of symbiotic stars. The UV continuum and the He II lambda-1640 recombination line turned out to be particularly sensitive to the hot star's parameters. For one object ROSAT observations of photospheric X-ray emission exist. They confirm the results obtained from IUE spectra. Unfortunately, only a subset of the symbiotics are bright enough to be observable in the UV, and moreover, the outburst of most symbiotic novae started long before the advent of space observatories. Possibilities to use optical spectroscopy are explored for these cases. T* can be derived from the observed ionization stages, and L* can be estimated from UBV magnitudes. The main result is displayed in Figure 1. Typically, the hot component of a symbiotic system has a radius ~0.1~R⊙, a surface temperature ~100,000~K, and a luminosity ~1000~L⊙. They occupy the same portion of the HR diagram as nova remnants and central stars of planetary nebulae. Like these, they seem to be basically white dwarfs with a hot atmosphere. They have not yet cooled or have been re-heated due to accretion of matter from the red giant. In particular, the results for the outbursts of symbiotic novae confirm the commonly adopted scenario of a thermo-nuclear event in an accreted layer on the surface of a white dwarf, largely analogous to a classical nova outburst. Unlike in classical novae, the cool component remains outside the outburst event, and the accreted matter slowly burns out over decades. While classical novae are hard to investigate when fading, the evolution of symbiotic novae can be tracked with the help of the subsisting nebular emission from the ionized part of the red giant's wind. The evolutionary paths roughly follow the theoretical models for novae. During the outburst of a symbiotic nova an energy around ~1047~erg is released, i.e. ~10-5 M⊙ of hydrogen is burnt. Besides supernovae, the outburst of symbiotic novae are the most energetic stellar events. The thesis is written in German.