Temperatures and luminosities of symbiotic novae.

Symbiotic novae occur in double star systems, where mass lost by a red giant is accumulated by wind accretion onto a wite dwarf. When a critical mass has been reached, a nova-like outburst of long duration sets in, the outbursts lasts many dozons of years. We determine the evolution of luminosity and temperature during the outburst. We find slowly increasing temperatures, up to 200 000 K. Peak luminosities are in the range L = 10^{4.0 +/- 0.5} solar luminosity. Symbiotic novae remain at such a level for at least one decade. A significant decline in luminosity is observed in the three oldest objects. The total radiation output of a symbiotic nova is of the order of 10⁴⁷ erg. This exceeds the energy set free in classical novae. From all eruptive stellar events only supernovae are more energetic than symbiotic novae. For the accreting star the core-mass luminosity relation gives white dwarfs masses in the range 0.6 less than M/solar mass less than 1.2. We also find the the white dwarfs in symbiotic nova systems are unlikely to reach the Chandrasekhar mass limit. We find that the relative slowness in the intitial brightness increase of symbiotic novae need not necessarily be due to a slow thermonuclear event. In the appropriate environment it can just as well be due to the time needed for the expansion and growth of the ionization front. Two of our objects may qualify for that possibility. For deriving the temperature of the ionizing source in a photo-ionized gaseous nebula we suggest a formula of the form T_* (1000 K) = chi_max (eV), where chi_max is the highest observed ionization potential.