Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apjs..105..145i&link_type=abstract
Astrophysical Journal Supplement v.105, p.145
Astronomy and Astrophysics
Astronomy
78
Accretion, Accretion Disks, Stars: Evolution, Stars: Binaries: Symbiotic, Stars: White Dwarfs, Stars: Novae, Cataclysmic Variables
Scientific paper
On comparison with the observed properties of symbiotic stars and cataclysmic variables, theoretical models of accreting degenerate dwarfs allow one to estimate the evolutionary status of the accretor and to explore the role of physical processes such as wind mass loss and element mixing (which are not yet properly taken into account in the models) in affecting the evolution of real accreting white dwarfs. An examination of existing numerical models of cooling and shell nuclear-burning degenerate dwarfs of different masses reveals that there is no single, unique relationship between the maximum (plateau) luminosity and dwarf mass. Instead, particularly when the dwarf is of low mass (≤0.7 Msun), the relationship depends significantly on the thermal history of the dwarf. For example, an accreting cold dwarf of mass 0.3 Msun can reach 3000 Lsun during a hydrogen-burning phase, whereas a hot dwarf must have a mass larger than ˜0.55 Msun to achieve this same luminosity during hydrogen burning. For cold white dwarf accretors, a relationship between plateau luminosity and dwarf mass exists and, when applied to symbiotic novae for which plateau luminosities are known, it yields an average mass of 0.77 Msun.
During a nova-like outburst of a symbiotic star, the phase of hydrogen burning in a shell at high luminosity is broken into two parts, one at large radius and one at small radius. Comparison of the temporal evolution of symbiotic stars with that of theoretical models suggests that a stellar wind emitted by the accretor shortens the duration of the large-radius part of the high-luminosity phase. If the dwarf mass is larger than ˜0.55 Msun, the surface temperature during the hydrogen-burning phase at small radius is larger than 105 K (dwarf radius ≤0.16 Rsun), and the binary contributes to the population of ultrasoft X-ray sources. For stars with helium-burning shells, the lower limit on surface temperature is Te ˜35,000 K, corresponding to a radius of ˜6 Rsun. Whereas in symbiotic stars the accumulation efficiency depends on wind mass loss from the accretor, m cataclysmic variables the accumulation efficiency depends primarily on the Roche-lobe radius of the accretor.
A qualitative analysis of the chemical composition of matter ejected during nova outbursts supports in most respects the existing theoretical picture of shell outbursts in symbiotic and classical novae. It is proposed that a shell helium-burning outburst (after a sufficient number of hydrogen shell flashes in the case of low-mass accretors) or wind mass loss during the pre-accretion phase (in the case of high-mass accretors) removes a helium buffer layer which otherwise inhibits the mixing together of hydrogen-rich matter and white dwarf matter in cataclysmic variables, thereby making possible classical nova outbursts with large enhancements of heavy elements. A helium shell flash may not be attainable in symbiotic stars because accretion from the wind of a giant companion is inefficient and of short duration. A test of this conjecture requires observations which can permit an unambiguous estimate of the composition of the ejecta of the outbursting star in symbiotic novae.
Iben Icko Jr.
Tutukov Aleksandr V.
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