S. Starrfield1, C. Iliadis2, F. X. Timmes1, W. R. Hix3,
W. D. Arnett4, C. Meakin5 and W. M. Sparks5
Abstract.
We review our current knowledge about the thermonuclear processing
that occurs during the evolution of accretion onto white dwarfs
(WDs) both with and without the mixing of core with accreted
material. We present a brief summary of the single degenerate
scenario for the progenitors of Type Ia supernovae in which it is
assumed that a low mass carbon-oxygen white dwarf is growing in mass
as a result of accretion from a secondary star in a close binary
system. The growth in mass requires that more material remain on a
white dwarf after a thermonuclear runaway than is ejected by the
explosion. Recent hydrodynamic simulations of accretion of solar
material onto white dwarfs without mixing always produce a
thermonuclear runaway and ``steady burning'' does not occur. For a
broad range in WD mass (0.4 M☉ to 1.35 M☉, the
maximum ejected material occurs for the 1.25M☉ sequences and
then decreases as the white dwarf mass decreases. Therefore, the
white dwarfs are growing in mass as a consequence of the accretion
of solar material, and as long as there is no mixing of accreted
material with core material. In contrast, a thermonuclear runaway in
the accreted hydrogen-rich layers on the low luminosity WDs in
close binary systems where mixing of core matter with accreted
material has occurred is the outburst mechanism for classical (CN),
recurrent, and symbiotic novae. The differences in characteristics
of these systems is likely the WD mass and mass accretion rate. The
high levels of enrichment of CN ejecta in elements ranging from
carbon to sulphur confirm that there is dredge-up of matter from the
core of the WD and enable them to contribute to the chemical
enrichment of the interstellar medium. Therefore, studies of
classical novae can lead to an improved understanding of Galactic
nucleosynthesis, some sources of pre-solar grains, and the
Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and
the chemical composition of both the accreting material and WD
material. The properties of the outburst also depends on when, how,
and if the accreted layers are mixed with the WD core and the mixing
mechanism is still unknown.
Keywords: stars: novae, cataclysmic variables -- stars: dwarf novae -- stars: supernovae: general
1School of Earth and Space Exploration, P.O. Box 871404, Arizona State University, Tempe, AZ 85287-1404, USA
2Dept. of Physics & Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
3Dept. of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996-1200, USA
4Dept. of Astronomy, University of Arizona, Tucson, AZ, 85721, USA
5Alamos Alamos National Laboratory, Los Alamos, NM, 87545, USA