Abstract
Accreting white dwarfs (WDs) in cataclysmic variables (CVs) provide crucial insights about the accretion of mass and angular momentum in all types of binaries, including accreting NSs and BHs. Accreting WDs are the critical component in the single degenerate pathway to SNe Ia, along with the double degenerate merger pathway, they are the standard candles of cosmology proving that the universe is accelerating and the existence of dark energy. Another key question is whether the WD in a CV can grow in mass despite the mass loss due to thousands of nova explosions in its lifetime. Angular momentum loss drives CV evolution and accreting WDs offer critically needed WD masses from Gaia distances and reliable surface temperatures to derive the most accurate accretion rates. We review the studies on accreting WDs, including WD masses, accurate rotational velocities and chemical abundances of elements. Most of the progress that has been made is based upon Hubble Space Telescope spectroscopy and FUSE spectroscopy in the UV spectral region during dwarf nova quiescence and the low states of novalike variables, when the accreting WD dominates the UV spectral range.
Highlights
A cataclysmic variable (CV) is a compact binary in which the primary, a white dwarf (WD) star, accretes matter and angular momentum from the secondary star, a main sequence-like object, filling its Roche lobe
Since the time-averaged accretion rate of CVs manifests the rate at which system angular momentum is lost, the authors of [26] found that by including an additional angular momentum loss mechanism associated with the mass transfer process itself, known as empirical consequential angular momentum loss, which is more efficient at lower white dwarf mass, into their Binary Population Synthesis Models (BPS) models, much better agreement was found with the observations
The Pala et al [16] study found no difference between the average masses of CV white dwarfs above the period gap and the average mass of CVs below the period gap
Summary
A cataclysmic variable (CV) is a compact binary (with an orbital period 1 day) in which the primary, a white dwarf (WD) star, accretes matter and angular momentum from the secondary star, a main sequence-like object, filling its Roche lobe. If a white dwarf in a CV is born massive (Mwd > 0.8M ) and grows in mass despite hundreds to thousands of nova explosions, instantaneous collapse and thermonuclear detonation will occur if the CV WD with a C-O core reaches the Chandrasekhar limit: the Type Ia supernovae: SNe Ia. we present the current state of knowledge on these vital parameters that will deepen our insights into the physics of accretion and the structure and evolution of cataclysmic variables. If an accreting white dwarf, born with a mass 0.8M (or larger) with a carbon-oxygen core grows in mass due to accretion from the Roche lobe-filling donor, it may reach the Chandrasekhar limit and undergo a Type Ia supernova explosion (SNe Ia; [3,4])—this is known as the single degenerate (SD) scenario to SN Ia. SN Ia are the standard candle providing evidence that the expansion of the universe is accelerating [5,6], implying the presence of dark energy. The availability of precise white dwarf masses will help make it possible to constrain the single degenerate pathway and potentially constrain the double degenerate pathway
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