The accretion of solar material onto white dwarfs: No mixing with core material implies that the mass of the white dwarf is increasing

Abstract

Cataclysmic Variables (CVs) are close binary star systems with one component a white dwarf (WD) and the other a larger cooler star that fills its Roche Lobe. The cooler star is losing mass through the inner Lagrangian point of the binary and some unknown fraction of this material is accreted by the WD. One consequence of the WDs accreting material, is the possibility that they are growing in mass and will eventually reach the Chandrasekhar Limit. This evolution could result in a Supernova Ia (SN Ia) explosion and is designated the Single Degenerate Progenitor (SD) scenario. One problem with the single degenerate scenario is that it is generally assumed that the accreting material mixes with WD core material at some time during the accretion phase of evolution and, since the typical WD has a carbon-oxygen (CO) core, the mixing results in large amounts of carbon and oxygen being brought up into the accreted layers. The presence of enriched carbon causes enhanced nuclear fusion and a Classical Nova (CN)explosion. Both observations and theoretical studies of these explosions imply that more mass is ejected than is accreted, and that the process repeats. Thus, the WD in a Classical Nova system is decreasing in mass and cannot be a SN Ia progenitor. However, the composition in the nuclear burning region is important and, in new calculations reported here, the consequences to the WD of no mixing of accreted material with core material have been investigated and it is assumed that the material involved in the explosion has only a Solar composition. WDs with a large range in initial masses and mass accretion rates have been evolved. I find that once sufficient material has been accreted, nuclear burning occurs in all evolutionary sequences and continues until a thermonuclear runaway (TNR) occurs and the WD either ejects a small amount of material or its radius grows to about 10 12 cm and the calculations are stopped. In all cases where mass ejection occurs, the mass of the ejecta is far less than the mass of the accreted material. Therefore, all the WDs are growing in mass. It is also found that the accretion time to explosion can be sufficiently short for a 1.0M ⊙ WD that a recurrent nova explosion can occur on a WD that is lower than typically assumed for the WDs in these systems. Finally, the predicted surface temperatures when the WD is near the peak of the explosion imply that only the most massive WDs will be significant X-ray emitters at this time.

White dwarf masses in cataclysmic variables

“…This contradicts standard theories for nova outbursts (Prialnik 1986;Prialnik & Kovetz 1995;Yaron et al 2005). WD masses can only grow in CVs if no mixing of core matter with accreted matter is assumed (Williams 2013;Starrfield 2015), which is unrealistic. However, given the large discrepancy between the observed and predicted WD mass distributions, we need to investigate all possible scenarios.…”

Constraining the role of novae as progenitors of type Ia supernovae

“…However, the main aspects of this picture have been contested by several authors. First, the existence of the stability strip has been questioned (e.g., Idan et al 2013;Starrfield 2014;Hillman et al 2015). In fact, no stable burning is reported in the Yaron et al (2005) tables either, although the ejected mass becomes zero at the highest mass-accretion rates.…”

“…Second, it has been suggested that mass accumulation at appreciable rates is possible in the broad range of the mass-accretion rates, including those traditionally considered to be associated with the nova regime. In fact, it has been claimed that any mass accumulation by the WD is associated with regular thermonuclear explosions (e.g., Starrfield 2014;Hillman et al 2015). In a related development, progenitor models have been proposed that involve WDs accreting in the presumably unstable nuclear burning regime throughout or at least a part of their accretion history (e.g., Starrfield et al 1985;Hachisu & Kato 2001;Hillman et al 2015).…”

“…However, close to the stability line, M ∼ 10 −7 M yr −1 , nova explosions are relatively weak and are not accompanied by significant mass loss; therefore, mass accumulation by the WD may be possible in this regime (Hillman et al 2015). Moreover, work by Starrfield (2014), Hillman et al (2015) and others has contested the entire existence of the stability strip. Instead, they suggested that nuclear burning is unstable in the entire mass accretion rate range up to very high rates, where common envelopes form.…”

“…In particular, Gilfanov & Bogdán (2010) demonstrated that the observed soft X-ray luminosity of early-type galaxies is too low to allow a significant population of hot (T > ∼ 5 × 10 5 K) accreting WDs with stable nuclear burning, which are required to explain the SN Ia rates in these galaxies. Populations of cooler WDs 1 have been constrained by Woods & Gilfanov (2013, 2014 and Johansson et al (2014) using the strength of He II recombination lines in the line emission spectra of passively evolving galaxies. Their work effectively excluded the parts of the parameter space corresponding to accreting WDs in and above the stability strip, at least in passively evolving galaxies.…”