Hydrogen Burning on Accreting White Dwarfs: Stability, Recurrent Novae, and the Post-Nova Supersoft Phase

Abstract

We examine the properties of white dwarfs (WDs) accreting hydrogen-rich matter in and near the stable burning regime of accretion rates as modeled by time-dependent calculations done with Modules for Experiments in Stellar Astrophysics (MESA). We report the stability boundary for WDs of masses between 0.51 M and 1.34 M as found via time-dependent calculations. We also examine recurrent novae that are accreting at rates close to, but below, the stable burning limit and report their recurrence times. Our dense grid in accretion rates finds the expected minimum possible recurrence times as a function of the WD mass. This enables inferences to be made about the minimum WD mass possible to reach a specific recurrence time. We compare our computational models of post-outburst novae to the stably burning WDs and explicitly calculate the duration and effective temperature (T eff ) of the post-novae WD in the supersoft phase. We agree with the measured turnoff time -T eff relation in M31 by Henze and collaborators, infer WD masses in the 1.0-1.3 M range, and predict ejection masses consistent with those observed. We close by commenting on the importance of the hot helium layer generated by stable or unstable hydrogen burning for the short-and long-term evolution of accreting white dwarfs.

Observational signatures of SNIa progenitors, as predicted by models

“…All these estimates rely on the assumption of steady state burning. Wolf et al (2013) define steady state burning as stable when an increase in temperature causes the cooling rate to increase more than the energy generation rate. In their models, the lower limit for steady state burning is 3.8 × 10 −7 M⊙yr −1 for a 1.34M⊙ WD and lower for less massive WDs.…”

Constraining the role of novae as progenitors of type Ia supernovae

“…The agreement is quite good even between authors drawing opposite conclusions regarding the existence of the stability strip. In the parameter range of interest, there is a good agreement between analytical (Fujimoto 1982) and more sophisticated numerical calculations, as well as between single flash (Fujimoto 1982;Starrfield 2014) and multicycle (Yaron et al 2005;Wolf et al 2013) calculations. We note that the latter two probably provide a more accurate representation of the accreting WD, and their results agree to much better than a factor of ∼2.…”

“…In the classical picture, copious amounts of material are lost in the nova explosion, especially at the lower mass-accretion rates (Ṁ < ∼ 10 −7 −10 −8 M yr −1 in the Yaron et al 2005 calculations, for example), rendering the growth of the WD mass impossible or insignificant in the context of SN Ia progenitors. Importantly, several authors have found that the transition from unstable to stable burning is sharp -there is a discontinuity in the stability of burning, with the large amplitude flashes occurring very close to the stability strip (e.g., Wolf et al 2013;Kato et al 2014). However, the main aspects of this picture have been contested by several authors.…”

“…This quantity is derived from sufficiently well understood physical principles and has been computed by a number of groups (for example , Fujimoto 1982;Townsley & Bildsten 2004;Yaron et al 2005;Wolf et al 2013;Kato et al 2014 and others). A comparison of some of the results is shown in Fig.…”

“…Their calculations only covered the mass-accretion rates typical for classical novae and did not extend beyond 10 −8 M yr −1 . However, at high mass-accretion rates, > ∼ 10 −7 M yr −1 , the properties of nova outbursts do not strongly depend on the core temperature because the overlying hot He layer from previous outbursts acts as a heat barrier (Townsley & Bildsten 2004;Wolf et al 2013). We therefore assumed a core temperature of 10 7 K in our baseline configuration.…”