2017ApJ...845...97B

White dwarfs (WDs) that accrete helium at rates ∼10^–8 M_☉ yr^–1, such as those in close binaries with sdB stars, can accumulate large ( 0.1 M_☉) helium envelopes, which are likely to detonate. We perform binary stellar evolution calculations of sdB+WD binary systems with MESA, incorporating the important reaction chain ¹⁴N(e^–,ν)¹⁴C(α,γ)¹⁸O (NCO), including a recent measurement for the ¹⁴C(α,γ)¹⁸O rate. In large accreted helium shells, the NCO reaction chain leads to ignitions at the dense base of the freshly accreted envelope, in contrast to 3αignitions, which occur away from the base of the shell. In addition, at these accretion rates, the shells accumulate on a timescale comparable to their thermal time, leading to an enhanced sensitivity of the outcome on the accretion rate history. Hence, time dependent accretion rates from binary stellar evolution are necessary to determine the helium layer mass at ignition. We model the observed sdB+WD system CD -30^○11223 and find that the inclusion of these effects predicts ignition of a 0.153 M_☉ helium shell, nearly a factor of two larger than previous predictions. A shell with this mass will ignite dynamically, a necessary condition for a helium shell detonation.