Electron Captures on 14N as a Trigger for Helium Shell Detonations

White dwarfs (WDs) that accrete helium at rates similar to 10(-8) M-circle dot yr(-1), such as those in close binaries with sdB stars, can accumulate large (greater than or similar to 0.1 M-circle dot)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 N-14(e(-), v) C-14(alpha, gamma)O-18(NCO), including a recent measurement for the C-14(alpha, gamma)O-18 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 3 alpha ignitions, 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 degrees 11223 and find that the inclusion of these effects predicts ignition of a 0.153 M-circle dot 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.