UNSTABLE HELIUM SHELL BURNING ON ACCRETING WHITE DWARFS

Abstract

AM Canum Venaticorum (AM CVn) binaries consist of a degenerate helium donor and a helium, C/O, or O/Ne white dwarf accretor, with accretion rates of . For accretion rates <10−6 M☉ yr−1, the accreted helium ignites unstably, resulting in a helium flash. As the donor mass and decrease, the ignition mass increases and eventually becomes larger than the donor mass, yielding a "last-flash" ignition mass of ≲0.1 M☉. Bildsten et al. have predicted that the largest outbursts of these systems will lead to dynamical burning and thermonuclear supernovae. In this paper, we study the evolution of the He-burning shells in more detail. We calculate maximum achievable temperatures as well as the minimum envelope masses that achieve dynamical burning conditions, finding that AM CVn systems with accretors ≳0.8 M☉ will undergo dynamical burning. Triple-α reactions during the hydrostatic evolution set a lower limit to the 12C mass fraction of 0.001–0.05 when dynamical burning occurs, but core dredge-up may yield 12C, 16O, and/or 20Ne mass fractions of ∼0.1. Accreted 14N will likely remain 14N during the accretion and convective phases, but regardless of 14N's fate, the neutron-to-proton ratio at the beginning of convection is fixed until the onset of dynamical burning. During explosive burning, the 14N will undergo 14N(α, γ)18F(α, p)21Ne, liberating a proton for the subsequent 12C(p, γ)13N(α, p)16O reaction, which bypasses the relatively slow α-capture onto 12C. Future hydrodynamic simulations must include these isotopes, as the additional reactions will reduce the Zel'dovich–von Neumann–Döring length, making the propagation of the detonation wave more likely.