The first binary star evolution model producing a Chandrasekhar mass white dwarf

Today, Type Ia supernovae are essential tools for cosmology, and recognized as major contributors to the chemical evolution of galaxies. The construction of detailed supernova progenitor models, however, was so far prevented by various physical and numerical difficulties in simulating binary systems with an accreting white dwarf component, e.g., unstable helium shell burning which may cause significant expansion and mass loss. Here, we present the first binary evolution calculation which models both stellar components and the binary interaction simultaneously, and where the white dwarf mass grows up to the Chandrasekhar limit by mass accretion. Our model starts with a 1.6 Msun helium star and a 1.0 Msun CO white dwarf in a 0.124 day orbit. Thermally unstable mass transfer starts when the CO core of the helium star reaches 0.53 Msun, with mass transfer rates of 1...8 times 10^{-6} Msun/yr. The white dwarf burns the accreted helium steadily until the white dwarf mass has reached ~ 1.3 Msun and weak thermal pulses follow until carbon ignites in the center when the white dwarf reaches 1.37 Msun. Although the supernova production rate through this channel is not well known, and this channel can not be the only one as its progenitor life time is rather short (~ 10^7 - 10^8 yr), our results indicate that helium star plus white dwarf systems form a reliable route for producing Type Ia supernovae.