A new evolutionary path to type Ia supernovae: A helium-rich supersoft x-ray source channel

We have found a new evolutionary path to Type Ia supernovae (SNe Ia) that has been overlooked in previous work. In this scenario, a carbon-oxygen white dwarf (C+ O WD) is originated not from an asymptotic giant branch star with a C+ O core but from a red giant star with a helium core of ∼ 0.8-2.0 M_⊙. The helium star, which is formed after the first common envelope evolution, evolves to form a C + O WD of ∼0.8-1.1 M_⊙, transferring a part of the helium envelope onto the secondary mainsequence star. This new evolutionary path, together with the optically thick wind from mass-accreting white dwarf, provides a much wider channel to SNe Ia than previous scenarios. A part of the progenitor systems are identified as luminous supersoft X-ray sources or recurrent novae such as U Sco, which are characterized by the accretion of helium-rich matter. The white dwarf accretes hydrogen-rich, helium-enhanced matter from a lobe-filling, slightly evolved companion at a critical rate and blows excess matter into the wind. The white dwarf grows in mass to the Chandrasekhar mass limit and explodes as an SN Ia. A theoretical estimate indicates that this channel contributes a considerable part of the inferred rate of SNe Ia in our Galaxy, i.e., the rate is about 10 times larger than the previous theoretical estimates for white dwarfs with slightly evolved companions.