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 h as been overlooked in previous work. In this scenario, a carbon-oxygen whit e dwarf (C+O WD) is originated not from an asymptotic giant branch star wit h a C+O core but from a red giant star with a helium core of similar to 0.8 -2.0 M.. The helium star, which is formed after the first common envelope e volution, evolves to form a C+O WD of similar to 0.8-1.1 M., transferring a part of the helium envelope onto the secondary main-sequence star. This ne w evolutionary path, together with the optically thick wind from mass-accre ting white dwarf, provides a much wider channel to SNe Ia than previous sce narios. A part of the progenitor systems are identified as luminous superso ft 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-r ich, 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 t heoretical 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 wi th slightly evolved companions.