A Wide Symbiotic Channel to Type Ia Supernovae

As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a symbiotic binary system consisting of a white dwarf (WD) and a low mass red-giant (RG), where strong winds from the accreting WD play a key role to increase the WD mass to the Chandrasekhar mass limit. However, the occurrence frequency of SNe Ia through this channel has been still controversial. Here we propose two new evolutionary processes which make the symbiotic channel to SNe Ia much wider. (1) We first show that the WD + RG close binary can form from a wide binary even with such a large initial separation as ai ∼< 40000R⊙. Such a binary consists of an AGB star and a low mass main-sequence (MS) star, where the AGB star is undergoing superwind before becoming a WD. If the superwind at the end of AGB evolution is as fast as or slower than the orbital velocity, the wind outflowing from the system takes away the orbital angular momentum effectively. As a result the wide binary shrinks greatly to become a close binary. Then the AGB star undergoes a common envelope (CE) evolution. After the CE evolution, the binary becomes a pair of a carbon-oxygen WD and the MS star. When the MS star evolves to a RG, a WD + RG system is formed. Therefore, the WD + RG binary can form from much wider binaries than our earlier estimate which is constrained by ai ∼< 1500R⊙. (2) When the RG fills its inner critical Roche lobe, the WD undergoes rapid mass accretion and blows a strong optically thick wind. Our earlier analysis has shown that the mass transfer is stabilized by this wind only when the mass ratio of RG/WD is smaller than 1.15. Our new finding is that the WD wind can strip mass from the RG envelope, which could be efficient enough to stabilize the mass transfer even if the RG/WD mass ratio exceeds 1.15. If this mass-stripping effect is strong enough, though its efficiency is subject to uncertainties, the symbiotic channel can produce SNe Ia for a much (ten times or more) wider range of the binary parameters than our earlier estimation. With the above two new effects (1) and (2), the symbiotic channel can account for the inferred rate of SNe Ia in our Galaxy. The immediate progenitor binaries in this symbiotic channel to SNe Ia may be observed as symbiotic stars, luminous supersoft X-ray sources, or recurrent novae like T CrB or RS Oph, depending on the wind status. Subject headings: binaries: symbiotic — stars: individual (T CrB, RS Oph) — stars: mass-loss — stars: novae — supernovae: general — X-rays: stars