A Wide Symbiotic Channel to Type Ia Supernovae

Abstract

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) in which strong winds from the accreting WD play a key role increasing the WD mass to the Chandrasekhar mass limit. However, the occurrence frequency of SNe Ia through this channel is still controversial. Here we propose two new evolutionary processes that 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 an initial separation as large as a i ≲ 40,000 R ☉ . Such a binary consists of a low-mass main-sequence (MS) star and an asymptotic giant branch (AGB) star that is undergoing a 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 to form a common envelope (CE) evolution. After the CE evolution, the binary becomes a pair consisting of a carbon-oxygen WD and an MS star. When the MS star evolves to an RG, a WD+RG system is formed. Therefore, the WD+RG binary can form from much wider binaries than our earlier estimate, which was constrained by a i ≲ 1500 R ☉ . (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 the RG to the 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-to-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 (10 or more times) wider range of the binary parameters than our earlier estimate predicted. 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, such as T CrB or RS Oph, depending on the wind status.