If accreting white dwarfs in binary systems are to produce type Ia supernovae, they must grow beyond the Chandrasekhar mass limit. Proving conclusively that a white dwarf has grown substantially since its birth is challenging. Slow accretion of hydrogen leads to the erosion, rather than the growth of WDs. Rapid hydrogen accretion does lead to growth of a helium layer, due to both decreased degeneracy and the inhibition of mixing of the accreted hydrogen with the underlying WD. However, until recently, simulations of helium-accreting WDs all claimed to show the explosive ejection of a helium envelope once it exceeded 0.1 M\sun. Hillman et al. (2016) have now demonstrated that tens of such eruptions monotonically heat the underlying WD, decrease its degeneracy, the explosive power of subsequent flashes, and the amount of mass subsequently ejected all the way down to zero. This mechanism allows WDs to grow in mass unchecked. An exemplar of this process may be a nearly-Chandrasekhar mass WD in a nova that recurs every six months in M31 (Darnley et al. 2015). Because CO WDs cannot be born with masses above ~1.1 M\sun, any such object, in excess of ~1.2 M\sun, must have grown substantially. We demonstrate that the WD in RS Oph is in the mass range 1.2-1.4M\sun}. We compare UV spectra of RS Oph with those of ONe novae, and with novae erupting on CO WDs. The RS Oph WD is clearly CO, demonstrating that it has grown substantially since birth, and a prime SNIa candidate.
J. Mikolajewska and M. Shara
Wed, 1 Mar 17
Comments: 8 pages, 3 figures, submitted to ApJ