The fraction of type Ia supernovae exploding inside planetary nebulae (SNIPs)

Using three independent directions, we estimate that the fraction of type Ia supernovae (SNe Ia) exploding inside planetary nebulae (PNe), termed SNIPs, is at least~20 per cent. Our three directions are: (i) Taking the variable sodium absorption lines in some SNe Ia to originate in massive circum-stellar matter (CSM), as has been claimed recently, we use the results of Sternberg et al. to imply that ≳ 20 per cent of SNe Ia occur inside a PN (or a PN descendant), hence classify them as SNIPs. (ii) We next use results that show that whenever there are hydrogen lines in SNe Ia, the hydrogen mass in the CSM is large, ≳1M^⊙, hence the explosion is a SNIP. We make the simplest assumption that the probability for explosion is constant in time for up to about 10⁵ yr after the merger of the core with the white dwarf (WD) in the core-degenerate scenario. The result is that at least a few tens of per cent of SNe Ia may have an SNIP origin. (iii) We examine the X-ray morphologies of 13 well-resolved close-by type Ia SN remnants (SNRs) and derive a crude upper limit, according to which 10-30 per cent of all SNRs Ia possess opposite ear-like features, which we take as evidence of SNIP origin. Our results, together with several other recent results, lead us to conclude that the two scenarios contributing most to SNe Ia are the core-degenerate and the double-degenerate scenarios. Together these two account for >95 per cent of all SNe Ia.