A new estimation of mass accumulation efficiency in helium shell flashes toward type Ia supernova explosions

We have calculated the mass accumulation efficiency during helium shell flashes to examine whether or not a carbon-oxygen white dwarf (C+O WD) grows up to the Chandrasekhar mass limit to ignite a Type Ia supernova (SN Ia) explosion. It has been frequently argued that luminous supersoft X-ray sources (SSSs) and symbiotic stars are progenitors of SNe Ia. In such systems, a C+O WD accretes hydrogen-rich matter from a companion and burns hydrogen steadily on its surface. The WD develops a helium layer underneath the hydrogen-rich envelope and undergoes periodic helium shell flashes. Using the OPAL opacity, we have reanalyzed a full cycle of helium shell flashes on a 1.3 M_⊙ C+O WD and confirmed that the helium envelope of the WD expands to blow a strong wind. A part of the accumulated matter is lost by the wind. The mass accumulation efficiency in helium shell flashes is estimated as η_He = -0.175(log M + 5.35)² + 1.05 for -7.3 < log M < -5.9 and η_He = 1 for -5.9 ≤ log Ṁ ≲ -5, where the mass accretion rate M is in units of M_⊙ yr ¹. In relatively high mass accretion rates, as expected in recent SN Ia progenitor models, the mass accumulation efficiency is large enough for C+O WDs to grow to the Chandrasekhar mass, i.e., η_He = 0.9 for log Ṁ = -6.3 and η_He = 0.57 for log Ṁ = -7.0. The wind velocity (∼1000 km s^-1) is much faster than the orbital velocity of the binary (≲300 km s^-1), and therefore the wind cannot be accelerated further by the companion's motion. We suggest observational counterparts of helium shell flashes in relation to long-term variations in supersoft X-ray fluxes of SSSs and symbiotic stars.