Carbon Ignition in an Accreting White Dwarf Occurs Far Below the Electron-Branch Bound

In the single-degenerate channel for Type Ia supernovae, a carbon-oxygen white dwarf gains mass from a companion until carbon ignites in its centre and the star is destroyed. This paper compares the densities at which ignition occurs, 1e12 to 5e12 kg per cubic metre, with the electron-branch density bound of 2.87e+48 g per cubic centimetre, and computes the growth times involved. At a representative ignition density of 2e12 kg per cubic metre the dwarf has reached a fraction 6.96e-40 of the bound, leaving a gap of 1.4e+39. Growth from 1.20 to 1.38 solar masses takes 1.80e+07 years at an accretion rate of 1e-8 solar masses per year, or 1.80e+06 years at 1e-7. Ignition is a thermal runaway decided by carbon fusion and neutrino losses, and it pre-empts the density bound completely. This study is simulated from stated assumptions, not measured.