Helium ignition on accreting neutron stars with a new triple-α reaction rate

Abstract

We investigate the effect of a new triple-α reaction rate from Ogata et al. on helium ignition conditions on accreting neutron stars and on the properties of the subsequent type I X-ray burst. We find that the new rate leads to significantly lower ignition column density for accreting neutron stars at low accretion rates. We compare the results of our ignition models for a pure helium accretor to observations of bursts in ultracompact X-ray binaries (UCXBs), which are believed to have nearly pure helium donors. For ^._m > 0.001 ^._m_(Edd), the new triple-α reaction rate from Ogata et al. predicts a maximum helium ignition column of ~3 × 10^9 g cm^(−2), corresponding to a burst energy of ~4 × 10^(40) erg. For ^._m ~ 0.01 ^._m_(Edd) at which intermediate long bursts occur, the predicted burst energies are at least a factor of 10 too low to explain the observed energies of such bursts in UCXBs. This finding adds to the doubts cast on the triple-α reaction rate of Ogata et al. by the low-mass stellar evolution results of Dotter & Paxton.