The Effect of Composition on Nova Ignitions

The accretion of hydrogen-rich matter onto C/O and O/Ne white dwarfs in binary systems leads to unstable thermonuclear ignition of the accreted envelope, triggering a convective thermonuclear runaway and a subsequent classical, recurrent, or symbiotic nova. Prompted by uncertainties in the composition at the base of the accreted envelope at the onset of convection, as well as the range of abundances detected in nova ejecta, we examine the effects of varying the composition of the accreted material. For high accretion rates and carbon mass fractions < 2×10, we find that carbon, which is usually assumed to trigger the runaway via proton captures, is instead depleted and converted to N. Additionally, we quantify the importance of He, finding that convection is triggered by He+He reactions for He mass fractions > 2 × 10. These different triggering mechanisms, which occur for critical abundances relevant to many nova systems, alter the amount of mass that is accreted prior to a nova, causing the nova rate to depend on accreted composition. Upcoming deep optical surveys such as Pan-STARRS-1, Pan-STARRS-4, and the Large Synoptic Survey Telescope may allow us to detect the dependence of nova rates on accreted composition. Furthermore, the burning and depletion of He with a mass fraction of 10, which is lower than necessary for triggering convection, still has an observable effect, resulting in a pre-outburst brightening in disk quiescence to > L⊙ and an increase in effective temperature to 6.5× 10 K for a 1.0 M⊙ white dwarf accreting at 10 −8 M⊙ yr . Subject headings: accretion, accretion disks — binaries: close — instabilities — novae, cataclysmic variables — nuclear reactions, nucleosynthesis, abundances — white dwarfs