THE EFFECT OF COMPOSITION ON NOVA IGNITIONS

The accretion of hydrogen-rich matter onto C/O and O/Ne white dwarfs (WDs) in binary systems may lead 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 carbon mass fractions <2 × 10−3 and the high accretion rates ⩾10−9 M☉ yr−1 that we consider, we find that carbon, which is usually assumed to trigger the runaway via proton captures, is instead depleted and converted to 14N. Additionally, we quantify the importance of 3He, finding that convection is triggered by 3He+3He reactions for 3He mass fractions >2 × 10−3. 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 the composition of the material accreted from the companion. 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 3He with a mass fraction of 10−3, 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 × 104 K for a 1.0 M☉ WD accreting at 10−8 M☉ yr−1.