ON THE EROSION OF THE HELIUM LAYER IN WHITE-DWARF NOVA PROGENITORS

Abstract

ABSTRACT Hydrodynamic simulations of nova outbursts--accretion, thermonuclear runaway, mass loss and decline--have been carried out, assuming white dwarf (WD) progenitors of carbon and oxygen with a relatively thick helium outer layer. Evolutionary sequences (of several cycles) have been computed for WD masses (MWD) of 0.75 solar mass, 1.00 solar mass, and 1.25 solar mass and for accretion rates between M = 10-8 solar mass yr-1 and M = 10-11 solar mass yr-1. Mixing between accreted mass and the underlying WD material is assumed to occur only via the diffusion induced convection mechanism. The ejected mass is found to exceed the accreted mass whenever MWD > 0.75 solar mass or M < 10-8 solar mass yr-1. Thus, the helium outer layer is gradually eroded, exposing the carbon oxygen core. Until the erostion is completed (some 108 yr) the ejecta is only enriched in helium; thereafter, dredge-up from the carbon-oxygen core will produce the high heavy element abundances typical of most observed nova ejecta. We expect that if mass loss is enhanced due to the drag energy deposited by the secondary orbiting deep inside the nova envelope, even low-mass WDs, accreting at very high rates would eventually lose their initial outer helium layer. Moreover, the erosion times would be shorter. We conclude that the observed heavy element enrichments in novae can be accounted for by the diffusion induced convection mechanism, without invoking potentially stronger alternative mechanisms.