AN EXTENDED GRID OF MULTICYCLE NOVA EVOLUTION MODELS

摘要

According to the nova theory, observed characteristics of novae may be reproduced by varying the values of three basic and independent parameters: the accreting white dwarf's mass M_(WD), its temperature T_(WD), and the mass transfer rate M. Calculations performed to date have, however, left wide regions of the parameter space unexplored. We carry out a systematic study involving calculations of evolutionary sequences of nova outbursts through several cycles, for 64 parameter combinations spanning the entire parameter space, assuming CO white dwarfs (WDs). An updated stellar evolution code is used, including an extended nuclear reactions network, new opacities (OPAL), diffusion of all elements and the effect of radiation pressure on mass loss. We find that the entire range of observed nova characteristics can be accounted for, including recurrent and symbiotic novae. Recurrent novae may be obtained on relatively low-mass WDs (～1 solar mass). Accretion at rates M ≥ 10~(-7) solar lum yr~(-1) invariably results in an increase of M_(WD) and may, eventually, lead to a type Ia supernova. For accretion rates M ≤ 10~(-9) solar mass yr~(-1), M_(WD) decreases under all circumstances. The overall dependence of nova characteristics on the basic parameters is analyzed. Observed correlations between nova properties, as well as the conspicuous lack of correlation between other properties, are verified by the theoretical results. Among all the observed properties of novae there are three that appear to be independent of each other: the time of decline by 3 magnitudes t_3, the heavy element abundance of the ejecta Z_(ej), and their helium content Y_(ej). Our calculations yield t_3(M_(WD), T_(WD), M), Z_(ej)(M_(WD), T_(WD), M), Y_(ej)(M_(WD), T_(WD), M) at discrete points over the entire parameter space. By matching observed characteristics of a particular nova with calculated counterparts, it is possible to derive the WD's mass and temperature and the (average) accretion rate as well as additional observable properties. We find an excellent match for the measured expansion velocities, but the calculated ejected masses are generally smaller than those estimated from observations.