Evolutionary sequences of very hot, low-mass, accreting white dwarfs with application to symbiotic variables and ultrasoft/supersoft low-luminosity X-ray sources

We present the first detailed model results of quasi-static evolutionary sequences of very hot low-mass white dwarfs accreting hydrogen-rich material at rates between 1 × 10^-7 and 1 × 10^-9 M_⊙ yr^-1. Most of the sequences were generated from starting models whose core thermal structures were not thermally relaxed in the thermal pulse cycle-averaged sense of an asymptotic giant branch stellar core. Hence, the evolution at constant accretion rate was not invariably characterized by series of identical shell flashes. Sequences exhibiting stable steady state nuclear burning at the accretion supply rate as well as sequences exhibiting recurrent thermonuclear shell flashes are presented and discussed. In some cases, the white dwarf accretors remain small (< 10¹¹ cm) and very hot even during the shell flash episode. They then experience continued but reduced hydrogen shell burning during the longer quiescent intervals while their surface temperatures increase both because of compressional heating and envelope structure readjustment in response to accretion over thousands of years. Both accretion and continued hydrogen burning power these models with luminosities of a few times 10³⁷ ergs s^-1. We suggest that the physical properties of these model sequences are of considerable relevance to the observed outburst and quiescent behavior of those symbiotic variables and symbiotic novae containing low-mass white dwarfs. We also suggest that our models are relevant to the observational characteristics of the growing class of low-luminosity, supersoft/ultrasoft X-ray sources in globular clusters, and the Magellanic Clouds.