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 evolutiona ry sequences of very hot low-mass white dwarfs accreting hydrogen-rich material at rates between 1 x 10(-7) and 1 x 10(-9) M. yr(-1). Most o f the sequences were generated from starting models whose core thermal structures were not thermally relaxed in the thermal pulse cycle-aver aged sense of an asymptotic giant branch stellar core. Hence, the evol ution at constant accretion rate was not invariably characterized by s eries of identical shell flashes. Sequences exhibiting stable steady s tate nuclear burning at the accretion supply rate as well as sequences exhibiting recurrent thermonuclear shell flashes are presented and di scussed. In some cases, the white dwarf accretors remain small(<10(11) cm) and very hot even during the shell flash episode. They then exper ience continued but reduced hydrogen shell burning during the longer q uiescent intervals while their surface temperatures increase both beca use of compressional heating and envelope structure readjustment in re sponse to accretion over thousands of years. Both accretion and contin ued hydrogen burning power these models with luminosities of a few tim es 10(37) ergs s(-1). We suggest that the physical properties of these model sequences are of considerable relevance to the observed outburs t and quiescent behavior of those symbiotic variables and symbiotic no vae 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 clus ters, and the Magellanic Clouds.