The lower main sequence and the nature of secondary stars in ultracompact binaries

The possible nature of the secondary stars in ultracompact binary stellar systems (with orbital periods of less than about 1 hr) are systematically investigated. Using a simplified stellar evolution code, which assumes isentropic stellar models, nearly 3000 separate models for hydrogen-burning main-sequence stars with masses less than 0.3 solar mass are generated. The effects of the (homogeneous) chemical composition on the properties of such stars and in, particular, on the minimum main-sequence mass are explored in detail. It is found that this minimum mass is a sensitive function of the hydrogen content and can be as small as 0.035 solar mass. The properties of fully degenerate low-mass stars, as well as low-mass stars that are not in thermal equilibrium are also studied. In particular, it is shown that the thermal time scale of such nonequilibrium stars may exceed 10 billion yr. The results also confirm the existence of a second branch of the main sequence and shed new light on the thermal instability of this branch. The available observational information on the three known ultracompact binary systems (4U 1626-67, G61-29, 4U 1916-05) is summarized, and the results of the stellar model calculations are combined with the empirical results to place constraints on the properties of the secondary stars.