Orbits of detached main-sequence eclipsing binaries of types late F to K. II. UV leonis, UV piscium, and BH virginis.

This is the second paper in a program to determine fundamental properties of main-sequence stars of types late F to K. Improved spectroscopic orbits are derived for the short-period detached eclipsing binaries UV Leo (G0+G2), UV Psc (G5+K3), and BH Vir (F8+G5). The spectra are obtained with the high-resolution Hamilton spectrometer on the 3 m Shane reflector of the Lick Observatory. A considerable body of photometry is available for each of the three systems, so that photometric parameters are known, allowing the properties of the stars to be evaluated with some confidence, despite intrinsic variations, presumably from star spots. The K3 secondary of UV Psc has a mass of 0.76 Msun, the smallest published in this series, although the secondaries of HP Aur and HS Aqr, binaries without adequate photometry, have masses 0.75 and 0.69 Msun, respectively, approaching that of the M dwarf, YY Gem, M=0.59 Msun. Most of the extant models of structure and evolution in the solar mass range do not extend to masses appreciably below 1 Msun because of the change of physical conditions at lower temperatures. Attempts to fit these models to binaries in which the two components differ appreciably in mass lead to contradictory values of the ages of the two components in each case, the secondaries appearing to be twice as old as the primaries or older. Available series of lower main-sequence models stop short of the 0.8 to 1 Msun region, so that their results are not available for comparison. It appears that the transition between the higher- and lower-mass regimes may be a difficult one because of the change of temperature regimes.