The Absolute Dimensions of Eclipsing Binaries. XXII. The Unevolved F-Type System HS Hydrae

We have redetermined the masses of the unevolved F-type eclipsing binary HS Hya (P = 1.57 days) from 70 observations with the CfA digital speedometers, combined with other observations from the literature. Analysis of our radial velocities determined with a two-dimensional cross-correlation technique shows the presence of a third star in the system, which we find to be most likely an MO dwarf with a mass of ∼0.5 M0 in a 190-day orbit of small eccentricity and apparently co-planar with the eclipsing system. We also reanalyze an earlier set of light curves in the uvby system, using modem analysis techniques and combining all available spectroscopic and photometric data in the analysis. The resulting masses for the eclipsing stars are M_A 1.255±0.008 M_sun and M_B1.219 ± 0.007 M_sun, ∼6% smaller and 6 times more accurate than previous values. The radii are R_A = 1.275±0.007 R_sun and R_B 1 216±0.007 R_sun, and the effective temperatures T_A = 6500±50 K, T_B = 6400±50 K further properties of the stars are given in Table 7. Current stellar evolution models for the observed metallicity (slightly below solar) match the observed properties of both stars in HS Hya very well for a single, but strongly model-dependent age of ∼1.0 Gyr. A precise spectroscopic determination of [Fe/H] for HS Hya is needed to further refine comparison with theoretical models.