U Geminorum: A Test Case for Orbital Parameter Determination

Abstract

High-resolution spectroscopy of U Gem was obtained during quiescence. We did not find a hot spot or gas stream around the outer boundaries of the accretion disk. Instead, we detected a strong narrow emission region near the location of the secondary star. We measured the radial velocity curve from the wings of the double-peaked Hα emission line and obtained a semiamplitude value that is in excellent agreement with that obtained from observations in the ultraviolet spectral region by Sion et al. We also present a new method to obtain K2, which enhances the detection of absorption or emission features arising in the late-type companion. Our results are compared with published values derived from the near-infrared Na I line doublet. From a comparison of the TiO band with those of late-type M stars, we find that a best fit is obtained for an M6 V star, contributing 5% of the total light at that spectral region. Assuming that the radial velocity semiamplitudes accurately reflect the motion of the binary components, then from our results Kem = 107 ± 2 km s-1 and Kabs = 310 ± 5 km s-1; using the inclination angle given by Zhang & Robinson, i = 69.7° ± 0.7°, the system parameters become MWD = 1.20 ± 0.05 M⊙, MRD = 0.42 ± 0.04 M⊙, and a = 1.55 ± 0.02 R⊙. Based on the separation of the double emission peaks, we calculate an outer disk radius of Rout/a ~ 0.61, close to the distance of the inner Lagrangian point L1/a ~ 0.63. Therefore, we suggest that, at the time of observations, the accretion disk was filling the Roche lobe of the primary and the matter leaving the L1 point was colliding with the disk directly, producing the hot spot at this location.