U Geminorum: a Test Case for Orbital Parameters 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 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 semi-amplitude value in excellent agreement with the ultraviolet results by Long & Gilliland (1999). We present also 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 NaI 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 a M6 V star, contributing 5% of the total light at that spectral region. Assuming that the radial velocity semi-amplitudes reflect accurately the motion of the binary components, then from our results: K_em = 108 ± 2 km s⁻¹; K_abs = 310 ± 5 km s⁻¹, and using the inclination angle by Zhang & Robinson (1987); i = 69^°.7 ± 0.7, the system parameters become: M_WD = 1.20 ± 0.05 M_⊙; M_RD = 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 R_out/a ∼ 0.63, 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 that the matter leaving the L₁ point was colliding with the disc directly, producing the hot spot at this location. Specific details not included in the printed version can be found in the Electronic Poster (EP).