U Geminorum: a Test Case for Orbital Parameters Determination

Due to its eclipsing nature and thorough observational studies, U Gem, in general, a good candidate for the analysis of standard and new methods in the determination of the orbital parameters in cataclysmic variables. Although in this interactive binary, these parameters are relatively well known, there are still discrepancies in the radial velocity semi-amplitude of the white dwarf, as obtained from the optical or the Ultraviolet data. Furthermore, the secondary star is not visible in the optical; consequently, its corresponding semi-amplitude has been derived from data obtained in the infrared region. For these reasons U Gem is an interesting case for testing new methods to derive orbital parameters based on optical observations only. High resolution spectroscopy of U Gem, covering the spectral region λ 5200-9100 Å, was obtained. The system was observed during quiescence, about 35 days after the onset of an outburst. 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 at 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 K[2], based on the detection of the TiO band around λ 7050 Å. 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 percent 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 km s^-1 and K[abs] = 310 km s^-1. For a revised inclination angle of i = 70^o (Zhang et al. 1987) the system parameters become; M[wd] = 1.20 ± 0.05 M[Θ]; M[rd] = 0.42 ± 0.04 M [Θ]; and a = 1.5 ± 0.02 R[Θ]. Based on the separation of the double emission peaks, we calculate an outer disk radius of R[out]/a ∼0.63 , similar to the distance of the inner Lagrange point L[1]/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 L[1 ]point was striking the disc directly, producing the hot spot near the L[1] location.