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 H$\alpha$ emission line, and obtained a semi-amplitude value that is in excellent agreement with the obtained from observations in the ultraviolet spectral region by Sion et al. (1998). We present also a new method to obtain K_2, 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 M6V 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 = 107+/-2 km/s; K_abs = 310+/-5 km/s, and using the inclination angle given by Zhang & Robinson(1987); i = 69.7+/-0.7, the system parameters become: M_WD = 1.20+/-0.05 M_sun,; M_RD = 0.42+/-0.04 M_sun; and a = 1.55+/- 0.02 R_sun. Based on the separation of the double emission peaks, we calculate an outer disk radius of R_out/a ~0.61, close to the distance of the inner Lagrangian 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 colliding with the disc directly, producing the hot spot at this location.

36 pages, 14 figures, ccepted for publication in AJ

References in corpus (6)

• Anomalous cooling of the massive white dwarf in U Geminorum following a narrow dwarf nova outburst
• Library of high and mid-resolution spectra in the CaII H & K, H{alpha}, H{beta}, NaI D{1}, D{2}, and HeI D{3} line regions of F, G, K and M field stars
• Evolution of spiral shocks in U Gem during outburst
• Optical spectroscopy of the dwarf nova U Geminorum
• The masses, radii and luminosities of the components of U Geminorum
• The October 1985 Long Outburst of U Geminorum: Revealing the Viscous Time Scale in Long Orbital Period Dwarf Novae

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