Optical spectroscopy of the dwarf nova U Geminorum

Abstract

U Gem is unique in having a direct measurement of K1 = 107 +- 2 km/s, Long et al. 1999). We present high-resolution optical spectra of the dwarf nova U Gem in quiescence taken to test the accuracy to which the HST value can be recovered from optical data. We find that, even with data of very high S/N we cannot recover Long et al.'s value to better than about 20% by any method. Contamination by neighbouring emission lines seems a likely culprit. Our data reveal a number of new features: Doppler tomograms show emission at low velocity, close to the centre of mass, and a transient, narrow absorption feature is seen in the Balmer lines near the line centres at the time of eclipse. We suggest that stellar prominences, as previously invoked for the dwarf novae IP Peg and SS Cyg in outburst, may explain both of these features. The He II 4686.75 A line emission is dominated by the gas stream/disc impact region. Two distinct spots are seen in Doppler maps, the first being very narrow and showing a velocity close to that of the accretion disc in the impact region, and the second much broader and located between the velocities of the (ballistic) stream and the (Keplerian) disc. We present tentative evidence of weak spiral structure. We find no evidence of stream-disc overflow in the system. Our data suggests an inclination angle > 70 degrees, adding to the evidence of a puzzle in the mass of U Gem's white dwarf. The mass donor is clearly seen in the Doppler maps, with emission concentrated towards its poles, and mainly on the side facing the white dwarf. This suggests irradiation with shielding by the disc from which we estimate an H/R ratio between 0.15 and 0.25.

References in corpus (10)

• Hydrodynamics of the stream-disk impact in interacting binaries
• HST Fine Guidance Sensor Astrometric Parallaxes for Three Dwarf Novae: SS Aurigae, SS Cygni, and U Geminorum
• The mass of the white dwarf in the old nova BT Mon
• On the observability of spiral structures in CV accretion discs
• Tidally distorted accretion discs in binary stars
• New results on GP Com
• The systemic velocities of four long-period cataclysmic variable stars
• Evolution of spiral shocks in U Gem during outburst
• The masses, radii and luminosities of the components of U Geminorum
• Numerical simulation of accretion discs in close binary systems and discovery of spiral shocks

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