HUBBLE-SPACE-TELESCOPE GODDARD HIGH-RESOLUTION SPECTROGRAPH OBSERVATION OF U-GEMINORUM DURING QUIESCENCE - EVIDENCE FOR A SLOWLY ROTATING WHITE-DWARF

We have obtained a pair of consecutive far-ultraviolet Goddard High-Resolution Spectrograph (GHRS) exposures of the Si iv region of the dwarf nova U Geminorum in early quiescence, 8 days after its return to optical quiescence when the underlying white dwarf dominates the ultraviolet light of the system. Our GHRS observation revealed a fully resolved line profile for the resonance doublet of Si IV. If it is associated with the white dwarf photosphere, then our best synthetic fits are consistent with T(eff) = 35,000 K-38,000 K, log g = 8, a rotational velocity of 50 to 100 km s-1, with a modestly enhanced silicon abundance (1.3-2.3 times solar) and our results suggest that at least in U Gem and perhaps in other similar dwarf novae, the missing boundary layer cannot be explained by rapid rotation of the white dwarf However, the gamma-velocity of the system remains uncertain. If the gamma-velocity is 43 km s-1 (Friend et al. 1990), then a gravitational redshift of approximately 50-60 km s-1 is implied for the white dwarf. If the gamma-velocity is 84 km s-1 (Wade 1981), then a gravitational redshift of only 10-30 km s-1 is indicated, which may imply that either the white dwarf has a low (0.5-0.6 M.) mass or an extended atmosphere (corona) due to the outburst heating 8 days earlier. The implications of our line-fitting results for the structure and temperature of boundary layers in cataclysmic variables are discussed.