Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...430l..53s&link_type=abstract
The Astrophysical Journal, Part 2 - Letters, vol. 430, no. 1, p. L53-L56
Other
52
Spectrum Analysis, Stellar Rotation, Ultraviolet Spectrometers, White Dwarf Stars, Angular Velocity, Hubble Space Telescope, Line Shape, Red Shift
Scientific paper
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 per sec, 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 per sec (Friend et al. 1990), then a gravitational redshift of approximately 50-60 km per sec is implied for the white dwarf. If the gamma-velocity is 84 km per sec (Wade 1981), then a gravitational redshift of only 10-30 km per sec is indicated, which may imply that either the white dwarf has a low (0.5-0.6 of Solar Mass) 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.
Huang Min
Long Knox S.
Sion Edward M.
Szkody Paula
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