HST eclipse mapping of dwarf nova OY carinae in quiescence: An "Fe II curtain" with mach ≃6 velocity dispersion veils the white dwarf

HST observations of the eclipsing dwarf nova OY Car in its quiescent state are used to isolate the ultraviolet spectrum (1150-2500 Å at 9.2 Å FWHM resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf dominates the ultraviolet flux, with the disk and bright spot contributions increasing longward of 2000 Å. The white dwarf spectrum has a Stark-broadened photospheric Lα absorption, but is veiled by a forest of blended Fe n features that we attribute to absorption by intervening disk material. A fit gives T_w ≃ 16.5 × 10³ K for the white dwarf with a solar-abundance, log g = 8 model atmosphere, and T ≃ 10⁴ K, n_e ≃ 10¹³ cm^-3, N_H ≃ 10²² cm^-2, and velocity dispersion ΔV ≃ 60 km s^-1 for the veil of homogeneous solar-abundance LTE gas. The veil parameters probably measure characteristic physical conditions in the quiescent accretion disk or its chromosphere. The large velocity dispersion is essential for a good fit; it lowers χ²/778 from 22 to 4. Keplerian shear can produce the velocity dispersion if the veiling gas is located at R ≃ 5R_w with ΔR/R ∼ 0.3, but this model leaves an unobscured view to the upper hemisphere of the white dwarf, incompatible with absorptions that are up to 80% deep. The veiling gas may be in the upper atmosphere of the disk near its outer rim, but we then require supersonic (Mach ≃ 6) but sub-Keplerian (ΔV/V_Kep ≃ 0.07) velocity disturbances in this region to produce both the observed radial velocity dispersion and vertical motions sufficient to elevate the gas to z/R = cos i = 0.12. Such motions might be driven by the gas stream, since it may take several Kepler periods to reestablish the disk's vertical hydrostatic equilibrium. The temperature and column density of the gas we see as Fe II absorption in the ultraviolet are similar to what is required to produce the strong Balmer jump and line emissions seen in optical spectra of OY Car and similar quiescent dwarf novae. The outer accretion disk is detected at mid-eclipse with a spectrum that rises from 0.05 to 0.3 mJy between 2000 and 2500 Å, consistent with combinations of cool blackbodies, blended Fe II emission lines, and Balmer continuum emission. The total disk flux density is 0.5 mJy at 2500 Å, and this shallow disk eclipse implies a roughly flat surface brightness distribution. The bright spot, somewhat bluer than the disk, has a flux density rising from 0.05 to 0.15 mJy between 1600 and 2500 Å. The C IV emission line has a broad shallow eclipse, but the radial velocity variations observed during the eclipse do not clearly distinguish between a disk or wind origin. The only possible indications of boundary layer emission are fast UV flares that appear to arise from near the central object - not from the bright spot.