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Session 79 - CVs and Novae.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center

[79.07] Time-Resolved UV Observations of DQ Herculis Revisited

A. Silber, S. Anderson, B. Margon (U. Wash.), R. Downes (STScI)

DQ Herculis was observed with the HST FOS for three satellite orbits in April 1995. The spectra were obtained in the ``Rapid'' mode with 4.1 second time resolution. The mean spectrum shows strong emission from NV, Si\,IV/O\,IV], C\,IV, and He\,II. Unlike our October 1993 observations in a larger aperture (Silber et al. 1995, ApJ, 461, in press), the Ly\alpha emission is weak, indicating that the a major source of Ly\alpha in the earlier observation was geocoronal, as we previously suggested. The other prominent lines are seen with similar strengths during the two epochs.

The eclipse of the accretion disk and white dwarf occurs during the second HST orbit. During the eclipse the UV continuum emission drops to zero; this emitting region must be quite compact. While the continuum is disappearing, C\,IV, the strongest line, and N\,V drop by \sim 4\times. He\,II and Si\,IV/O\,IV] are more strongly eclipsed, but even these lines are still seen at the center of the eclipse. Clearly the emission lines come from a region significantly more extended than the continuum.

Coherent pulsations at the 71 s spin period are seen in the UV continuum, but only during the first HST orbit. C\,IV is seen to pulse during the third HST orbit. The phasing of both of these pulsations is consistent with that of the optical pulsations. There is also a possible detection of pulsations in C\,IV during the eclipse ingress. Our previous observations of DQ Her also showed intermittent pulsations of the continuum and emission lines. These observations raise the question of what factor(s) control the complex behavior of the pulsation amplitudes.

ROSAT has detected faint X-ray emission from DQ Her, with L_x (0.1-2.0 keV) \sim10^30 erg s^-1. While the observed X-ray emission is consistent with an origin from the late-type secondary star, the strength of He\,II \lambda1640 suggests that there is an additional but obscured X-ray component of strength L_x \sim10^34 erg s^-1, comparable to that seen in other DQ Her stars.

This work was supported by NASA Grant NAG5-1630.

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