Kinematics of the helium accretor GP Com


We present time-resolved spectra of the double-degenerate binary star, GP Com. The spectra confirm the presence and period (46.5 min) of the `S'-wave feature found by Nather, Robinson and Stover. GP Com is erratically variable at X-ray and UV wavelengths. We have found the equivalent variability in our data, which, as also seen in UV data, is mostly confined to the emission lines. The HeII 4686 changes by the largest amount, consistent with X-ray driven photo-ionisation. The flaring part of the line profiles is broader than the average, as expected if they are dominated by the inner disc. The HeII 4686 profile is especially remarkable in that its blue-shifted peak is 1400 km/s from line centre compared to 700 km/s for the HeI lines (the red-shifted peak is blended with HeI 4713). We deduce that HeII 4686 emission is confined to the inner 1/4 of the disc. We suggest that the activity of the inner disc indicates that accretion is significant (and unstable) there, in contrast to quiescent dwarf novae, in support of models in which GP Com is in a (quasi-)steady-state of low mass transfer rate. GP Com shows triple-peaked lines profiles which consist of the usual double-peaked profiles from a disc plus a narrow component at line centre. We find evidence for both radial velocity and flux variability in this component, inconsistent with a nebula origin. The radial velocity amplitude and its phase relative to the `S'-wave are consistent with an origin on the accreting white dwarf, if the mass ratio, q = M2/M1, is of order 0.02, as expected on evolutionary grounds. However this explanation is still not satisfactory as the systemic velocity of the narrow component shows significant variation from line to line, and we have no explanation for this.Comment: 9 pages Latex, 9 figures. Accepted for publication in Monthly Notice

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