EMISSION-LINE MAPPING OF THE DWARF NOVA IP-PEGASI IN OUTBURST AND QUIESCENCE

Time-resolved spectroscopy covering half an orbit and one eclipse during an outburst of the dwarf nova IP Peg are presented and discussed in comparison with quiescent data. The outburst spectra show strong Balmer and He II λ4686 emission lines that are atypical of outbursting dwarf novae. Double-peaked velocity profiles and rotational distortions during eclipse show that the lines arise primarily in a Keplerian accretion disk. We construct Doppler maps of the disk which show that the Balmer and He II λ4686 emission surface bright-nesses vary roughly as R-2 and R-0.1 respectively, and that large-scale azimuthal structure is present in the outer disk. The He II line profile is filled in between the two disk peaks by non-Keplerian emission from close to the white dwarf, possibly indicative of a compact outflowing wind or inflowing magnetic accretion column. The secondary star displays Balmer, but not He II, emission in the polar region on the side facing toward the white dwarf. No evidence is seen for a stronger gas stream in outburst than quiescence. During outburst, an accretion rate Ṁ ≈ 1017 g s-1 is needed to account for the continuum emission of the disk, and the associated boundary layer generates soft X-ray and Lyman continuum fluxes sufficient to photoionize the disk's He II and Balmer emission-line regions. The disk appears to shield the red star totally from the soft X-rays, and in the equatorial region, from the Lyman continuum. By contrast, irradiation effects on the red star are absent in quiescence, and we conclude that a different mechanism must be sought to account for emission lines from quiescent disks. A sharp transient He II emission feature is observed through half a cycle along a sinusoidal velocity curve with a 32 minute period. Its velocity amplitude and short period are consistent with Keplerian motion near the rim of the disk and confirm a mass of about 1 M⊙ for the white dwarf. The phase and velocity at which the feature first appears suggests that it may be generated through the interaction of the gas stream with the disk.