DOPPLER IMAGING OF THE DWARF NOVA U-GEMINORUM

Spectra of the dwarf nova U Gem show narrow emission-line components which vary smusoidally in wavelength (S-waves), and which have previously been ascribed to emission from the gas stream or the bright spot. We apply Doppler tomography to data taken in 1983 and show that the secondary star also contributed to the Balmer lines. The bulk of the S-wave emission still comes from the bright-spot region where the gas stream hits the disk; however, its velocity is not consistent with either the gas stream or the disk but lies in between. We suggest that the bright spot's emission comes from a mixture of disk and stream material, each of which has passed through a shock in the bright-spot region. The velocity of the emission is then an average of the disk's and stream's velocities. The strength of emission from both the secondary star and the bright spot varies around the orbit. The secondary star's emission is concentrated upon its leading face and is most visible from phase 0.5 to 0.75. The emission from the bright spot is always visible but is strongest between phases 0.75 and 0, similar to its contribution to continuum light. We show that our results are consistent with earlier data. The accretion disk's surface brightness in the Balmer lines increases as ≈R-1.5 toward the center of the disk, with Hγ slightly more concentrated than Hβ. The edge of the disk is distorted and is not symmetrical about the line of centers of the two stars. The disk produces little He I λ4471 emission and is not detected in He II λ4686.