Spectroscopy of IY Ursae Majoris during Superoutburst

We report time-resolved spectroscopic observations of the recently discovered dwarf nova, IY Ursae Majoris, over 2 days during its 2000 January superoutburst. The spectra show prominent, broad, and double-peaked emission lines of Hα, Hβ, Hγ, Hδ, and He II and weaker He I lines. The radial velocity curve of the peaks of Hα and Hβ has an amplitude of 93 ± 7 km s-1 and a phase offset of 0.12 ± 0.03. From clear evidence for a precessing disk, we obtain a solution to an eccentric outer disk consistent with theoretical works, which demonstrates the validity of the relation between superhumps and tidal effects. The γ velocity of the binary is derived as -4 ± 32 km s-1 according to the solution. The inner part of the disk is also eccentric as evidenced by asymmetric wings in the lines. Therefore, the whole disk is eccentric. Finally, we find that a simple model can explain all asymmetric features of the emission lines.