Dramatic Spectral Evolution of WZ Sagittae during the 2001 Superoutburst

We carried out optical spectroscopic observations of the most enigmatic dwarf nova, WZ Sge, on 11 nights during the 2001 superoutburst. Our observations covered the period from the initial phase several hours before the maximum to the ninth maximum of the rebrightening phase. The first spectrum shows absorption lines of H I (except for Hα), He I, and Na I , as well as emission lines of He II , C III /N III, and Hα in doubly-peaked shapes. The same spectrum shows the emission lines of C IV and N IV , which are the first detection in dwarf novae. The spectral features dramatically changed on various time scales. For example, the peak separations of the emission lines of H I and He II changed from ∼ 700 s^-1 to ∼ 1300 km s^-1, and one of the peaks dominated over an orbital period in the genuine-superhump era, but the dominant peak remained interchanged with the orbital phase in the early-superhump era. The lines of H I and He I were in emission at the minima of the rebrightening phase (with no high-excitation lines, nor Na I ), while they became in absorption at the maxima. We report on the observational results in detail and their implications concerning the outburst mechanism, two types of superhumps, and the variation of the disk structure.