Drastic Spectroscopic Variability of the Be/X-Ray Binary Ariel 0535+262/V725 Tau during and after the 2009 Giant Outburst

We report on high-dispersion optical spectroscopic observations of the Be/X-ray binary A 0535$+$262/V725 Tau during a giant outburst in 2009 November/December, and after it. The observed emission line profiles, reflecting the structure of the geometrically thin circumstellar envelope of the Be star (Be disk), show drastic variabilities, and indicate the existence of a warped component. The enhanced blue shoulder seen after periastron passage implies a gas stream from a dense part of the Be disk to the neutron star.