2003ApJ...595..384H

High-resolution spectroscopy was obtained of the FU orionis stars FU Ori and V1057 Cyg between 1995 and 2002 with the SOFIN spectrograph at the Nordic Optical Telescope and with HIRES at Keck I. During these years FU Ori remained about 1 mag (in B) below its 1938-39 maximum brightness, but V1057 Cyg (B~10.5 at peak in 1970-1971) faded from about 13.5 to 14.9 and then recovered slightly. Their photospheric spectra resemble that of a rotationally broadened, slightly veiled supergiant of about type G0 Ib, with v_eqsini=70 km/s for FU Ori, and 55 km/s for V1057 Cyg. As V1057 Cyg faded, P Cyg structure in Hα and the IR Ca II lines strengthened and a complex shortward-displaced shell spectrum of low-excitation lines of the neutral metals (including Li I and Rb I) increased in strength, disappeared in 1999, and reappeared in 2001. Several SOFIN runs extended over a number of successive nights so that a search for rapid and cyclic changes in the spectra was possible. These spectra show rapid night-to-night changes in the wind structure of FU Ori at Hα, including clear evidence of sporadic infall. The equivalent width of the P Cyg absorption varied cyclically with a period of 14.8 days, with phase stability maintained over three seasons. This is believed to be the rotation period of FU Ori. The internal structure of its photospheric lines also varies cyclically, but with a period of 3.54 days. A similar variation may be present in V1057 Cyg, but the data are much noisier and that result uncertain. As V1057 Cyg has faded and the continuum level fallen, the emission lines of a preexisting low-excitation chromosphere have emerged. Therefore we believe that the ``line doubling'' in V1057 Cyg is produced by these central emission cores in the absorption lines, not by orbital motion in an inclined Keplerian disk. No convincing dependence of v_eqsini on wavelength or excitation potential was detected in either FU Ori or V1057 Cyg, again contrary to expectation for a self-luminous accretion disk. It was found also that certain critical lines in the near infrared are not accounted for by synthetic disk spectra. It is concluded that a rapidly rotating star near the edge of stability, as proposed by Larson, can better account for these observations. The possibility is also considered that FUor eruptions are not a property of ordinary T Tauri stars but may be confined to a special subspecies of rapidly rotating pre-main-sequence stars having powerful quasi-permanent winds.