Evolutionary changes in the optical spectrum of the peculiar supergiant IRC+10420

We present new spectroscopic observations of the peculiar supergiant IRC+10420. In 1997-2000, we obtained three high signal-to-noise ratio spectra of the object at 4300-8000 Angstrom with a spectral resolution of 15000 (20 km/s) using the 6-m telescope of the Special Astrophysical Observatory. From our 2000 spectrum, we estimate the spectral type of IRC+10420 to be A2, corresponding to a temperature of similar to9200 K. Many emission lines were detected, identified with lines of Fe I; Fe II, Ti II, Cr II, and Sc II ions; and [O I], [Fe II], and [Ca II] forbidden lines. The radial velocity derived from absorption lines without obvious emission components (He I lambda5876, O I, N I, Si II) and from absorption components of the Balmer lines is 93+/-1 km/s, The redshift of photospheric lines relative to the star's center-of-mass velocity is interpreted as a consequence of scattering in the expanding, optically thick dust envelope. Both emission and absorption lines show a correlation between radial velocity and oscillator strength. We found variability in the relative intensities of the H-alpha and H-beta emission components. We conclude that IRC+10420 is rapidly evolving towards a Wolf-Rayet stage; the current rate of the photospheric temperature increase is similar to120 K per year. Based on the intensity of the O I (lambda7773) triplet, we estimate the star's luminosity to be M-bol = -9.5(m). In all 1997-2000 spectra of IRC+10420, the He I lambda5876 line has a significant equivalent width of at least 200 m(A)over dot; this may be possible in the presence of such a low temperature due to the star's high luminosity and the enhanced helium abundance in the supergiant's atmosphere. (C) 2002 MAIK Nauka/Interperiodica.