Ultraviolet spectrum of FU Ori and a ``Compromise'' model of the FUor

Abstract

We have analyzed the Hubble Space Telescope spectrum of the young star FU Ori in the range 2300 3100 Å. The long-wavelength part of the spectrum is similar to the spectrum of a supergiant with T _eff ≃ 5000 6000 K, but the range of wavelengths shorter than ≃2600 Å is dominated by radiation from a region with T _eff ≃ 9000 K. We discuss the possibility of explaining these peculiarities of the spectrum, the Al II] 2669.2 emission line profile, and the results of X-ray observations for FU Ori in terms of an accretion disk model whose thickness increases as the star is approached starting from distances ≃10¹² cm. Near the star, the disk has the shape of a cone in which only the part of its surface on the far (from the observer) side is visible. The suggested model is a kind of a compromise between the models of a thin α-disk and a supergiant: basically, this is an accretion model, but it resembles a supergiant in observational manifestations. Numerous absorption lines originating in the disk wind are superimposed on the disk spectrum. The wind is a cold ( T ≃ 5000 K), dense ( N _e ≃ 10¹¹ cm^-3) gas. The number of wind absorption lines in the ultraviolet spectrum of FU Ori increases with decreasing wavelength. This causes a rapid decline in intensity in the short-wavelength part of the spectrum. As a result, the maximum temperature in the disk estimated from low-resolution IUE spectra has been underestimated.