Analysis of HST and IUE ultraviolet spectra for T Tauri stars: DF Tau

We analyze ultraviolet spectra of DF Tau, a binary system whose primary com ponent is a classical T Tauri star. The spectra were obtained from the Hubb le Space Telescope and the IUE satellite. The stellar emission in the wavel ength range covered is shown to originate in an accretion shock wave. The g as infall velocity is similar to 250 km s(-1). The accreted-gas density is typically N-0 less than or equal to 10(11) cm(-3), but it can occasionally be higher by one and a half orders of magnitude. The continuum intensity ne ar lambda = 1900 Angstrom was found to be virtually constant for such a sig nificant change in No. The star's photometric variability is probably attri butable to variations in accreted-gas density and velocity, as well as to v ariations in the area of a hot spot on the stellar surface and in its orien tation relative to the observer. The mean accretion rate is M similar to 3 x 10(-9) M-circle dot yr(-1).The interstellar extinction for DF Tau is A(V) similar or equal to 0.(m)5, the stellar radius is less than or equal to2 R -circle dot, and the luminosity of the primary component is most likely no higher than 0.3 L-circle dot. We argue that the distance to DF Tau is about 70 pc. Upper limits are placed on the primary's coronal emission measure: EM(T = 10(7) K) < 3 x 10(54) cm(-3) and EM(T = 1,3 x 10(6) K) < 3 x 10(55) cm(-3) Absorption lines originating in the stellar wind were detected in th e star's spectrum. Molecular hydrogen lines have essentially the same radia l velocity as the star, but their full width at half maximum is FWHM simila r or equal to 50 km s(-1) We failed to explain why the intensity ratio of t he C IV lambda 1550 doublet components exceeds 2. (C) 2001 MAIK "Nauka/lnte rperiodica".