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 component is a classical T Tauri star. The spectra were obtained from the Hubble Space Telescope and the IUE satellite. The stellar emission in the wavelength range covered is shown to originate in an accretion shock wave. The gas infall velocity is ∼250 km s−1. The accreted-gas density is typically N0≤1011 cm−3, but it can occasionally be higher by one and a half orders of magnitude. The continuum intensity near λ=1900 Å was found to be virtually constant for such a significant change in N0. The star’s photometric variability is probably attributable to variations in accreted-gas density and velocity, as well as to variations in the area of a hot spot on the stellar surface and in its orientation relative to the observer. The mean accretion rate is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$\dot M \sim 3 \times 10^{ - 9} M_ \odot yr^{ - 1}$$ \end{document}. The interstellar extinction for DF Tau is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$A_V \simeq 0\mathop .\limits^m 5$$ \end{document}, the stellar radius is ≤2R⊙, and the luminosity of the primary component is most likely no higher than 0.3 L⊙. 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=107 K)<3×1054 cm−3 and EM(T=1.3×106 K)<3×1055 cm−3. Absorption lines originating in the stellar wind were detected in the star’s spectrum. Molecular hydrogen lines have essentially the same radial velocity as the star, but their full width at half maximum is FWHM ≃50 km s−1. We failed to explain why the intensity ratio of the C IV λ1550 doublet components exceeds 2.