Observations of T-Tauri Stars using HST-GHRS: I. Far Ultraviolet Emission Lines

We have analyzed GHRS data of eight CTTS and one WTTS. The GHRS data consists of spectral ranges 40 Å wide centered on 1345, 1400, 1497, 1550, and 1900 Å. These UV spectra show strong Si IV, and C IV emission, and large quantities of sharp (∼ 40 km s) H2 lines. All the H2 lines belong to the Lyman band and all the observed lines are single peaked and optically thin. The averages of all the H2 lines centroids for each star are negative which may indicate that they come from an outflow. We interpret the emission in H2 as being due to fluorescence, mostly by HLyα and identify seven excitation routes within 4 Å of that line. We obtain column densities (10 to 10cm) and optical depths (∼ 1 or less) for each exciting transition. We conclude that the populations are far from being in thermal equilibrium. We do not observe any lines excited from the far blue wing of HLyα, which implies that the molecular features are excited by an absorbed profile. Si IV and C IV (corrected for H2 emission) have widths of ∼ 200 km s, and an array of centroids (blueshifted lines, centered, redshifted). These characteristics are difficult to understand in the context of current models of the accretion shock. For DR Tau we observe transient strong blueshifted emission, perhaps the a result of reconnection events in the magnetosphere. We also see evidence of multiple emission regions for the hot lines. While C IV is optically thin in most stars in our sample, Si IV is not. However, C IV is a good predictor of Si IV and H2 emission. We conclude that most of the flux in the hot lines may Astronomy Dept., Univ. of California, Berkeley, CA 94720, [email protected] Astronomy Dept., Univ. of California, Berkeley, CA 94720, [email protected] Department of Physics and Astronomy, SUNY, Stony Brook NY 11794-3800, [email protected] Space Telescope Science Institute, Baltimore, MD 21218, [email protected] Space Sciences Laboratory, Berkeley, CA 94720, [email protected]