T Tauri Stars in Blue

We present an atlas of 96 low dispersion spectra of T Tauri stars, both classical and weak-lined. The flux-calibrated spectra extend from H-Beta to well blueward of the Balmer jump. Observed equivalent widths and rough blue veilings are presented. We model stars with excess emission as a combination of a weak-lined T Tauri star of a similar spectral type and a slab of hot hydrogen. We tabulate the physical parameters of the slab, and give both intrinsic and observed Balmer jumps. The region responsible for the excess blue continuum has a high density and small surface area, in agreement with previous work. This is consistent with the boundary layer hypothesis, but not unique to it. We suggest that the emission region may actually be the stellar photosphere, heated by accretion shocks at the footprints of magnetic loops extending to the disk. We find that observed infrared excesses are often many times higher than predicted by simple accretion disk models (including reprocessing). The blue excess is more likely to be diagnostic of the actual accretion rate onto the star than the infrared. Typical values for this are a few 10^-8 solar mass/yr. Some accreting material may not make it to the star, but is turned around into a wind near the star. The line strengths from the model agree qualitatively with observations of the upper Balmer lines, but are increasingly underestimated for the lower Balmer lines. Including both low and high optical depth components yields better agreement. No weak-lined stars are found to have a Balmer emission jump, confirming that an accretion disk is required to produce this feature in pre-main-sequence stars.