H$_{\bf\alpha}$ emission in pre-main sequence stars. I. An atlas of line profiles

We present an atlas of very high resolution (R ~ 50000) Hα line profiles of 63 pre-main sequence stars, divided among 43 T Tauri stars, 18 Herbig Ae/Be stars, and 2 FU Orionis objects. Hα emission is the most common and prominent spectroscopic feature of pre-main sequence stars, and although it is optically very thick it is still the most frequently modelled emission line in young stars. In T Tauri stars the principal models involve magnetically driven winds, and more recently the role of infalling magnetospheric material has been explored. For Herbig Ae/Be stars a variety of models have been proposed, current emphasis is directed towards obscuration by clumpy circumstellar disk structures. In order to provide constraints on such models, we have made a statistical analysis of the 63 high resolution profiles. We here ignore the considerable variability of the Hα emission, which is discussed in detail in a second paper. Most of our observed lines show complex profiles due to an interplay between emission and absorption features, and we suggest a two-dimensional classification scheme to describe these line profiles, based on the relative height of a secondary peak to the primary peak, as well as whether the absorption is blue- or red-shifted. Among T Tauri stars, 25% have symmetric profiles, 49% have blueshifted absorption dips, and 5% have P Cygni profiles; the remaining 21% show a variety of redshifted absorptions. For Herbig Ae/Be stars symmetric lines are quite rare (11%), indeed almost all of these stars have deep and prominent central absorptions. We have measured the extent of the line wings for all of our stars at the Imax/40 level, and find that almost all have very extended wings, with typical extents of ±350 km/s, but in high spectra the wings can be traced to lower intensities, and velocities as high as ±900 km/s have been observed. Pronounced asymmetries of these extended wings are found for many stars, suggesting the possibility that the highest velocity material could be non-uniformly distributed. The equivalent widths of the Hα emission in our sample of stars span two orders of magnitude, with a distribution that increases with decreasing equivalent width.