SPECTROSCOPIC EVIDENCE FOR MAGNETOSPHERIC ACCRETION IN CLASSICAL, T-TAURI STARS

We have conducted a high resolution spectroscopic survey of 15 classical T Tauri stars covering a broad range of inferred disk accretion rates to study mass infall to the stellar surface. Spectral diagnostics of mass infall are line profiles exhibiting either redshifted absorption minima at velocities comparable to free fall or asymmetric emission profiles with more blue than red emission. Echelle spectra from the KPNO 4 m have been used both (1) to extract residual line profiles of unparalleled sensitivity, free from contamination from underlying photospheric features, and (2) to determine the optical continuum ''veiling'' flux. Residual emission profiles of Balmer, Na D and He I lines are presented, along with simultaneously determined relative veiling fluxes. In addition to spectral signatures of mass outflow in many stars, 14/15 of our sample exhibit one of our two spectral diagnostics of mass infall. Redshifted absorption minima are found in 13/15 stars, in at least one of the lines examined, with radial velocities of the redward edge of the absorption minimum between 200 to 300 km s(-1). Among these lines, redshifted absorption minima appear least often at Ha (2/15) and most frequently at HS (8/15) and Na D (9/15). The appearance of redshifted absorption minima is more likely at Ha when continuum veiling fluxes are low, and at He I lambda 5876 Angstrom when continuum veiling fluxes are high. Although the upper Balmer lines show resolved redshifted minima in only half the sample of 15 stars, 13/15 stars exhibit a significant blueward asymmetry in these lines, with a ratio of emission blueward to redward of line center ranging from 1.1 to 2.4. A similar blueward asymmetry is found at Ha; when the sample is restricted to stars with low continuum veiling fluxes. Together, these results suggest that the Balmer emission lines form predominantly in infalling zones in most classical T Tauri stars. However, the near ubiquity of redshifted absorption minima, both in snapshot views of a large sample of stars and in temporal coverage of a limited number of stars, suggests an extensive magnetospheric structure which keeps infall zones in our view at all times.