THE NARROW EMISSION-LINES OF T-TAURI STARS

We present the first comprehensive study of the narrow emission lines of T Tauri stars (TTS). These narrow lines have been reported in the l iterature as originating in the stellar atmosphere and having Gaussian -type profiles centered at the stellar rest velocity, with a base widt h not larger than 50 km s(-1). Here, we concentrate on the Ca II lines lambda lambda 8498, 8542, and 8662 and the helium line lambda 5876. A fter applying veiling corrections, the average narrow component line e mission is found to be larger than that found in active main-sequence stars: up to several times larger for classical T Tauri stars with str ong rates of disk accretion. More striking is the finding that the res ulting line emission strengths of these lines correlate with veiling. The correlation is confirmed on individual stars for which observation s at several epochs exist and for which veiling varies widely on relat ively short timescales. We also find a correlation between the narrow emission fluxes and the near-infrared excesses for stars with low leve ls of veiling, which includes the few weak-lined TTS of the sample. We discuss possible formation sites for the narrow emission lines in the classical TTS, and we present simple models to explain the observatio ns. In these models, the excess line emission found for the stars with higher accretion rates is assumed to originate in localized regions n ear the magnetic footpoints of the accretion column. We refer to these hypothetical regions in the atmosphere collectively as the ''hot chro mosphere'' since we assume they are additionally heated by the reproce ssed energy of the colliding gas in the accretion process. Computing t wo chromospheric models, one representing the typical weak TTS chromos phere and the other representing the best guess at the ''hot chromosph ere,'' we find the following. The ''hot chromosphere'' is characterize d by a steep temperature gradient beginning at low continuum optical d epths in order to give simultaneously the large observed central flux and the relatively narrow baselines( 50-60 km s(-1)). The chromosphere temperature rise is not similar to the earlier deep chromosphere mode ls in which a sudden chromospheric temperature rise is appended to the photosphere at relatively large mass column. For the most extreme cas es(i.e., largest line fluxes), 20%, at most, of the star's surface mus t be covered by ''hot chromospheric'' regions.