Fe I fluorescence in T Tauri stars

A statistical equilibrium analysis for the selective excitation of AA4063, 4132 of Fe 1 shows: (a) selective excitation by Ca ii or HE can reproduce the line intensities observed in T Tauri stars; (b) when enhanced AA4063, 4132 emission is observed, an upper limit on the electron density and/or hydrogen density as a function of distance from the star is obtained; (c) it is sometimes also possible to obtain an upper limit on the local gas kinetic temperature and a lower limit on the hydrogen density divided by the dilution factor W. Upper limits obtained for RWAurigae are: N6/W 10' and NHIW 1016 if H li/H I 0.1, for TE = 3000K. Also, NH/W> 10' if the lines are formed less than 1000 stellar radii from the star. A qualitative model is suggested for RW Aurigae where the Fe I lines are formed in an expanding envelope, beyond 10 stellar radii from the central star. Although Ca ii H is a stronger emission line than HE in some T Tauri stars, considerations of the velocity shifts expected for different parts of the envelope indicate that HE may be responsible for the selective excitation of AA4063, 4132. Subject headings: atomic processes - emission-line stars - pre-main-sequence stars