Photospheric abundance analysis of low mass pre-main sequence stars

The atmospheres of low mass pre-main sequence stars are extremely valuable tools for probing the composition of the star-forming interstellar medium. This thesis investigation uses photospheric abundance analysis techniques to determine metal abundances both for individual weakly active T Tauri stars and the molecular clouds with which these stars are associated. Using the high resolution echelle spectrographs of the Palomar 1.5m and the Las Campanas Observatory 2.5m telescopes, high signal-to-noise ratio spectra have been obtained for 53 low mass pre-main sequence stars in six major northern and southern hemisphere star formation regions. Spectra were also taken of 14 main sequence stars with known effective temperatures and metallicity to serve as spectral standards. Equivalent widths of a large number of Fe I absorption lines have been measured, as well as lines of Ca I, Al I, V I, Ni I, Ti I, K I, and Li I. Temperature-sensitive line ratios of neutral iron and vanadium calibrated against the spectral standards are used to derive effective temperatures for 30 weak-line T Tauri stars, which are much improved over the low resolution spectral classifications found for these objects in the literature. Micro-turbulent velocities have also been found for 16 of the pre-main sequence stars in the sample. Several stars with abnormally high microturbulences for their spectral type have been found. Using these improved stellar parameters, metallicities of a large number of T Tauri stars have been determined for the first time. Abundance analyses of iron, aluminum, calcium, nickel, and titanium are performed on 30 low mass, pre-main sequence stars. In the star-formation regions of Taurus-Auriga, Orion, Chamaeleon, and Ophiuchus, which each contributed five or more association members to the young star survey, bulk metallicities have been determined. The results indicate that the current metal abundances in these clouds are near or slightly above solar values. Cloud material metallicities are then used to calibrate ultraviolet line-ofsight metal depletion studies for the Orion and ρ Ophiuchi clouds. The final chapter of this work is a lithium abundance analysis for the entire sample of 53 pre-main sequence stars. Statistics performed on a subset of 37 T Tauri stars without veiling give a mean lithium abundance of 3.60 ± 0.07 on the logarithimic scale relative to log N(H) = 12, and a few stars have derived abundances in excess of 4.0. The mean lithium values among pre-main sequence stars are considerably higher than the maximum values of 3.1 - 3.2 derived for young main sequence cluster stars, indicating that the processes of lithium destruction are quite rapid for low mass stars during the transition onto the main sequence. Although an appreciable spread in lithium abundances are found among the stars in each pre-main sequence star association, no significant difference in mean or maximum lithium abundances are seen between the clusters.