A search for low mass companions and a new determination of effective temperatures for T-Tauri stars
Hartigan, Patrick M.
Doctor of Philosophy thesis
I present an analysis of over 300 spectra of the photospheres of young stars in order to address three outstanding questions of current interest: (1) whether or not the youngest stars are accompanied by massive substellar companions, (2) the degree to which starspots influence radial velocity measurements in young stars, and (3) establishment of a scale to convert spectral observations to effective temperatures and determining effective temperature for a large sample of stars. The first question is essential for understanding the star and planet formation process, the second question sets fundamental limits on ability of radial velocity surveys to detect young planets, and the third is a key for getting accurate mass and age estimates. Results from a new radial velocity survey of T Tauri stars focus on three specific stars---BP Tau, DN Tau, and LkCa19---with the largest number of observations. All the objects show far greater radial velocity variability than our radial velocity standards. Using a new simulation of the effect of star spots on the photometric and radial velocity variability of T Tauri star, I show the data for LkCa19 is fully consistent with variations caused by large star spots present on this rapidly rotating young star. In the case of BP Tau and DN Tau, the origin of the velocity variability is currently not well established. I also present the results of a new method to determine the effective temperature for weak-lined T Tauri stars, from the direct fitting of TiO bands near 7000 to NextGen synthetic spectra. The new temperatures are consistently warmer (by as much as 400K) than previous determinations based on spectral type. For the lowest mass young stars, the revised effective temperature doubles the mass inferred from evolutionary models.