Identification and characterization of young, nearby, solar-type stars

Abstract

Post-T Tauri stars (PTTSs) are low-mass, pre-MS stars which have ceased accreting, and are not necessarily near star-forming molecular clouds. Historically, they have been difficult to identify due to their benign spectroscopic signatures. With recent all-sky X-ray surveys and proper motion catalogs, it is now possible to find PTTSs in large numbers. The nearest PTTSs will be important targets for future imaging surveys characterizing dust disks and planetary systems around young solar analogs. The goal of this work is to systematically identify samples of PTTSs, investigate the evolution of circumstellar disks, to infer the fossil star-formation history of molecular clouds, and to estimate kinematic distances to young stars lacking trigonometric parallaxes. We present the results of a spectroscopic survey which identified 110 PTTS members of the nearest OB association (Sco-Cen). We find that 2/3rds of the low-mass star-formation in each OB subgroup occurred in <5 Myr, and that only ∼1% of solar-type stars with mean age ∼13 Myr shows signs of accretion from a circumstellar disk. In order to assess how long circumstellar material is detectable around PTTSs, we conducted a 10 μm imaging survey of post-T Tauri members of the ∼30-Myr-old Tuc-Hor association. The goal was to find evidence of either remnant accretion disks or dusty debris disks with orbital radii of ≲10 AU. Combined with data from other surveys, we conclude that mid-IR emission from warm dust grains in the terrestrial planet zones around young stars become undetectable compared to the stellar photosphere for nearly all stars by age ∼20 Myr. Lastly, we present a technique for calculating distances isolated young field stars that currently lack trigonometric parallax measurements. The technique is a generalization of the classical cluster parallax method, but can handle anisotropic velocity dispersions and non-zero Oort parameters. Distances and isochronal ages are estimated for a subsample of PTTSs included in the Formation and Evolution of Planetary Systems (FEPS) Spitzer Space Telescope (SST) Legacy Science program. The techniques developed in this thesis will allow one to efficiently conduct a systematic survey to identify the nearest, youngest stars to the Sun using existing databases.