Emission from Hot Gas in Pre-Main Sequence Objects: The Accretion Shock and the Inner Disk.

[摘要] High energy (X-ray and ultraviolet) emission traces hot gas produced by energetic phenomena in T Tauri stars.These phenomena include magnetic heating of the stellar atmosphere and magnetospheric accretion of disk gas onto the stellar surface.High energy emission irradiates the planet-forming disk during a key time for the origin of planets, so an understanding of these energetic processes and their evolution is crucial for theories of disk evolution and planet formationIn this thesis, I analyze X-ray and ultraviolet observations of young stars to study high energy events and follow their evolution.I confirm that X-ray emission is saturated during the T Tauri phase and suggest that far ultraviolet emission is also saturated at this age, possibly because the same mechanism heats both the chromosphere and corona.I compare accretion diagnostics for a large sample of T Tauri stars to characterize the properties of magnetospheric accretion.For the first time, I use models of the accretion emission which have contributions from multiple accretion hot spots, characterized by varying energy fluxes in the accretion columns.Models of T Tauri magnetospheres and the magnetic footprints on the star physically motivate this multi-component description of accretion. For RECX-11, a source near the final stage of disk depletion, I show that it has a very low mass accretion rate.If theories that predict the circumstellar disk is losing mass at very high rates are correct, the disk of RECX-11 would have a gap or hole in it, which it does not.I also present observations of hot H2 gas in the inner circumstellar disk.I find that the strength of the H2 emission is correlated with the accretion luminosity of the T Tauri star and show that for young stars in which accretion has ceased, there is no H2 left in the inner disk.I show this is true even for non-accreting young stars at 1-3 Myr, indicating rapid disk removal.Observations of circumstellar gas, combined with knowledge of the radiation fields, are crucial for studying disk evolution.Here, I provide timescales for gas depletion and constraints to disk dissipation models.

[发布机构] University of Michigan

[效力级别] Astronomy

[关键词] Star Formation;Astronomy;Science;Astronomy and Astrophysics