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

Abstract

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 formation

In 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.