Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21932703i&link_type=abstract
American Astronomical Society, AAS Meeting #219, #327.03
Astronomy and Astrophysics
Astronomy
Scientific paper
Gas in the inner circumstellar disk may be essential for circularizing the orbits of terrestrial planets, but disk gas is difficult to observe so its evolution is not well characterized. Intricately related to the gas evolution is the evolution of the accretion of inner disk gas onto the central star. High energy emission produced in an accretion shock at the stellar surface irradiates and heats the inner disk. I show that the properties of disk gas, in turn, affect the rate of accretion. Observations of H2 show that the depletion of gas in the inner disk eventually turns off accretion. These final stages of the gas have important implications for theories of disk dissipation, in particular photoevaporation by radiation from the central star. Recent models suggest that mass loss rates by photoevaporation may reach 10-8 solar masses per year, comparable to measured accretion rates for T Tauri stars. I will discuss one source which has an evolved disk, based on its infrared spectral energy distribution, yet does not exhibit the signatures of photoevaporation which are traced by dust emission (e.g. a gap or hole in the disk). With an upper limit on the accretion rate of 3x10-10 solar masses per year, regions of dust in the disk should be cleared, yet this is not observed. Clearly photoevaporation at such high rates is not feasible in this case. Finally, I discuss the evolution of high energy X-ray and far-ultraviolet emission, which drive photoevaporation, from the star and the accretion shock. I show that X-ray emission is consistently high throughout the disk lifetime, yet the far-ultraviolet emission decreases on timescales consistent with the end of accretion. These observations constrain photoevaporation models by providing the radiation field present and an upper limit on the mass loss rate via the mass accretion rate.
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