| Description |
Protoplanetary gas and dust disks around T Tauri stars are the most important elements in the process of planet formation. In particular, the dissipation time scale of disks sets a critical time limit for planet formation. The mechanisms of disk dissipation are poorly understood, however. Apart from the planet formation process itself, photoevaporation and disk instabilities induced by the magneto- rotational instability are key candidates. In both cases, heating and ionisation by stellar short-wavelength radiation plays a crucial role. High-energy stellar radiation (X-rays, extreme ultraviolet) also drives chemical networks in circumstellar disks, forming key molecules that eventually become important for the formation of life in habitable zones. However, there is little information on the X-ray radiation spectrum impinging on the disk, given various sources of photoelectric absorption. We will study disk ionisation and X-ray driven chemistry by observing X-ray sensitive lines in an outstanding and unique sample of three classical T Tauri stars in which X-rays appear to be blocked before reaching the disk. We will compare the results with a control sample of disks that appear to be irradiated by high levels of stellar X-ray radiation. Our X-ray sensitive lines include transitions from HCN, HCO+, and ortho-NH3. The interpretation of our results will be based on thermo-chemical codes developed in the proposing team. |
