17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

Type: Oral

Topic: Formation and Evolution of Cool Stars and Brown Dwarfs

Abstract
The closest accreting T Tauri star, TW Hya was intensively and continuously observed over $\sim$17 days with spectroscopy and photometry in conjunction with CHANDRA X-ray observations. For the first time we are able to trace an accretion event signaled by an hour-long enhancement of X-rays from the accretion shock and revealed through substantial changes in optical emission line profiles. Downflowing turbulent material appears in H$\alpha$, H$\beta$, and He D3 emission. Two hours after the X-ray accretion event, the optical veiling increases due to continuum emission from the hot splashdown region. The response of the stellar coronal emission to the heated photosphere follows about 2.4 hours later, giving direct evidence that the stellar corona is heated in part by accretion. Then, the stellar wind becomes re-established. A model that incorporates the dynamics of this sequential series of events includes: an accretion shock, a cooling downflow in a supersonically turbulent region, followed by photospheric and later, coronal heating. This model naturally explains the presence of broad optical and ultraviolet lines, and affects the mass accretion rates currently determined from emission line profiles. Additional spectroscopic measures in the near infrared and the optical over several years reveal both the structures and variations in the wind from TW Hya. These results coupled with the large heated coronal region revealed from X-ray diagnostics suggest that our current models are not adequate to explain the accretion process in young stars.