[Normal Stars and WD -- Oral ]

Discovery of an Accretion-Fed Corona in an Accreting Young Star

Nancy Brickhouse, Smithsonian Astrophysical Observatory
S. R. Cranmer (SAO), A. K. Dupree (SAO), G. J. M. Luna (SAO), S. Wolk (SAO)

A deep (489 ks) Chandra High Energy Transmission Grating spectrum of the classical T Tauri star TW Hydrae shows a new type of coronal structure that is produced by the accretion process. In the standard model for a stellar dipole, the magnetic field truncates the disk and channels the accreting material onto the star. The He-like diagnosticlines of Ne IX provide excellent agreement with the shock conditions predicted by this model, with an electron temperature of 2.5 MK and electron density of 3 times 10\^{12}$ cm$\^{-3}$ (see also Kastner et al. 2002).However, the standard model completely fails to predict the post-shock conditions, significantly overpredicting both the density and absorption observed at O VII. Instead the observations require a second "post-shock" component with 30 times more mass and 1000 times larger volume than found at the shock itself. We note that in the standard model, the shocked plasma is conveniently located near both closed (coronal) and open (stellar wind) magnetic structures, as the magnetic field connecting the star and disk also separates the open and closed field regions on the stellar surface. The shocked plasma thus can provide the energy to heat not only the post-shock plasma, but also adjacent regions (i.e. an "accretion-fed corona") and drive stellar material into surrounding coronal structures. These observations provide new clues to the puzzling soft X-ray excess found in accreting systems, which depends on both the presence of accretion and the level of coronal activity (Guedel and Telleschi 2007). This Large Program with Chandra demonstrates the value of high signal-to-noise, high resolution spectroscopy for understanding the complex interaction of magnetic and accretion processes in late-typestar formation.