[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.