  We present ASCA and ROSAT X-ray observations of the classical T Tauri star TW
Hya, the namesake of a small association that, at a distance of ~ 50 pc,
represents the nearest known region of recent star formation. Analysis of ASCA
and ROSAT spectra indicates characteristic temperatures of ~ 1.7 MK and ~ 9.7
MK for the X-ray emitting region(s) of TW Hya, with emission lines of highly
ionized Fe dominating the spectrum at energies ~1 keV. The X-ray data show
variations in X-ray flux on <~ 1 hr timescales as well as indications of
changes in X-ray absorbing column on timescales of several years, suggesting
that flares and variable obscuration are responsible for the large amplitude
optical variability of TW Hya on short and long timescales, respectively. The
ASCA spectra of TW Hya, which is ~ 20 Myr old, has a mass of ~ 0.7 M_sun, and
is surrounded by a dusty circumstellar disk, offer perhaps the best indication
yet of the X-ray spectrum of the young Sun during the epoch of Jovian planet
building. Comparison with model calculations suggests that TW Hya produces
sufficient hard X-ray flux to produce significant ionization of molecular gas
within its circumstellar disk; such X-ray ionization may regulate both
protoplanetary accretion and protoplanetary chemistry.
