X-ray spectroscopy of the nearby, classical T Tauri star TW hydrae

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 similar to 50 pc, represents the nearest known region of recent star formation. Ana lysis of ASCA and ROSAT spectra indicates characteristic temperatures of si milar to 1.7 and similar to 9.7 MK for the X-ray-emitting region(s) of TW H ya, with emission lines of highly ionized Fe dominating the spectrum at ene rgies of similar to 1 keV. The X-ray data show variations in X-ray flux on timescales of less than or similar to 1 hr as well as indications of change s in the X-ray-absorbing column on timescales of several years, suggesting that flares and variable obscuration are responsible for the large-amplitud e optical variability of TW Hya on short and long timescales, respectively. Comparison with model calculations suggests that TW Hya produces sufficien t hard X-ray flux to produce significant ionization of molecular gas within its circumstellar disk; such X-ray ionization may regulate both protoplane tary accretion and protoplanetary chemistry.