SIMBAD references

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 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 of ∼1 keV. The X-ray data show variations in X-ray flux on timescales of ≲1 hr as well as indications of changes in the 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. 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.