2000A&A...354..621C

We have analysed newly calibrated IUE and ROSAT data for three T Tauri stars from different subclasses: TW Hya, V410 Tau and CS Cha, a Classical T Tauri star (CTTS), a Weak T Tauri star (WTTS) and a T Tauri star intermediate between CTTS and WTTS, respectively. In the ultraviolet the continuum seems to be well explained by the sum of the stellar black body emission plus a hydrogenic component with temperature in the range 1.4 to 5x10⁴K. TW Hya requires additionally a third component at 7900K covering approximately 5% of the stellar surface. Using UV line fluxes, we have also analysed the energy emitted by these stars up to temperatures of 10⁵K. All three stars show emission strongly enhanced relatively to the Sun and peaking at temperatures characteristic of the transition region. In this respect the behaviour of the three stars seems to extend in a harmonic way from the ultraviolet into the X-ray band. The X-ray spectral analyses are consistent with emission from a two-temperature, optically thin plasma in collisional equilibrium. The lowest temperature plasma is at about 10⁶K while the highest reaches 10⁷K. We have also checked for and found no evidence for flaring activity in our sources at the time of the observations. Nevertheless, some level of variability is present for the stars in the sample. The ROSAT datasets for V410 Tau show a variation in terms of count rate on timescales of months to years.