Chandra and ASCA observations of the X-ray brightest T Tauri stars in the ρ Ophiuchi cloud

We present the Chandra Advanced CCD Imaging Spectrometer and ASCA Gas Imaging Spectrometer results for a series of four long-term observations on DoAr 21, ROXs 21, and ROXs 31, the X-ray brightest T Tauri stars in the rho Ophiuchi cloud. In the four observations with a net exposure of similar to600 ks, we found six, three, and two flares from DoAr 21, ROXs 21, and ROXs 31, respectively; hence, the are rate is fairly high. The spectra of DoAr 21 are well fitted with a single-temperature plasma model, while those of ROXs 21 and ROXs 31 need an additional soft plasma component. Since DoAr 21 is younger (similar to10(5) yr) than ROXs 21 and ROXs 31 (similar to10(6) yr), these results may indicate that the soft component gradually increases as T Tauri stars age. The abundances are generally subsolar and vary from element to element. Both high first ionization potential ( FIP) and low-FIP elements show enhancement over the mean abundances. An unusual giant are is detected from ROXs 31. The peak luminosity and temperature are similar to10(33) ergs s(-1) and similar to10 keV, respectively. The temperature reaches its peak value before the flux maximum and is nearly constant (4-5 keV) during the decay phase, indicating successive energy release during the are. The abundances and absorption show dramatic variability from the quiescent to are phase.