2002ApJ...572..300I

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 ρ Ophiuchi cloud. In the four observations with a net exposure of ∼600 ks, we found six, three, and two flares from DoAr 21, ROXs 21, and ROXs 31, respectively; hence, the flare 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 (∼10⁵ yr) than ROXs 21 and ROXs 31 (∼10⁶ 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 flare is detected from ROXs 31. The peak luminosity and temperature are ∼10³³ ergs.s^–1 and ∼10 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 flare. The abundances and absorption show dramatic variability from the quiescent to flare phase.