ASCA observations of the T Tauri star SU Aurigae and the surrounding L1517dark cloud

We present the results of a approximate to 40 ks pointed ASCA observation o f the L1517 star-forming region, centered on the X-ray-bright T Tauri star SU Aurigae. This star has the highest X-ray luminosity of any classical T T auri star in the Taurus-Auriga region, and its optical spectra show evidenc e for both mass inflow and outflow. Strong X-ray emission was detected from SU Aur (L-x = 10(30.9) ergs s(-1)) as well as weaker emission from five other pre-main-sequence stars. Althoug h no large-amplitude flares were detected, the X-ray emission of SU Aur sho wed clear variability in the form of a slow decline in count rate during th e 1.3 day observation. We provide the first direct comparison of the coronal differential emission measure (DEM) distribution of a classical T Tauri star with that of a youn g main-sequence star of similar spectral type. The DEM distributions of SU Aur (G2; age 3 Myr) and the young solar-like star EK Draconis (G0 V; age 70 Myr) are qualitatively similar, with both showing a bimodal temperature di stribution characterized by a cool plasma component peaking at similar to 8 -9 MK and a hot component peaking at similar to 20-21 MK. However, there is a striking difference in the relative proportion of plasma at high tempera tures in the two stars, with hot plasma (greater than or equal to 20 MK) ac counting for approximate to 80% of the volume emission measure of SU Aur, c ompared to only approximate to 40% in EK Dra. These results provide new insight into the changes that will occur in the c orona of a T Tauri star as it descends onto the main sequence. A sharp decl ine in the fraction of coronal plasma at flarelike temperatures will occur during the late-T Tauri and post-T Tauri phases, and other recent X-ray stu dies have shown that this decline will continue after the young solar-like star reaches the main sequence.