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K. Koyama (Kyoto University)
We first review the pioneering works with ASCA and ROSAT, the discovery of heavy absorbed X-rays and flares from class Is, and implications on the stellar evolution and structure. We then discuss on the results of the Chandra X-ray Observatory (CXO): the X-ray detections from 16 out of 22 class Is near the center of the Rho-Oph cloud. Surprisingly about half of the 16 class Is exhibited a clear flare within the exposure window of 100 k-sec. With the deep exposure on the Orion 2/3 cloud, CXO found highly absorbed X-rays from 7 of the 36 molecular cores (Class 0 candidates). This indicates that X-rays can be emitted from very early phase of the stellar evolution (class 0s) with ages younger than 105 year.