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Y. Tsuboi (Penn State), K. Hamaguchi, K. Imanishi, K. Koyama (Kyoto), Y. Sekimoto, K. Tatematsu (NRO), J. Bally, B. Reipurth (Colorado)
A young stellar object (YSO) evolves from a molecular cloud core through the protostellar and T Tauri phases to a main sequence star. In order to search when a star begins to emit X-rays, we observed \rho Oph molecular cloud (d~160 pc) and Orion Molecular Cloud (OMC) 2/3 regions (d~450 pc) with ACIS-I instrument onboard Chandra X-ray Observatory (CXO).
Using hard X-ray band (> 2 keV) of CXO with up to sub-arcsec resolution, we detected more than 70 % of the 22 Class I protostars, in a 17' \times 17' field of view at the \rho Oph molecular cloud. The high detection rate of Class I protostars in this Chandra observation changed our understanding qualitatively: Most Class I protostars have emitted prominent X-rays already. Seven of the detected Class I protostars showed nine X-ray flares during the 100 ksec observation time. This indicates that Class I protostars are magnetically very active.
In the OMC-2/3 regions, we further discovered very hard X-rays (> 3 keV) from two sources which is most likely Class 0 protostars, earlier phase of protostars. The X-ray spectra of these sources show absorption column of as much as (1--3) \times1023 H cm-2. This result indicates for the first time that protostars can emit X-rays preceding Class I phase certainly.