THE FIRST DEFINITE DETECTION OF X-RAYS FROM AN EXTREMELY YOUNG PROTOSTAR

摘要

Class I protostars exhibit powerful X-ray emission. Their X-ray activity can exceed those of older stars such as T-Tauri and main-sequence stars. Without surface convection to drive a solar-type dynamo mechanism, X-ray activity is suspected to be driven by magnetic activity linked to the mass accretion process. Mass accretion is thought to be more intense in the earliest Class 0 protostar phase, but X-ray emission from them has not been conclusively detected so far. This could be due to stronger X-ray absorption to their protostellar cores, or a certain transition in high energy activity between the Class 0 and Class I phases. With two XMM-Newton observations on March 2003, we detected for the first time strong X-ray emission from an extremely embedded source in the R Corona Australis star forming core, IRS 7 region (Hamaguchi et al., 2005). The source has the radio counterpart 10E (IRS7B) but no near-IR counterpart. These facts plus the strong X-ray absorption of N{sub}H ～3×10{sup}23cm{sup}(-2) (equivalent to A{sub}V ～180{sup}m) indicate that the source is a Class 0 or perhaps a Class 0/I protostar. The X-ray spectrum showed thermal emission with kT=3-4keV with the luminosity up to 10{sup}31.2 ergs s{sup}(-1). The light curve showed gradual flux increase by a factor of two during 30 ksec, unlike solar-type magnetically driven X-ray flares which have smaller variation timescales. The source was 10-100 times fainter during Chandra observations which occurred before and after the XMM-Newton observations. The flux enhancement on month timescales might be driven by sporadic mass accretion episode, while the short-term variation during the XMM-Newton observation could be related to the proto-stellar core rotation.