AN X-RAY SUPERFLARE FROM AN INFRARED PROTOSTAR

Class I protostars(1) are very young, low-mass stellar objects that ar e, according to current models, composite: they include a central star (still in the process of formation) surrounded by an accretion disk s imilar to 10-100 AU in radius and embedded in an extended, infalling e nvelope of gas and dust up to similar to 104 AU in size(2). X-ray emis sion from such protostars has recently been reported(3,4), suggesting that X-ray ionization of gas and heating of dust could profoundly infl uence the physical and chemical properties of young stellar systems. B ut these observations did not have the resolution necessary to rule ou t a non-protostar origin for the emissions. Here we report X-ray obser vations of one of the nearest star-forming regions-the rho Ophiuchi cl oud-that clearly show an intense X-ray flare associated with a deeply embedded protostar. The peak X-ray luminosity, after correcting for ex tinction, is greater than or equal to 10-100 times the Sun's bolometri c luminosity. The behaviour and intensity of the flare can be modelled as arising from a magnetically confined, low-density plasma bubble si milar to 0.05-0.3 Au in diameter (much larger than the star itself), a nd the X-ray luminosity equals or exceeds the bolometric luminosity of the forming star. Taken together the evidence suggests that the X-ray s are not created by the type of magnetic activity seen on the Sun or on other young low-mass stars, but rather are associated with processe s in the circumstellar accretion disk or within the envelope.