Reflected infrared spectrum of a massive protostar in Orion.

The infrared source IRc2 in the star-forming region Orion-KL is generally believed to contain a massive and very young star. Its nature and evolutionary status, however, are difficult to determine because it is hidden from direct view by a dense disklike envelope of gas and dust. Here we report observations of infrared radiation (at a wavelength of about 2 microm) that has escaped the surrounding dust in the polar direction, perpendicular to the plane of the disk, and then been reflected towards us by dust farther away from the star. The reflected spectrum contains absorption lines of neutral metallic atoms and carbon monoxide, which we interpret as indicating a source temperature of about 4,500 K. But, given the luminosity of the source, its radius must be at least 300 solar radii-too large to be attained with the modest gas-accretion rates in existing theories of massive-star formation. Whether the infrared radiation is coming from the protostar itself or the self-luminous accretion disk around it, the accretion rate must be around (5-15) x 10(-3) solar masses per year, at least two orders of magnitude greater than is commonly assumed in models of star formation.