A Giant Outburst at Millimeter Wavelengths in the Orion Nebula

Abstract

Berkely-Illinois-Maryland Association (BIMA) array observations of theOrion nebula discovered a giant flare from a young star previouslyundetected at millimeter wavelengths. The star briefly became thebrightest compact object in the nebula at 86 GHz. Its flux densityincreased by more than a factor of 5 on a timescale of hours, to a peakof 160 mJy. This is one of the most luminous stellar radio flares everobserved. Remarkably, the Chandra X-Ray Observatory was in the midst ofa deep integration of the Orion nebula at the time of the BIMAdiscovery; the source's X-ray flux increased by a factor of 10approximately 2 days before the radio detection. Follow-up radioobservations with the VLA and BIMA showed that the source decayed on atimescale of days, then flared again several times over the next 70days, although never as brightly as during the discovery. Circularpolarization was detected at 15, 22, and 43 GHz, indicating that theemission mechanism was cyclotron. VLBA observations 9 days after theinitial flare yield a brightness temperatureTb>5×107 K at 15 GHz. Infraredspectroscopy indicates that the source is a K5 V star with faint Brγ emission, suggesting that it is a weak-line T Tauri object.Zeeman splitting measurements in the infrared spectrum find B~2.6+/-1.0kG. The flare is an extreme example of magnetic activity associated witha young stellar object. These data suggest that short observationsobtained with the Atacama Large Millimeter Array will uncover hundredsof flaring young stellar objects in the Orion region.