A Giant Outburst at Millimeter Wavelengths in the Orion Nebula

BIMA observations of the Orion nebula discovered a giant flare from a youngstar previously undetected at millimeter wavelengths. The star briefly becamethe brightest 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 peak of 160mJy. This is one of the most luminous stellar radio flares ever observed.Remarkably, the Chandra X-ray observatory was in the midst of a deepintegration of the Orion nebula at the time of the BIMA discovery; the source'sX-ray flux increased by a factor of 10 approximately 2 days before the radiodetection. Follow-up radio observations with the VLA and BIMA showed that thesource decayed on a timescale of days, then flared again several times over thenext 70 days, although never as brightly as during the discovery. Circularpolarization was detected at 15, 22, and 43 GHz, indicating that the emissionmechanism was cyclotron. VLBA observations 9 days after the initial flare yielda brightness temperature Tb > 5 x 10^7 K at 15 GHz. Infrared spectroscopyindicates the source is a K5V star with faint Br gamma emission, suggestingthat it is a weak-line T Tauri object. Zeeman splitting measurements in theinfrared spectrum find B ~ 2.6 +/- 1.0 kG. The flare is an extreme example ofmagnetic activity associated with a young stellar object. These data suggestthat short observations obtained with ALMA will uncover hundreds of flaringyoung stellar objects in the Orion region.Comment: 29 pages, 7 figures, accepted for publication in Ap