Constraining the Rate of Protostellar Accretion Outbursts in the Orion Molecular Clouds

Outbursts due to dramatic increases in the mass accretion rate are the most extreme type of variability in young stellar objects. We searched for outbursts among 319 protostars in the Orion molecular clouds by comparing 3.6, 4.5, and 24 micron photometry from the Spitzer Space Telescope to 3.4, 4.6, and 22 micron photometry from the Wide-field Infrared Survey Explorer (WISE) obtained ~ 6.5 yr apart. Sources that brightened by more than two standard deviations above the mean variability at all three wavelengths were marked as burst candidates, and they were inspected visually to check for false positives due primarily to the reduced angular resolution of WISE compared to Spitzer. We recovered the known burst V2775 Ori (HOPS 223) as well as a previously unknown burst, HOPS 383, which we announced in an earlier paper. No other outbursts were found. With observations over 6.5 yr, we estimate an interval of about 1000 yr between bursts with a 90% confidence interval of 690 to 40,300 yr. The most likely burst interval is shorter than those found in studies of optically revealed young stellar objects, suggesting that outbursts are more frequent in protostars than in pre-main-sequence stars that lack substantial envelopes.