2015ApJ...808...68H

The FU Orionis class of young stellar objects is enigmatic and rare. The members are interpreted to be ''outbursting,'' that is, they are currently in a state of enhanced accretion by several orders of magnitude relative to the more modest disk-to-star accretion rates measured in typical T Tauri stars. They are key to our understanding of the history of stellar mass assembly and pre-main-sequence evolution, and are also critical when considering the chemical and physical evolution of the circumstellar environment–where planets form. A common supposition is that all T Tauri stars repeatedly undergo such outbursts, more frequently in their earlier evolutionary stages when the disks are more massive, so as to build up the requisite amount of stellar mass on the required timescale. However, the actual data supporting this traditional picture of episodically enhanced disk accretion are limited, and the observational properties of the known sample of FU Ori objects are quite diverse. To improve our understanding of these rare objects, we outline the logic used to meaningfully constrain the rate of FU Ori outbursts and present numbers to guide parameter choices in the analysis of time domain surveys.