# # Program information file # PROGRAM_ID 2023B043 PROGRAM_TITLE A ToO study of young stars with major eruptions PROGRAM_INV1 Bo Reipurth PROGRAM_INV2 Michael Connelley PROGRAM_INV3 PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG Newborn stars gain their mass by accretion from their circumstellar environment. This was theoretically predicted to occur in a gradually declining manner, but observations of young low-mass stars in major FUor and EXor outbursts have shown that up to 50% of a star's mass may be gained in sudden major accretion events. Such events, during which a T Tauri star increases 5-6 magnitudes in brightness, remain a mystery, with many flavors of accretion events suggested as triggering mechanisms, divided into two classes: disk instability scenarios and external perturbations from stars, brown dwarfs, or giant planets. Given the importance of such events in the build-up of a star as well as our current state of ignorance, we propose a long-term ToO program to obtain a SpeX spectrum as early as possible in the ascending branch of a FUor or EXor event, which is when the signature of the triggering event is most likely to be observable. A major study of all known FUors using SpeX in cross-dispersed mode across the full z,J,H,K range exists, so it is possible to pick out discrepancies between spectra taken on the ascending branch and later in the elevated state. Two major all-sky surveys, ATLAS and ASAS-SN, can produce alerts when certain conditions are met. Our triggering threshold is r>3 mag for optical events [and K>2 mag for embedded objects in case such an object is discovered serendipitously during other observations]. Based on [very incomplete] statistics of such previous rare events, we estimated that we have a 50% chance to witness an eruption per semester. We have submitted this proposal in 6 semesters and have successfully observed 5 events. PROGRAM_ABSTRACT_END