# # Program information file # PROGRAM_ID 2022A075 PROGRAM_TITLE Studying the Formation and Dissipation of Transient Decretion Disks in Newly Discovered Be Stars PROGRAM_INV1 Guy Stringfellow PROGRAM_INV2 PROGRAM_INV3 PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG Be stars are relatively rare objects believed to be rapidly rotating B-stars undergoing non-radial pulsations that eject matter into the circumstellar environment temporarily forming a decretion disk. How this process works and the physics and time scales driving the formation of decretion disks remain elusive. Historically, classification as a Be-star if the observed stellar optical spectrum resembles that of a B-type star displaying mostly normal absorption lines, but with some [primarily H-alpha or other Balmer lines] displaying [line-core] emission. Disks are now being found around hot massive stars of other spectral types, ranging from A-O. Further, many previously observed normal B-stars have subsequently been found to display emission lines, emphasizing the transient nature of these decretion disks. Large surveys are discovering hundreds of previously unknown Be-stars, making their prevalence and importance in the evolution of massive stars more impactful. These serendipitous discoveries often have no previous spectra so their classification and evolutionary state remain unclear. To make progress in understanding the underlying physics responsible for the formation and dissipation of these decretion disks, the intervals of quiescence between outbursts, the duration or longevity of these disks, the formation and dissipation events need to be well studied. This requires spectral classification and concerted time domain studies with cadences enabling coverage of the events. Also required are coordinated photometric and spectroscopic observations over a wide wavelength range. We will carry out monitoring observations of a sample of [newly discovered] Be-stars already displaying evidence of transitory disks over the monitoring time scales proposed along with high-resolution optical spectroscopy and optical photometric monitoring. This unprecedented dataset will enable the formation and dissipation of these disks to be studied through measured emission line ratios and structural analysis, rotation/angular momentum diagnostics, and their evolution through decretion disk formation and dispersal in massive star populations. PROGRAM_ABSTRACT_END