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Mining the Roman GBTDS for Ultracool Dwarfs: Evolving Variability and Transiting Companions
Program ID 19106
Science Category Stellar Physics
Program Type Analysis
Category Small
Principal Investigator Mary Anne Limbach
PI Institution University of Michigan
Co-Investigators
  • Brooke Kotten (University of Michigan)
  • Rohan Gupta (University of Michigan)
Abstract We propose an analysis of the Roman Galactic Bulge Time Domain Survey (GBTDS) to enable the first population-level, long-baseline time-domain study of nearby (within 500 pc) substellar objects, including ultracool dwarfs spanning the late-M, L, T, and Y spectral classes and planetary-mass objects. The GBTDS provides long (70–90 days per season), high-cadence light curves in the Roman WFI F146 wide filter, sampled every 12.1 minutes, together with repeated F087 (Z) and F213 (K) observations at lower cadence. This combination opens a new regime for studying rotationally modulated variability driven by heterogeneous cloud structures and atmospheric circulation. We will extract multi-band lightcurves for all sources in the survey and identify ~100 nearby substellar objects using variability, color, time-domain diagnostics, and machine-learning classification. These data will enable transformative science: probing atmospheric variability, including enhanced variability at the L/T transition associated with cloud clearing, measuring rotation periods, and tracking variability evolution over many cycles at unprecedented sensitivity and temporal baseline. As part of this effort, we will develop and validate methods to robustly identify ultracool dwarf sources in crowded, confusion-limited fields. We will also search for transiting companions among this population. While transit probabilities are low for older brown dwarfs, the sample size provides a realistic opportunity to detect a small number of systems. Given the scarcity of planets known around very low-mass hosts, even a few detections would constrain occurrence rates, and the most compelling systems will be prime targets for JWST follow-up. We will release a uniform catalog of nearby substellar objects with variability metrics, transit candidates, and rotation constraints, providing a benchmark sample of field-age ultracool dwarfs spanning a wide range of spectral types, surface gravities, and atmospheric regimes.