McCarthy et al. Map Three HST Initiatives for Substellar Atmosphere Evolution Through 2030s

The paper synthesizes prior HST and ground-based results showing clouds, disequilibrium chemistry, and auroral emission in L/T dwarfs. It proposes three large initiatives: multi-epoch UV-optical monitoring for thermal structure, coordinated multi-facility campaigns to link HST data with JWST and ground-based infrared spectra, and targeted UV auroral searches in objects with known radio emission. Each program leverages HST’s unique access to 0.2–0.8 µm wavelengths unavailable to other current facilities.

These observations directly test atmospheric circulation models and cloud sedimentation timescales across 10–1000 Myr ages, providing empirical benchmarks for retrieval codes that will be applied to Habitable Worlds Observatory spectra. The roadmap explicitly connects substellar science to exoplanet characterization pipelines, noting that brown dwarfs supply high-signal-to-noise templates free of host-star contamination.

The main limitation is the absence of guaranteed HST time allocation beyond Cycle 32; the authors therefore recommend Director’s Discretionary large programs. Strengthening the case requires demonstrated pilot observations in Cycle 31 that achieve 3–5 % photometric precision over multiple rotations.