A Search for Variability of Ultracool Dwarfs with the Zwicky Transient Facility

TL;DR

This study uses ZTF optical light curves to identify rotation periods of ultracool dwarfs, revealing trends in rotation with spectral type and providing new data for understanding their atmospheric properties.

Contribution

First large-scale search for ultracool dwarf rotation periods using ZTF data, identifying 226 periodic variables including new and previously known periods.

Findings

Identified 226 periodic ultracool dwarfs, with 12 new period detections.

Found a trend of decreasing rotation periods toward later spectral types.

Results broadly consistent with angular-momentum-conservation models for brown dwarfs.

Abstract

Rotationally modulated photometric variability of ultracool dwarfs encodes key information about cloud structure and temperature contrasts. Large homogeneous optical datasets are crucial for linking atmospheric heterogeneity to fundamental parameters such as rotation, mass, and age. We present a search for rotation periods in ultracool dwarfs using Zwicky Transient Facility (ZTF) optical light curves. By propagating the coordinates to the ZTF epoch and applying Lomb-Scargle analysis, we identified 226 periodic variables, including 32 robust detections and 194 tentative cases. Among the robust detections, 12 have no previously published periods, while 20 have literature counterparts, most of which are consistent with the published values. Most robust detections are M dwarfs, reflecting the optical sensitivity limits of ZTF. We find a trend of decreasing periods toward later spectral types in relatively old dwarfs (> 100 Myr), suggesting faster rotation for late-M types than for mid-M types. The age-period relation of our sample is broadly consistent with angular-momentum-conservation models at higher-mass regime of brown dwarfs, consistent with the M-dwarf bias of our catalog. Many additional candidates remain to be confirmed due to sparse sampling or low S/N. Future high-cadence, multi-wavelength monitoring and systematic mining of ZTF and upcoming surveys will be crucial for validating these periods, extend sensitivity to later (L/T) types, and better connect rotation with cloud physics across the stellar-substellar boundary.