A Panchromatic JWST Spectrum of a Giant Starspot on the Fully Convective M-dwarf TOI-3884

TL;DR

This study uses JWST observations of a giant starspot on an M-dwarf to empirically measure its spectrum, revealing discrepancies with models and emphasizing the importance of empirical data for exoplanet atmosphere analysis.

Contribution

First empirical panchromatic spectrum of an M-dwarf starspot obtained with JWST, providing a benchmark for stellar atmosphere models in the fully convective regime.

Findings

Starspot is 185K cooler than the photosphere.

Models match long-wavelength contrasts but underpredict short-wavelength contrasts.

Empirical spot spectra are crucial for accurate exoplanet atmosphere interpretation.

Abstract

TOI-3884 b is a rare super-Neptune transiting a fully convective M dwarf that hosts a persistent giant polar spot. Because the planet occults this active region during every transit, the system offers a unique laboratory to directly probe the stellar surface and spot properties. We present seven James Webb Space Telescope (JWST) transits of TOI-3884 b observed with NIRISS and NIRSpec, spanning 0.5--5.3$\mu$m. While all visits show a recurring spot-crossing signature, each transit exhibits a distinct spot-crossing morphology, enabling us to infer a stellar rotation period of $P$=11.102$\pm$0.003d and tightly constrain the pole-on stellar orientation ($i_{*}$=40.8$\pm$0.3$^{\circ}$, $\lambda_{*}$=148.9$\pm$0.4$^{\circ}$) and spot properties ($R_{\rm{spot}}=0.576^{+0.006}_{-0.005}$R$_{*}$, $\phi_{\rm{spot}}$=84.69$\pm$0.12$^{\circ}$). We leverage this orbital configuration to measure the first empirical panchromatic spectrum of an M-dwarf starspot with JWST, establishing a direct observational benchmark for stellar atmosphere models in the fully convective regime. Comparison with 1D NewEra and SPHINX atmosphere models indicates that the spot is 185$\pm$2K cooler than the photosphere, consistent with previous ground-based measurements and expectations for mid-M-dwarf spot contrasts. While the models reproduce the observed contrasts at wavelengths longer than 1$\mu$m, they significantly underpredict the contrasts at shorter wavelengths. These results demonstrate that M-dwarf stellar atmosphere models alone may not fully capture the wavelength dependence of stellar contamination in transmission spectra and highlight the importance of empirical spot spectra for robust interpretation of planetary atmospheres, particularly in the optical.