Flares on TRAPPIST-1 reveal the spectrum of magnetic features on its surface

TL;DR

This study uses JWST observations to measure the spectrum of magnetic features on TRAPPIST-1, revealing flare-induced magnetic feature disappearance and its impact on stellar brightness, which is crucial for exoplanet atmosphere studies.

Contribution

We introduce a novel method to measure magnetic feature spectra on an M8 dwarf using time-resolved JWST data, linking stellar flares to magnetic surface changes.

Findings

Detected persistent post-flare spectral flux enhancement on TRAPPIST-1.

Linked flux changes to disappearance of magnetic features similar to solar phenomena.

Estimated magnetic features are cooler than the stellar photosphere by a few hundred kelvins.

Abstract

TRAPPIST-1 is an M8 dwarf hosting seven known exoplanets and is currently one of the most frequently observed targets of the James Webb Space Telescope (JWST). However, it is notoriously active, and its surface is believed to be covered by magnetic features that contaminate the planetary transmission spectra. The radiative spectra of these magnetic features are needed to clean transmission spectra, but they currently remain unknown. Here, we develop a new approach for measuring these spectra using time-resolved JWST/NIRISS observations. We detect a persistent post-flare enhancement in the spectral flux of TRAPPIST-1. Our analysis rules out lingering flare decay as the cause of the flux enhancement and, thus, points to structural changes on the stellar surface induced by flares. We suggest that the flaring event triggers the disappearance of (part of) a dark magnetic feature, producing a net brightening. This suggestion is motivated by solar data: flare-induced disappearance of magnetic features on the solar surface has been directly detected in high spatial resolution images, and our analysis shows that this process produces changes in solar brightness very similar to those we observe on TRAPPIST-1. The proposed explanation for the flux enhancement enables, to our knowledge, the first measurement of the spectrum of a magnetic feature on an M8 dwarf. Our analysis indicates that the disappearing magnetic feature is cooler than the TRAPPIST-1 photosphere, but by at most a few hundred kelvins.