Characterizing the Near-infrared Spectra of Flares from TRAPPIST-1 During JWST Transit Spectroscopy Observations

Abstract

We present the first analysis of JWST near-infrared spectroscopy of stellar flares from TRAPPIST-1 during transits of rocky exoplanets. Four flares were observed from 0.6--2.8 $\hat{I}¼$m with NIRISS and 0.6--3.5 $\hat{I}¼$m with NIRSpec during transits of TRAPPIST-1b, f, and g. We discover P$\hat{I}\pm$ and Br${\hat{I}}^2$ line emission and characterize flare continuum at wavelengths from 1--3.5 $\hat{I}¼$m for the first time. Observed lines include H$\hat{I}\pm$, P$\hat{I}\pm$-P$\hat{I}\mu$, Br${\hat{I}}^2$, He I $\hat{I}»$0.7062$\hat{I}¼$m, two Ca II infrared triplet (IRT) lines, and the He I IRT. We observe a reversed Paschen decrement from P$\hat{I}\pm$-P${\hat{I}}^3$ alongside changes in the light curve shapes of these lines. The continuum of all four flares is well-described by blackbody emission with an effective temperature below 5300 K, lower than temperatures typically observed at optical wavelengths. The 0.6--1 $\hat{I}¼$m spectra were convolved with the TESS response, enabling us to measure the flare rate of TRAPPIST-1 in the TESS bandpass. We find flares of 10${}^{30}$ erg large enough to impact transit spectra occur at a rate of 3.6$\begin{array}{c}+2.1\\ -1.3\end{array}$ flare d${}^{-1}$, $\sim$10$\times$ higher than previous predictions from K2. We measure the amount of flare contamination at 2 $\hat{I}¼$m for the TRAPPIST-1b and f transits to be 500$\pm$450 and 2100$\pm$400 ppm, respectively. We find up to 80% of flare contamination can be removed, with mitigation most effective from 1.0--2.4 $\hat{I}¼$m. These results suggest transits affected by flares may still be useful for atmospheric characterization efforts.

29 pages, 17 figures, 3 tables, accepted to The Astrophysical Journal