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 μm with the Near Infrared Imager and Slitless Spectrograph and 0.6–3.5 μm with the Near Infrared Spectrograph during transits of TRAPPIST-1b, f, and g. We discover Pα and Brβ line emission and characterize flare continuum at wavelengths from 1–3.5 μm for the first time. Observed lines include Hα, Pα–Pϵ, Brβ, He i λ0.7062 μm, two Ca ii infrared triplet (IRT) lines, and the He i IRT. We observe a reversed Paschen decrement from Pα–Pγ 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 the temperatures typically observed at optical wavelengths. The 0.6–1 μm spectra were convolved with the Transiting Exoplanet Survey Satellite (TESS) response, enabling us to measure the flare rate of TRAPPIST-1 in the TESS bandpass. We find flares of 1030 erg, large enough to impact transit spectra occur at a rate of 3.6 − 1.3 + 2.1 flare day−1, ∼10× higher than previous predictions from K2. We measure the amount of flare contamination at 2 μm for the TRAPPIST-1b and f transits to be 500 ± 450 and 2100 ± 400 ppm, respectively. We find up to 80% of flare contamination can be removed, with mitigation most effective from 1.0–2.4 μm. These results suggest transits affected by flares may still be useful for atmospheric characterization efforts.

Muted Features in the JWST NIRISS Transmission Spectrum of Hot Neptune LTT 9779b

“…Other mass-loss tracers like Lyα (the system is only at a distance of 80 pc, and interstellar medium absorption might not be completely optically thick) or lines of ionized metal (Linssen & Oklopčić 2023) may prove to be more fruitful tracers of atmospheric escape around Sun-like stars. SOSS is also potentially able to probe atmosphere loss via Hα (Vidal-Madjaret al 2003;Howard et al 2023;Lim et al 2023) though we also find no evidence for excess Hα absorption in our data. Thus, as proposed by Fernández Fernández et al (2024), due to the weak high-energy emission of the host star, LTT 9779 b may simply not be undergoing sufficiently significant photoevaporative mass loss for any of the aforementioned tracers to be observable.…”

Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Transmission Spectroscopy

“…The simulated JWST transmission spectra of Gliese 12 b demonstrate detectable atmospheric features for a range of compositions, underscoring the potential to characterize this terrestrial exoplanet using a modest amount of JWST time. Furthermore, transit photometry with JWST for an active star like TRAPPIST-1 is disturbed by the contamination of flares (Howard et al 2023), which should be less problematic for Gliese 12 due to its quiescent nature.…”

Time-dependent Stellar Flare Models of Deep Atmospheric Heating

“…The v1.0 model grid has a large number of valuable applications. These models have already been leveraged in a variety of ways: studies of the merging of the Balmer series (Kowalski et al 2017b) for NUV exoplanet habitability assessments (Kowalski 2022), the energy partition in NUV and optical photometry (Brasseur et al 2023), infrared flaring in JWST spectra and the response of the He I λ10830 line (Howard et al 2023), solar-stellar comparisons (Monson et al 2024), and the relations among gas, color, and radiation temperatures in flare models (Kowalski 2023). Many further applications of the grid models are possible.…”

“…The detailed spectra and the hydrodynamic output are made publicly available to facilitate comparisons to and interpretations of stellar flare data. The grid spectra have already been used for a wide range of purposes (Kowalski et al 2017b;Namekata et al 2020;Kowalski 2022Kowalski , 2023Brasseur et al 2023;Howard et al 2023;Monson et al 2024).…”