Lower-than-expected flare temperatures for TRAPPIST-1

Abstract

Although high energetic radiation from flares is a potential threat to exoplanet atmospheres and may lead to surface sterilization, it might also provide the extra energy for low-mass stars needed to trigger and sustain prebiotic chemistry. We investigate two flares on TRAPPIST-1, an ultra-cool dwarf star that hosts seven exoplanets of which three lie within its habitable zone. The flares are detected in all four passbands of the MuSCAT2 allowing a determination of their temperatures and bolometric energies. We analyzed the light curves of the MuSCAT1 and MuSCAT2 instruments obtained between 2016 and 2021 in $g,r,i,z_{\mathrm{s}}$-filters. We conducted an automated flare search and visually confirmed possible flare events. We studied the temperature evolution, the global temperature, and the peak temperature of both flares. For the first time we infer effective black body temperatures of flares that occurred on TRAPPIST-1. The black body temperatures for the two TRAPPIST-1 flares derived from the SED are consistent with $T_{\mathrm{SED}}=7940_{-390}^{+430}$K and $T_{\mathrm{SED}}=6030_{-270}^{+300}$K. The flare black body temperatures at the peak are also calculated from the peak SED yielding $T_{\mathrm{SEDp}}=13620_{-1220}^{1520}$K and $T_{\mathrm{SEDp}}=8290_{-550}^{+660}$K. We show that for the ultra-cool M-dwarf TRAPPIST-1 the flare black body temperatures associated with the total continuum emission are lower and not consistent with the usually adopted assumption of 9000-10000 K. This could imply different and faster cooling mechanisms. Further multi-color observations are needed to investigate whether or not our observations are a general characteristic of ultra-cool M-dwarfs. This would have significant implications for the habitability of exoplanets around these stars because the UV surface flux is likely to be overestimated by the models with higher flare temperatures.

17 pages, 11 figures, 2 tables, accepted for publication in A&A

References in corpus (22)

• The Transiting Exoplanet Survey Satellite
• The K2 Mission: Characterization and Early results
• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• Constraining the Age-Activity Relation for Cool Stars: The SDSS DR5 Low-Mass Star Spectroscopic Sample
• The Solar Neighborhood XVII: Parallax Results from the CTIOPI 0.9m Program -- Twenty New Members of the RECONS 10 Parsec Sample
• Kepler Flares I. Active and Inactive M dwarfs
• The Kepler Catalog of Stellar Flares
• Kepler Flares II: The Temporal Morphology of White-Light Flares on GJ 1243
• The habitability of Proxima Centauri b. I. Irradiation, rotation and volatile inventory from formation to the present
• Magnetism, rotation, and nonthermal emission in cool stars -- Average magnetic field measurements in 292 M dwarfs
• Global Energetics of Solar Flares: V. Energy Closure in Flares and Coronal Mass Ejections
• Quasi-periodic pulsations in solar and stellar flares: an overview of recent results
• The first observed stellar X-ray flare oscillation: Constraints on the flare loop length and the magnetic field
• Multiwavelength observations of a giant flare on CN Leonis I. The chromosphere as seen in the optical spectra
• EvryFlare III: Temperature Evolution and Habitability Impacts of Dozens of Superflares Observed Simultaneously by Evryscope and TESS
• Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER and OISTER
• The Mega-MUSCLES Spectral Energy Distribution Of TRAPPIST-1
• MuSCAT2 multicolour validation of TESS candidates: an ultra-short-period substellar object around an M dwarf
• Giant white-light flares on fully convective stars occur at high latitudes
• The Statistical Relationship between White-light Emission and Photospheric Magnetic Field Changes in Flares
• Evryscope and K2 Constraints on TRAPPIST-1 Superflare Occurrence and Planetary Habitability
• Surface and oceanic habitability of Trappist-1 planets under the impact of flares

Cited by in corpus (2)

• Stellar flare morphology with TESS across the main sequence
• Separating flare and secondary atmospheric signals with RADYN modeling of near-infrared JWST transmission spectroscopy observations of TRAPPIST-1