# Lower-than-expected flare temperatures for TRAPPIST-1

**Authors:** A. J. Maas, E. Ilin, M. Oshagh, E. Pallé, H. Parviainen, K. Molaverdikhani, A. Quirrenbach, E. Esparza-Borges, F. Murgas, V. J. S. Béjar, N. Narita, A. Fukui, C. -L. Lin, M. Mori, P. Klagyivik  
**arXiv:** [2210.11103](https://arxiv.org/abs/2210.11103)  
**Date:** 2022-10-20  
**Summary page:** https://www.summarizepaper.com/en/paper/2210.11103v1/

## Key points

- Researchers investigated two flares on TRAPPIST-1, an ultra-cool dwarf star that hosts seven exoplanets.
- Flares are high energetic radiation events that can potentially threaten exoplanet atmospheres and lead to surface sterilization.
- The flares were detected in all four passbands of the MuSCAT2 instrument, allowing for a determination of their temperatures and bolometric energies.
- Effective black body temperatures of flares that occurred on TRAPPIST-1 were inferred by the researchers for the first time.
- The study showed that for ultra-cool M-dwarf TRAPPIST-1, the flare black body temperatures associated with total continuum emission are lower than expected and not consistent with usually adopted assumptions of 9000–10000 K. This could imply different and faster cooling mechanisms.
- Further multi-color observations are needed to investigate whether or not these observations are a general characteristic of ultra-cool M dwarfs.

## Summary

In a recent study, researchers investigated two flares on TRAPPIST-1, an ultra-cool dwarf star that hosts seven exoplanets, three of which lie within its habitable zone. Flares are high energetic radiation events that can potentially threaten exoplanet atmospheres and lead to surface sterilization. However, they may also provide the extra energy for low-mass stars needed to trigger and sustain prebiotic chemistry. The flares were detected in all four passbands of the MuSCAT2 instrument, allowing for a determination of their temperatures and bolometric energies. The researchers analyzed the light curves of the MuSCAT1 and MuSCAT2 instruments obtained between 2016 and 2021 in $g$, $r$, $i$, and $z_\mathrm{s}$-filters. They conducted an automated flare search and visually confirmed possible flare events. For the first time, effective black body temperatures of flares that occurred on TRAPPIST-1 were inferred by the researchers. The black body temperatures for the two TRAPPIST-1 flares derived from the spectral energy distribution (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 peak were also calculated from the peak SED yielding $T_\mathrm{SEDp} = 13620_{-1220}^{1520}$K and $T_\mathrm{SEDp} = 8290_{-550}^{+660}$K. The study showed that for ultra-cool M-dwarf TRAPPIST-1, the flare black body temperatures associated with total continuum emission are lower than expected and not consistent with usually adopted assumptions of 9000–10000 K. This could imply different and faster cooling mechanisms. Further multi-color observations are needed to investigate whether or not these 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 models with higher flare temperatures.