UCD flares: similar characteristics to FGKM flares but cannot drive abiogenesis

In this contribution, we present the characterization of the photometric variability of 208 ultra-cool dwarfs (UCDs) with spectral types between M4 and L4 using 2-min cadence data from the space mission TESS. We determined the rotation period of 87 UCDs and detected 778 flares in 103 of them. Through the analysis of these data, we were able to conclude that the correlations among the measured flare characteristics, namely the slope of the cumulative flare frequency distribution, flare amplitude, duration and energy, are consistent with those measured for dwarf stars of earlier spectral types. Our results indicate that UCD flares are similar to those produced by FGK and earlier M dwarfs. According to traditional understanding, magnetic fields in stars with convective envelopes are seated at the tachocline, which fully-convective UCDs do not have. In this context, our results suggest that the physical mechanism that produces flares might be similar in these very differently structured dwarfs, for example if driven by magneto-instabilities near their surfaces. Flares are thought to initiate abiogenesis in terrestrial planets; we find that the UV energy emitted during the UCD flares in our sample is not enough to drive prebiotic chemistry on any terrestrial planet potentially orbiting them.

Theme(s): The Sun and Cool Stars in the Time Domain, Brown Dwarfs and Giant Exoplanets: Future Prospects and Thirty Years of Discovery