A Study of Flares in the Ultra-Cool Regime from SPECULOOS-South

We present a study of photometric flares on 154 low-mass ($\leq 0.2
\textrm{M}_{\odot}$) objects observed by the SPECULOOS-South Observatory from
1st June 2018 to 23rd March 2020. In this sample we identify 85 flaring
objects, ranging in spectral type from M4 to L0. We detect 234 flares in this
sample, with energies between $10^{29.2}$ and $10^{32.7}$ erg, using both
automated and manual methods. With this work, we present the largest
photometric sample of flares on late-M and ultra-cool dwarfs to date. By
extending previous M dwarf flare studies into the ultra-cool regime, we find
M5-M7 stars are more likely to flare than both earlier, and later, M dwarfs. By
performing artificial flare injection-recovery tests we demonstrate that we can
detect a significant proportion of flares down to an amplitude of 1 per cent,
and we are most sensitive to flares on the coolest stars. Our results reveal an
absence of high-energy flares on the reddest dwarfs. To probe the relations
between rotation and activity for fully convective stars, we extract rotation
periods for fast rotators and lower-bound period estimates of slow rotators.
These rotation periods span from 2.2 hours to 65 days, and we find that the
proportion of flaring stars increases for the very fastest rotators. Finally,
we discuss the impact of our flare sample on planets orbiting ultra-cool stars.
As stars become cooler, they flare less frequently; therefore, it is unlikely
that planets around the very reddest dwarfs would enter the `abiogenesis' zone
or drive visible-light photosynthesis through flares alone.