HiPERCAM and TESS observations of the rapidly rotating M7V star LP 89--187

The discovery of a significant number of rapidly rotating low mass stars showing no or few flares in TESS observations was a surprise as rapid rotation has previously been taken as implying high stellar activity. Here we present TESS and HiPERCAM $u_s{g}_s{r}_s{i}_s{z}_s$ observations of one of these stars LP 89–187 which has a rotation period of 0.117 d. TESS data covering three sectors (64.6 d) only show three flares which have energies a few $\times {10}^{33}$ erg, whilst HiPERCAM observations, which cover 0.78 of the rotation period, show no evidence for flares more energetic than $\sim {10}^{31}$ erg. Intriguingly, other surveys show LP 89–187 has shown weak H$\alpha$ in emission. We compare the flare energy distribution of LP 89–187 with low mass stars in the $\beta$ Pic moving group, which have an age of $\sim$24 Myr. We find LP 89–187 has a lower flare rate than the $\beta$ Pic stars. In addition, we find that TRAPPIST-1 analogue stars, which are likely significantly older than the $\beta$ Pic stars, show fewer flares with energies $>{10}^{33}$ erg in TESS data. We examine the relationship between amplitude and period for a sample of low mass stars and find that more rapid rotators have a higher amplitude.