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 linked to high stellar activity. Here we present TESS and HiPERCAM usgsrsiszs 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 of a few times 1033 erg, whilst HiPERCAM observations, which cover 0.78 of the rotation period, show no evidence of flares more energetic than approximately 1031 erg. Intriguingly, other surveys show that LP 89-187 has a weak Hα emission. We compared the flare energy distribution of LP 89-187 with low-mass stars in the β Pic moving group, which have an age of approximately 24 Myr. We find that LP 89-187 has a lower flare rate than the β Pic stars. In addition, we find that TRAPPIST-1 analogue stars, which likely are significantly older than the β Pic stars, show fewer flares with energies greater than 1033 erg in TESS data. We examined the relationship between amplitude and period for a sample of low-mass stars and find that more rapid rotators have a higher amplitude.