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

¹ Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, N. Ireland, UK
² Astrophysics Research Cluster, School of Mathematical and Physical Sciences, University of Sheffield, Sheffield S3 7RH, UK
³ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁴ GRANTECAN, Cuesta de San José s/n, 38712, Breña Baja, La Palma, Spain

^★ Corresponding author: gavin.ramsay@armagh.ac.uk

Received: 26 February 2025
Accepted: 18 September 2025

Abstract

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 u_sg_sr_si_sz_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 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.