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

TL;DR

This study investigates why an ultra-fast-rotating M7V dwarf, LP 89--187, shows few flares despite its rapid rotation. By combining TESS photometry across three sectors with HiPERCAM high-speed, multi-band observations, the authors detect three flares with $E\gtrsim\!10^{33}$ erg in TESS data but find no flares above $\sim10^{31}$ erg with HiPERCAM, implying a low rate of low- and high-energy flares. Comparisons with young $\beta$ Pic group stars and TRAPPIST-1 analogues suggest flare activity declines with age, while an observed correlation between rotation amplitude and period indicates more pronounced spot inhomogeneity for faster rotators. A polar-spot–dominant magnetic topology is proposed as a plausible mechanism for reduced flare production in ultra-fast rotators, highlighting the complexity of magnetic activity in fully convective stars.

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 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 $\times10^{33}$ erg, whilst HiPERCAM observations, which cover 0.78 of the rotation period, show no evidence for flares more energetic than $\sim10^{31}$ erg. Intriguingly, other surveys show LP 89--187 has shown weak H$α$ in emission. We compare the flare energy distribution of LP 89--187 with low mass stars in the $β$ Pic moving group, which have an age of $\sim$24 Myr. We find LP 89--187 has a lower flare rate than the $β$ Pic stars. In addition, we find that TRAPPIST-1 analogue stars, which are likely significantly older than the $β$ 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.

Figures (5)

• Figure 1: Differential $g_{s}$ (colour coded green), $r_{s}$ (yellow), $i_{s}$ (orange) and $z_{s}$ (red) photometry of LP 89--187 obtained using HiPERCAM. The differential flux for each 0.74 s exposure has been normalised by dividing by the mean in each band and each subsequent filter has been shifted vertically by 0.5. Time zero corresponds to MJD=60350.902. The residual features, including the flare-like features in the $g_{s}$ band, are due to poorer conditions. The signature of the stars rotation has largely been removed due to the large colour difference between the target and the two comparison stars.
• Figure 2: The three flares detected in TESS observations of LP 89--187 using 2 min cadence data. The start time of each flare is, from left to right, MJD=59581.12, 59592.19 (both Sector 47) and 59942.01 (Sector 60).
• Figure 3: The bolometric flare frequency distibution of LP 89--187 as derived from three sectors of TESS data shown in blue. The blue arrow indicates the upper limit derived from the HiPERCAM observations. We also show the bolometric flare frequency distribution of stars in the $\beta$ Pic moving group in orange (spectral type $\buildrel {\hbox{$<$}} \over {\hbox{$\sim$}}$ M2) and in red (spectral type $\buildrel {\hbox{$>$}} \over {\hbox{$\sim$}}$ M3). We also show flare frequency distribution of TRAPPIST-1 analogue stars (black dashed line) and TRAPPIST-1 using K2 data (brown dashed line) taken from Figure 21 of Seli2021.
• Figure 4: The folded light curves of LP 89--187 in the three TESS sectors. The phase has been shifted so that flux maximum is at $\phi$=0.0.
• Figure 5: The amplitude of the modulation versus period of stars taken from Ramsay2024 which have a low spread in their period between TESS sectors ($\delta_{LS}$ < 0.05). LP 89--187 is shown as a blue cross and the slope has an index of -0.19 and significance of $p$=0.0016.