K2 Ultracool Dwarfs Survey – V. High superflare rates on rapidly rotating late-M dwarfs

• Published in: Monthly notices of the Royal Astronomical Society Vol. 486; no. 1; pp. 1438 - 1447
• Main Authors: Paudel, R R, Gizis, J E, Mullan, D J, Schmidt, S J, Burgasser, A J, Williams, P K G, Youngblood, A, Stassun, K G
• Format: Journal Article
• Language: English
• Published: Oxford University Press 11.06.2019
• Subjects: stars: flare; stars: individual: (2MASS J08371832+2050349, 2MASS J08315742+2042213, 2MASS J08312608+2244586, TRAPPIST-1); stars: low-mass; stars: activity
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stz886

Abstract We observed strong superflares (defined as flares with energy in excess of 1033 erg) on three late-M dwarfs: 2MASS J08315742+2042213 (hereafter 2M0831+2042; M7 V), 2MASS J08371832+2050349 (hereafter 2M0837+2050; M8 V), and 2MASS J08312608+2244586 (hereafter 2M0831+2244; M9 V). 2M0831+2042 and 2M0837+2050 are members of the young (∼700 Myr) open cluster Praesepe. The strong superflare on 2M0831+2042 has an equivalent duration (ED) of 13.7 h and an estimated energy of 1.3 × 1035 erg. We observed five superflares on 2M0837+2050, on which the strongest superflare has an ED of 46.4 h and an estimated energy of 3.5 × 1035 erg. This energy is larger by 2.7 orders of magnitude than the largest flare observed on the older (7.6 Gyr) planet-hosting M8 dwarf TRAPPIST-1. Furthermore, we also observed five superflares on 2M0831+2244 which is probably a field star. The estimated energy of the strongest superflare on 2M0831+2244 is 6.1 × 1034 erg. 2M0831+2042, 2M0837+2050, and 2M0831+2244 have rotation periods of 0.556 ± 0.002, 0.193 ± 0.000, and 0.292 ± 0.001 d, respectively, which we measured by using K2 light curves. We compare the flares of younger targets with those of TRAPPIST-1 and discuss the possible impacts of such flares on planets in the habitable zone of late-M dwarfs.