EDEN: Flare Activity of the Nearby Exoplanet-hosting M Dwarf Wolf 359 Based on K2 and EDEN Light Curves

Abstract

We report the flare activity of Wolf 359, the fifth closest star to the Sun and a candidate exoplanet-hosting M dwarf. The star was a target of the Kepler/K2 mission and was observed by the EDEN project, a global network of 1–2 m class telescopes for detection and characterization of rocky exoplanets in the habitable zones of late-M dwarfs within 50 light year from the solar system. In the combination of the archived K2 data and our EDEN observations, a total of 872 flares have been detected, 861 with the K2 (860 in the short-cadence and 18 in the long-cadence data, with 17 long-cadence events having short-cadence counterparts) and 11 with EDEN. Wolf 359 has relatively strong flare activity even among flaring M dwarfs, in terms of the flare activity indicator (FA) defined as the integrated flare energy relative to the total stellar bolometric energy, where FA = ∑E f /∫L bol dt ∼ 8.93 × 10−5 for the long-cadence flares, whereas for K2 short cadence and EDEN flares, the FA values are somewhat larger, FA ≈ 6.67 × 10−4 and FA ≈ 5.25 × 10−4, respectively. Such a level of activity, in accordance with the rotation period (P rot), suggests the star to be in the saturation phase. The size of the starspots is estimated to be at least 1.87% ± 0.59% of the projected disk area of Wolf 359. We find no correlation of FA with the stellar rotational phase. Our analysis indicates a flare frequency distribution in a power-law form of dN/dE∝E−α with α = 2.13 ± 0.14, equivalent to an occurrence rate of flares E f ≥ 1031 erg about once per day and of superflares with E f ≥ 1033 erg approximately 10 times per year. These superflares may impact the habitability of system in multiple ways, the details of which are topics for future investigations.