EvryFlare. II. Rotation Periods of the Cool Flare Stars in TESS across Half the Southern Sky

Abstract

We measure rotation periods and sinusoidal amplitudes in Evryscope light curves for 122 two-minute K5-M4 TESS targets selected for strong flaring. The Evryscope array of telescopes has observed all bright nearby stars in the south, producing 2-minute cadence light curves since 2016. Long-term, high-cadence observations of rotating flare stars probe the complex relationship between stellar rotation, starspots, and superflares. We detect periods from 0.3487 to 104 days and observe amplitudes from 0.008 to 0.216 g′ mag. We find that the Evryscope amplitudes are larger than those in TESS with the effect correlated to stellar mass (p-value=0.01). We compute the Rossby number (R o ) and find that our sample selected for flaring has twice as many intermediate rotators (0.04<R o <0.4) as fast (R o <0.04) or slow (R o >0.44) rotators; this may be astrophysical or a result of period detection sensitivity. We discover 30 fast, 59 intermediate, and 33 slow rotators. We measure a median starspot coverage of 13% of the stellar hemisphere and constrain the minimum magnetic field strength consistent with our flare energies and spot coverage to be 500 G, with later-type stars exhibiting lower values than earlier-type stars. We observe a possible change in superflare rates at intermediate periods. However, we do not conclusively confirm the increased activity of intermediate rotators seen in previous studies. We split all rotators at R o ∼0.2 into bins of P Rot <10 days and P Rot >10 days to confirm that short-period rotators exhibit higher superflare rates, larger flare energies, and higher starspot coverage than do long-period rotators, at p-values of 3.2×10 −5 , 1.0×10 −5 , and 0.01, respectively.Unified Astronomy Thesaurus concepts: Starspots (1572); Optical flares (1166); Sky surveys (1464); Stellar rotation (1629); Time series analysis (1916)

Stellar flares detected with the Next Generation Transit Survey

“…This can be explained by us overestimating the spot temperature in equation (5). Our magnetic field results agree with those of Howard et al (2020b), who performed the same analysis using the EvryScope flare sample for cool stars. Our results are also consistent with previous observations of kilogauss magnetic field strengths for K and M stars (e.g.…”

K2 Ultracool Dwarfs Survey – VI. White light superflares observed on an L5 dwarf and flare rates of L dwarfs

“…For example, TESS data for Proxima Centauri (Vida et al 2019) indicate that with spot areal coverage of 5 per cent or more, the observed flare energies can be powered by fields in the range 1-2 kG. And data from EVRYSCOPE (Howard et al 2019) also indicate that flares on stars with spectral types ranging from K7 to M4 have energies such that, with spot areal coverages of at least 4-5 per cent, the most energetic flares can be powered by magnetic fields with strengths of 1.5 kG.…”

No Such Thing as a Simple Flare: Substructure and QPPs Observed in a Statistical Sample of 20 Second Cadence TESS Flares

“…The flaring properties of these M-dwarfs have been extensively studied at 2 min cadence as part of the EvryFlare Survey (e.g. Howard et al 2018Howard et al , 2019Howard et al , 2020a). The sample is composed of very nearby (23 +27 −12 pc) and bright (g =12.3 +1.4 −1.0 ) flare stars, where the quoted spreads are 1σ standard deviations.…”

“…Wakeford et al 2019;Teske et al 2020). Spectral types are updated from those of Howard et al (2020a) with SIMBAD (Wenger et al 2000) classifications where available. Flare stars were selected to cross the fullyconvective boundary with 146 M-dwarfs of types ≈M0-M3, 47 ∼M4 dwarfs, and 16 M5-M6 dwarfs.…”