EvryFlare II: Rotation Periods and Starspot Coverage of Cool Active Stars Throughout the Spin-down Process [SSA]

http://arxiv.org/abs/1907.10735


We measure photometric rotation periods and starspot amplitudes in Evryscope light curves for 113 cool stars selected to have strong flaring. The Evryscope array of small telescopes has observed all bright nearby stars in the Southern sky, producing two minute cadence light curves since 2016. Long-term, high-cadence observations allow detection of rare flare events and long-term periodic variability, and provide tools to probe the complex relationship between stellar rotation, starspots, and superflares. We detect precise rotation periods from 0.3487 to 104 days with a median period uncertainty of 0.0024 days. We observe sinusoidal amplitudes from 0.008 to 0.216 g’ magnitudes. We fold the TESS light curves of each target to the Evryscope-detected period and compare rotational amplitudes. TESS amplitudes are less than or equal to Evryscope amplitudes. We compute the Rossby number (R_o) for each star, finding 25 of our flare stars to be fast rotators (R_o<0.04), 55 to be intermediate-period rotators (0.04<R_o<0.4), and 33 to be slow rotators (R_o>0.44). We measure the photometric starspot coverage of each flare star, and find a median spot coverage of 14% of the stellar hemisphere. We constrain the minimum surface magnetic field strength consistent with our flare energies and spot coverage fraction to be 500 G, with later-type stars exhibiting lower minimum field strengths than earlier-type stars. We observe a possible change in starspot coverage at intermediate periods. However, we do not conclusively confirm the increased activity of intermediate rotators seen in previous studies. We then split all rotators at R_o ~ 0.2 into P_Rot<10 d and P_Rot>10 d bins and confirm short-period rotators exhibit higher superflare rates, larger flare energies observed, and higher starspot coverage fractions than do long-period rotators, at p-values of 7.3 X 10^-5, 1.2 X 10^-6 and 0.0097, respectively.

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W. Howard, H. Corbett, N. Law, et. al.
Fri, 26 Jul 19
42/84

Comments: 16 pages, 9 figures, 3 tables. Ancillary machine-readable files included. Submitted to AAS journals