KEPLERFLARES. I. ACTIVE AND INACTIVE M DWARFS

Abstract

We analyzed Kepler short-cadence M dwarf observations. Spectra from the ARC 3.5m telescope identify magnetically active (Hα in emission) stars. The active stars are of mid-M spectral type, have numerous flares, and well-defined rotational modulation due to starspots. The inactive stars are of early-M type, exhibit less starspot signature, and have fewer flares. A Kepler to U-band energy scaling allows comparison of the Kepler flare frequency distributions with previous ground-based data. M dwarfs span a large range of flare frequency and energy, blurring the distinction between active and inactive stars designated solely by the presence of Hα. We analyzed classical and complex (multiple peak) flares on GJ 1243, finding strong correlations between flare energy, amplitude, duration and decay time, with only a weak dependence on rise time. Complex flares last longer and have higher energy at the same amplitude, and higher energy flares are more likely to be complex. A power law fits the energy distribution for flares with log E Kp > 31 ergs, but the predicted number of low energy flares far exceeds the number observed, at energies where flares are still easily detectable, indicating that the power law distribution may flatten at low energy. There is no correlation of flare occurrence or energy with starspot phase; the flare waiting time distribution is consistent with flares occurring randomly in time; and the energies of consecutive flares are uncorrelated. These observations support a scenario where many independent active regions on the stellar surface are contributing to the observed flare rate.

LO Peg: surface differential rotation, flares, and spot-topographic evolution

“…Most of the flares are ∼1 h long with a minimum and maximum flare duration of ∼12 min and ∼3.4 h. Flare observed on SV Cam (Patkos, 1981), XY UMa (Zeilik, Elston & Henson, 1982), DK CVn (Dal, Sipahi &Özdarcan, 2012), FR Cnc (Golovin et al, 2012), AB Dor (Lalitha et al, 2013), and DV Psc (Pi et al, 2014) studied in optical bands also lie in the same range. Several flares observed on M dwarfs by Hawley et al (2014) show similar feature but with a flare duration of ∼2 min. The derived flare amplitudes on LO Peg were found to be higher in shorter wavelengths than that in longer wavelengths (see two representative flares in Fig.…”

“…Seventeen out of twenty flares having energy less than 10 33 erg, signifies more energetic flares are less in number. With Kepler data Hawley et al (2014) also detected most of the flares on M dwarf GJ 1243 having energy of the order of 10 31 erg. One flare (F13) is found to have total energy more than 10 34 erg, therefore, this flare can be classified as a Superflare (see Candelaresi et al, 2014).…”

“…Thus, a flare detected in one band is not necessarily detected in each of the optical bands. Due to the sparse data, we have not followed the usual flare detection methods as described in Osten et al (2012), Hawley et al (2014) and Shibayama et al (2013). We have chosen different epochs such that, each epoch contains a continuous single night observation with at least 16 data points and minimum observing span of ∼ 1 h. A total of 501 epochs were found using the most populous V -band data, among which only 82 epochs have simultaneous observations with other three optical bands.…”

“…The local mean flux (F lm ) and standard deviation (σ ql ) of the flux were then computed at each time-sampled data set. To avoid misdetection of short stellar brightness enhancement as a flare, candidate flares were flagged as excursions of two or more consecutive data points above 2.5σ ql from F lm (see Hawley et al, 2014;Davenport et al, 2014;Lurie et al, 2015) with at least one of those points being 3σ ql above F lm in any of the optical band. Once the flare was detected using the above criteria, the flare segment was removed to calculate the exact value of σ ql , where most of the flares were identified above the 3σ ql from the quiescent state.…”

A Unified Computational Model for Solar and Stellar Flares

“…Active dMe stars are known to have flares much larger than those on the Sun. These stars spin faster and generate much larger magnetic fields (Hawley et al 2014, and references therein) resulting in flares some 10 3 times more energetic than typical solar flares (e.g., Hawley & Pettersen 1991;Hawley et al 2003;Kowalski et al 2010). In addition, the background intensity of dMe stars is much smaller than that of the Sun.…”