Photometric Study on Stellar Magnetic Activity. I. Flare Variability of Red Dwarf Stars in the Open Cluster M37

Abstract

Based on one-month long MMT time-series observations of the open cluster M37, we monitored light variations of nearly 2500 red dwarfs and successfully identified 420 flare events from 312 cluster M dwarf stars. For each flare light curve, we derived observational and physical parameters, such as flare shape, peak amplitude, duration, energy, and peak luminosity. We show that cool stars produce serendipitous flares energetic enough to be observed in the r-band, and their temporal and peak characteristics are almost the same as those in traditional U -band observations. We also found many large-amplitude flares with inferred ∆u > 6 mag in the cluster sample which had been rarely reported in previous ground-based observations. Following the ergodic hypothesis, we investigate in detail statistical properties of flare parameters over a range of energy (E r ≃ 10 31 -10 34 erg). As expected, there are no statistical differences in the distributions of flare timescales, energies, and frequencies among stars of the same age and mass group. We note that our sample tend to have longer rise and decay timescales compared to those seen in field flare stars of the same spectral type and be more energetic. Flare frequency distributions follow power-law distributions with slopes β ∼ 0.62-1.21 for all flare stars and β ∼ 0.52-0.97 for stars with membership information (P mem ≥ 0.2). These are in general agreement with previous works on flare statistics of young open clusters and nearby field stars. Our results give further support to the classical age-activity relations.

Photometric flaring fraction of M dwarf stars from the SkyMapper Southern Survey

“…Their appearance is most dramatic in blue passbands, but they are also found at longer wavelengths (e.g. Berger et al 2013;Chang et al 2015;Ho et al 2018;Soraisam et al 2018;van Roestel et al 2019).…”

“…Flare discovery as fast optical transients in time-domain surveys. References: Deep Lens Survey (DLS; Becker et al 2004); Sloan Digital Sky Survey Stripe 82 (SDSS; Kowalski et al 2009); Pan-STARRS 1 Medium Deep Survey (PS1; Berger et al 2013); MMT M37 Transit Survey (Chang et al 2015); intermediate Palomar Transient Factory (iPTF; Ho et al 2018); All Sky Automated Survey for SuperNovae (ASAS-SN; Schmidt et al 2019); SkyMapper (this work); Kepler primary mission (Davenport 2016); Transiting Exoplanet Survey Satellite (TESS; Günther et al 2019 (h) in that system would be inhospitable for life (MacGregor et al 2018;Howard et al 2018). Such dramatic magnitude variations from flares in ultracool dwarfs (from M7 to early-L-type) provides a fresh look into the underlying dynamo action and flare generation process in the regime of low stellar mass down to the hydrogenburning limit (e.g.…”

Comparison of Damped Oscillations in Solar and Stellar X-Ray Flares

“…The similarity in solar and stellar flares, including the QPPs may give us hints to understand the nature of the flares and associated oscillations, as well as coronal plasma properties for various stars including the Sun (Balona et al 2015;Chang et al 2015). In particular, properties of oscillating loops could be estimated by applying an appropriate model to the frequency and amplitude modulation of the perturbation, which are independent on the sharpness and strength of the sources.…”

Finding flares in Kepler data using machine-learning tools

“…We note that this last step was performed only after evaluating each light-curve segment. These criteria are basically the same as those defined by Equations 3 a-d of Chang et al (2015) and are consistent with adaptations made by Davenport (2016). When running FLATW'RM from command line, the user can change the number of flare points needed for a flare, the detection level of the flares, and the full with at half maximum (FWHM) that is used for the analytic fit of the events.…”

“…As a demonstration of the FLATW'RM code, we analyzed two data sets from the Kepler telescope: a short-cadence K2 observation of TRAPPIST-1, and a long-cadence light curve of KIC 1722506 (the data were obtained in the 420-900 nm wavelength range with the maximum spectral response at 575 nm). In the case of TRAPPIST-1, we set the minimum number of data points needed for a flare (N3 in Chang et al 2015) to 5, and the detec-tion limit to 5σ. In the case of KIC 1722506 we used N3=3 with a detection limit of 3σ.…”