M Dwarf Flare Continuum Variations on One-Second Timescales: Calibrating and Modeling of Ultracam Flare Color Indices*

Abstract

We present a large dataset of high-cadence dMe flare light curves obtained with custom continuum filters on the triple-beam, high-speed camera system ULTRACAM. The measurements provide constraints for models of the near-ultraviolet (NUV) and optical continuum spectral evolution on timescales of ≈1 s. We provide a robust interpretation of the flare emission in the ULTRACAM filters using simultaneously obtained low-resolution spectra during two moderate-sized flares in the dM4.5e star YZ CMi. By avoiding the spectral complexity within the broadband Johnson filters, the ULTRACAM filters are shown to characterize bonafide continuum emission in the NUV, blue, and red wavelength regimes. The NUV/blue flux ratio in flares is equivalent to a Balmer jump ratio, and the blue/red flux ratio provides an estimate for the color temperature of the optical continuum emission. We present a new "color-color" relationship for these continuum flux ratios at the peaks of the flares. Using the RADYN and RH codes, we interpret the ULTRACAM filter emission using the dominant emission processes from a radiative-hydrodynamic flare model with a high nonthermal electron beam flux, which explains a hot, T≈10 4 K, color temperature at blue-to-red optical wavelengths and a small Balmer jump ratio asobserved in moderate-sized and large flares alike. We also discuss the high time resolution, high signal-to-noise continuum color variations observed in YZ CMi during a giant flare, which increased the NUV flux from this star by over a factor of 100.

Pulsed Accretion in the T Tauri Binary TWA 3A

“…In moderatecadence photometry, however, these events would likely go undetected having similar amplitudes and colors as accretion variability. Only if a measurement were to contain a large flare or a flare peak, where the photometric color is typically bluer than accretion, would it stand out from the underlying accretion variability (Kowalski et al 2016). …”

Star–Disk Interactions in Multiband Photometric Monitoring of the Classical T Tauri Star GI Tau

“…In one case, the B-band brightness is consistent with a non-detection, so the minimum and maximum accretion rates before and during the burst are not reported. These short bursts are attributed here to accretion but could alternatively be attributed to stellar flares (e.g., Kowalski et al 2016;Tofflemire et al 2017aTofflemire et al , 2017b.…”

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

“…In the impulsive phase of impulsive-type flares from active M dwarf stars, the NUV and blue optical continuum distribution often exhibits a color temperature of T∼ 10,000-12,000 K, with a small Balmer jump in emission (Hawley & Pettersen 1991;Kowalski et al 2013Kowalski et al , 2016. Due to a low surface flux of an M dwarf in quiescence, subtracting an M dwarf pre-flare spectrum from a flare observation (to infer a color temperature) does not artificially produce a hot continuum in the blue and NUV as is possible for the Sun as discussed in Kleint et al (2016).…”

“…We repeat the calculations (L. Kleint 2016, private communication) using the data from Kretzschmar (2011) while adjusting for the fraction of the solar surface no longer emitting at pre-flare values during the flare (Equation3 of Kowalski et al 2016) for a direct comparison to a blackbody flare model.…”