A Cool Star Flare Reveals an Unexpectedly Hot Emission Component

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Introduction

The MUSCLES and Mega-MUSCLES Treasury Surveys are large observing programs that are using the Hubble Space Telescope, Chandra X-ray Observatory, XMM-Newton, the Neil Gehrels Swift Observatory, and a suite of ground-based optical telescopes to construct panchromatic (5 Å - 5.5 μm) spectral energy distributions for a representative sample of M dwarf stars (Refs [0],[1]). The primary goal of the surveys is to establish characterize the stellar energetic radiation environment and activity levels in dwarf stars and their effects on exoplanet atmospheres and habitability; however, the survey observations have also been excellent probes of the energetic properties of flares in dwarf stars, providing X-ray and ultraviolet (UV) time-resolved spectroscopy of targets.

One of the targets in the Mega-MUSCLES survey was the M2.5V (0.35 M⊙) star GJ674. It hosts a hot Neptune exoplanet in a 4.7 day orbit. GJ674 is classified as a "weakly active" star: it has regular starspot activity and shows emission lines of CaII H&K, but does not show Hα in emission, the characteristic definition of an active flare star. However, results from the MUSCLES survey have shown that even optically inactive dwarf stars show regular activity in the UV (Ref. [2]) and GJ 674 was no exception. Here, we present the properties of one particularly energetic flare and discuss early efforts to model the flare using radiative hydrodynamic models of chromospheric condensations in the flare.

Observations and Flare Properties

The survey programs are centered about far-ultraviolet spectroscopy and time-series monitoring of the stars using the Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph instruments on board the Hubble Space Telescope. During the observations with COS (Ref. [2]), GJ 674 experienced a large flare (Figure 1).

Figure 1: During FUV observations with the Hubble Space Telescope, the M2.5V dwarf star, GJ 674, exhibited several small flares and one large flare in the last orbit.

Figure 2: The black line shows the COS FUV spectrum of GJ674 in its normal state, while the red line shows the spectrum during the large flare shown in Figure 1.

Figure 3: The continuum emission during the flare is fit with a blackbody, showing that the color temperature of the flare emission was Tcolor ~ 40,000K.

Hydrodynamic Models

Figure 3=4: Flare model flux densities in the FUV and optical. The dashed line is parameterization of a F11 solar flare model multiplied by a factor of 200 to match the optical color temperature and Balmer jump parameters seen in M dwarf flares. The dotted line is a Tref = 75; 000 K, logmref = 􀀀1:7 downflowing compression layer (the stationary are layers are set to zero). The solid blue line is the superposition of the previous two models. The χflare values give the size of the Balmer jump ratio.

Conclusion

References

[0] The MUSCLES Treasury Survey. I. Motivation and Overview

[1] Flexing our MUSCLES: The HST Mega-MUSCLES Treasury Survey

[2] The MUSCLES Treasury Survey. V. FUV Flares on Active and Inactive M Dwarfs

[3] The Cosmic Origins Spectrograph: on-orbit instrument performance

[4] A Hot Ultraviolet Flare on the M Dwarf Star GJ 674

[5] Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres