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.

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.

==Hydrodynamic Models and

Conclusion

References

[0] The MUSCLES Treasury Survey. I. Motivation and Overview [1] Flexing our MUSCLES: The HST Mega-MUSCLES Treasury Survey [2] The Cosmic Origins Spectrograph: on-orbit instrument performance [3] A Hot Ultraviolet Flare on the M Dwarf Star GJ 674