A Cool Star Flare Reveals an Unexpectedly Hot Emission Component

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The flare spectrum showed a strong FUV continuum component. Time-resolved light curves of the lines and continuum during the flare showed that the continuum more tracked the time evolution of the chromospheric lines, peaking earlier and declining faster than
The flare spectrum showed a strong FUV continuum component. Time-resolved light curves of the lines and continuum during the flare showed that the continuum more tracked the time evolution of the chromospheric lines, peaking earlier and declining faster than
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the coronal [Fe XXI] emission line. The latter remained high well after the impulsive phase of the flare, indicating ongoing heating in the corona. We �fit a single temperature blackbody to the FUV flare continuum emission. The best fi�t, shown in Figure 3, has a color temperature of T<sub>color</sub> &#8773; 40000 K.
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the coronal [Fe XXI] emission line. The latter remained high well after the impulsive phase of the flare, indicating ongoing heating in the corona. We fit a single temperature blackbody to the FUV flare continuum emission. The best fit, shown in Figure 3, has a color temperature of T<sub>color</sub> &#8773; 40000 K.
[[Image:GJ674 flare continuum.png|300px|thumb|center|'''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.]]
[[Image:GJ674 flare continuum.png|300px|thumb|center|'''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==
==Hydrodynamic Models==
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[[Image:Rhd_model_gj674_data.png|400px|thumb|center|'''Figure 3=4''': Flare model flux densities in the FUV and optical. The dashed line is parameterization of a F11 solar flare model
[[Image:Rhd_model_gj674_data.png|400px|thumb|center|'''Figure 3=4''': Flare model flux densities in the FUV and optical. The dashed line is parameterization of a F11 solar flare model
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== Conclusion ==
== Conclusion ==
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The large flare we observed in the FUV with Hubble is unprecedented in several respects. In terms of the absolute flare energy, it is not particularly strong compared to historic M dwarf flare star observations. However, the equivalent duration of the flare places it in the realm of the highest amplitude flares observed. Moreover, the color temperature of the continuum, �fitt with a 40,000 K, is the hottest spectroscopic color temperature contribution to a broadband flare spectrum measured to date.
== References ==
== References ==

Revision as of 16:26, 29 August 2019


A Cool Star Flare Reveals an Unexpectedly Hot Emission Component
Number: 112
1st Author: C. S. Froning
2nd Author: A. Kowalski
Published: September 5, 2019
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Contents

Introduction

The MUSCLES and Mega-MUSCLES Treasury Surveys are large observing programs that have used 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 characterize the stellar energetic radiation environment and activity levels in dwarf stars and determine 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 exposure.

The flare energy was not particularly large compared to those seen in active flare stars (EFUV = 1030.75 ergs, compared to E>1033 ergs recently seen in a flare on a young M star (Ref. [5]). However, if normalized to the basal flux of the star, the flare was extraordinary. The equivalent duration, δ, of a flare is the length of time the star must emit in quiescence to equal the energy emitted in the flare. For GJ674, the equivalent duration was an impressive δ ≅ 30,000 sec (compared to 6700 sec for the energetic flare mentioned above), more than 4 times higher than any flare observed in the FUV by HST and comparable to the AD Leo "Great Flare" observed by IUE (Ref. [6]), which had δ > 40,000 sec. Thus, while an older star such as GJ 674 emits flares of lower absolute energies then young, active flare stars, it still emits flares that are a significant fraction of the overall energy budget of the stellar irradiance.

The flare on GJ 674 had other unique properties. During the flare, line emission was enhanced in a number of lines tracing chromospheric and transition region emission (e.g., C II, C III, Si III, Si IV, N V; with formation temperatures ranging from Tform ~ 3 x 104 K to Tform ~ 2 x 105 K) as well as lines tracing hotter coronal regions ([Fe XII], [Fe XIX], [Fe XXI]; Tform ~ 106 ~ 107 K; Fig. 2).

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.

In general, the lower temperature chromospheric lines showed similar peak to quiescent flux ratios to the largest flare in the MUSCLES sample of optically inactive M stars, with C III and Si III increasing in flux by a factor of ~100 and C II by 30-50. However, the higher formation temperature N V line and the FUV continuum emission increased by larger factors in the GJ 674 flare (by a factor of 10 vs. 5 for the largest flare seen in that survey) and the [Fe XXI] 1354 line, only marginally detected in the previous flare, increased by a factor of ~20 in GJ 674.

The flare spectrum showed a strong FUV continuum component. Time-resolved light curves of the lines and continuum during the flare showed that the continuum more tracked the time evolution of the chromospheric lines, peaking earlier and declining faster than the coronal [Fe XXI] emission line. The latter remained high well after the impulsive phase of the flare, indicating ongoing heating in the corona. We fit a single temperature blackbody to the FUV flare continuum emission. The best fit, shown in Figure 3, has a color temperature of Tcolor ≅ 40000 K.

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

The large flare we observed in the FUV with Hubble is unprecedented in several respects. In terms of the absolute flare energy, it is not particularly strong compared to historic M dwarf flare star observations. However, the equivalent duration of the flare places it in the realm of the highest amplitude flares observed. Moreover, the color temperature of the continuum, �fitt with a 40,000 K, is the hottest spectroscopic color temperature contribution to a broadband flare spectrum measured to date.

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] HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet

[6] The Great Flare of 1985 April 12 on AD Leonis

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

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