When "Boring" Stars Flare: The Ultraviolet Activity of GJ 887, a Bright M Star Hosting Newly Discovered Planets^*

Recently Jeffers et al. (2020) announced the discovery of two (possibly three) planets orbiting the brightest M dwarf in the sky, GJ 887 (G = 6.522, J = 4.34), noting that the host star is remarkable for its apparently low level of magnetic activity. The star is well into the main sequence, with an age of ∼3 Gyr estimated from stellar evolution models (Mann et al. 2015). Its $\mathrm{log}\,{R}_{\mathrm{HK}}^{{\prime} }$, an activity diagnostic that requires only optical spectra, is −4.805 (Boro Saikia et al. 2018). Meanwhile, it exhibited no detectable flares and very little spot-induced rotational variability (semi-amplitude 240 ppm) in 27 days of optical monitoring by the Transiting Exoplanet Survey Satellite (TESS; Jeffers et al. 2020).

Low magnetic activity favors the survival of atmospheres on GJ 887's planets, hinting that atmospheric erosion from high energy radiation, stellar winds, and stellar flares (e.g., Lammer et al. 2007; Bisikalo et al. 2018; Egan et al. 2019), could be modest in comparison to known active stars. The optical evidence that GJ 887 provides a quiescent space environment to its planets motivated extensive press coverage, such as Nature News' "Why boring could be good for this star's two intriguing planets" (Witze 2020).

However, evidence is emerging that M stars across a wide range of conventionally defined activity levels exhibit regular flares in FUV emission (Loyd et al. 2018a, 2018b). GJ 887 is no exception. FUV observations by the Hubble Space Telescope (HST; program 13650, PI France) in 2015 revealed two clear flares in 1.3 hr of exposure. These flares were discovered by Kruczek et al. (2017) as part of an analysis of M star H₂ emission. Since 2015, HST has visited GJ 887 twice again for FUV spectroscopy (program 15326, PI Wood) with no additional flare detections in the combined 1.5 hr of exposure. Figure 1 displays these data.

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We estimate the bolometric energy of these flares to have been a few × 10³⁰ erg and >a few × 10³¹ erg, respectively, using the energy budgets of well-observed flares on the M4 star AD Leo (Hawley et al. 2003). Solar flare scalings indicate that GJ 887's first flare was X class or greater (peak 1–8 Å soft X-ray flux of >10⁻⁴ W m⁻² at 1 au; Youngblood et al. 2017). Such flares occur only monthly during activity cycle maxima on the Sun (Veronig et al. 2002), yet likely occur hourly on GJ 887. X class solar flares are almost always accompanied by coronal mass ejections (CMEs; Yashiro et al. 2006), producing showers of energetic protons with fluences of 1–10⁴ pfu (proton flux unit; proton cm⁻² s⁻¹ sr⁻¹) at 1 au (Youngblood et al. 2017).

The equivalent durations of GJ 887's flares were 150 ± 20 s and >1300 s in He ii. FUV flares with equivalent durations (a measure of their energies relative to the star's quiescent emission) in this range occur every few hours on other early to mid M stars across a wide range of Ca ii K emission levels and ages (Loyd et al. 2018a, 2018b). GJ 887's similar rate of FUV flares strengthens the possibility that all early to mid M stars share the same FUV flare frequency distribution (FFD) when cast in equivalent duration. This universal M-star FFD implies that most of the molecule-splitting FUV photons M-star planets receive could be delivered in short, intense bursts that are not captured by sparse and brief FUV observations (Loyd et al. 2018a). Those modeling GJ 887 b and c's atmospheres can utilize the fiducial_flare code from Loyd et al. (2018a) to generate time-dependent FUV spectra from stochastic flares.⁴

GJ 887 is not remarkably quiescent based on optical or UV observations. Quiescent surface fluxes of the strongest FUV lines are near or above the median of field age, early M stars in the recent MUSCLES (France et al. 2016; Loyd et al. 2016; Youngblood et al. 2016) and HAZMAT (R.O.P. Loyd et al. 2020, in preparation) surveys. GJ 887's $\mathrm{log}\,{R}_{\mathrm{HK}}^{{\prime} }$ of −4.805 (Boro Saikia et al. 2018) is also typical for its age and type (Melbourne & Youngblood 2020). The star's quiescent UV flux decreased by a factor of two from the 2015 to the 2018 and 2019 observations. This hints that the 2015 flares could be associated with stellar faculae or a period of overall higher stellar activity.

The FUV observations of GJ 887 highlight a disparity between optical and FUV indicators of a star's magnetic activity. Just 2.8 hr of FUV HST monitoring captured two clear flares, whereas 27 days of TESS monitoring left undetected likely hundreds of similar events. The stochastic nature of GJ 887's FUV, and, we infer, EUV and X-ray radiation will drive episodic photochemistry, heating, and evaporation in the atmospheres of orbiting planets. As our community continues to explore the evolution and possible end states of M star planets, it will be important to characterize and account for the "hidden UV lives" of their hosts.

R.O.P.L. and E.S. gratefully acknowledge support from NASA HST Grant HST-GO-14784.001-A for this work.

Facility: Hubble Space Telescope. -