Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b

Abstract

Stellar flares are a frequent occurrence on young low-mass stars around which many detected exoplanets orbit. Flares are energetic, impulsive events, and their impact on exoplanetary atmospheres needs to be taken into account when interpreting transit observations. We have developed a model to describe the upper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. The model simulates thermal escape from the upper atmospheres of close-in EGPs. Ionisation by solar radiation and electron impact is included and photo-chemical and diffusive transport processes are simulated. This model is used to study the effect of stellar flares from the solar-like G star HD 209458 and the young K star HD 189733 on their respective planets, HD 209458b and HD 189733b. The Sun is used as a proxy for HD 209458, and Eridani, as a proxy for HD 189733. A hypothetical HD 209458b-like planet orbiting the very active M star AU Microscopii is also simulated. We find that the neutral upper atmosphere of EGPs is not significantly affected by typical flares on HD 209458 and HD 189733. Therefore, stellar flares alone would not cause large enough changes in planetary mass loss to explain the variations in HD 189733b transit depth seen in previous studies, although we show that it may be possible that an extreme stellar proton event could result in the required mass loss. Our simulations do however reveal an enhancement in electron number density in the ionosphere of these planets, the peak of which is located in the layer where stellar X-rays are absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flare levels and enhanced electron densities last from about 3 to 10 hours after the onset of the flare, depending on the composition of the ionospheric layer. The strength of the flare and the width of its spectral energy distribution affect the range of altitudes in the ionosphere that see enhancements in ionisation. A large broadband continuum component in the XUV portion of the flaring spectrum in very young flare stars, such as AU Mic, results in a broad range of altitudes affected in planets orbiting this star. Indeed, as well as the X-ray absorption layer, the layer in which EUV photons are absorbed is also strongly enhanced.

Extremely Irradiated Hot Jupiters: Non-oxide Inversions, H− Opacity, and Thermal Dissociation of Molecules

“…Atmospheric escape has been observed on a handful of hot Jupiters to date, namely HD209458b, HD189733b, and WASP-12b (Vidal-Madjar et al 2003;Lecavelier des Etangs et al 2010, 2012Fossati et al 2010). Models of more moderately irradiated hot Jupiters HD209458b and HD189733b indicate that atmospheric escape does not drastically alter the total mass of these planets throughout the planet's lifetime (e.g., Yelle 2004Yelle , 2006Murray-Clay et al 2009;Koskinen et al 2013;Chadney et al 2017). The same does not necessarily hold for extreme hot Jupiters such as WASP-12b that undergo significant Roche lobe overflow in addition to thermal escape due to their close orbit around the host star (Li et al 2010;Jackson et al 2017).…”

Exoplanet X-ray irradiation and evaporation rates with eROSITA

“…Lecavelier Des Etangs et al (2010) estimate the current mass loss rate to be in the range of 10 9 and 10 11 gs −1 . Other estimates for the mass loss have been made such as the model by Chadney et al (2017), who calculate an upper limit to the mass loss rate of HD 189733b of the order of 10 7 and 10 12 gs −1 during stellar flares with large scale proton events. Our calculation, based on the energy-limited escape model, estimates a current mass loss rate of about 1.8×10 11 gs −1 , which is at the upper end, but compatible with the estimates based on hydrogen Ly-α observations.…”

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

“…If the mass-loss efficiency for flares is similar to the canonical value of 0.1 for steadystate flux (e.g., Murray-Clay et al 2009), then the accumulated atmospheric "erosion" by such flares could be significant over timescales of hundreds of Myr. Chadney et al (2017) modeled the effects of a flare on mass loss from hot Jupiters and found that it could not explain variations seen in the Lyα transit of HD 189733b. More modeling is needed to determine the efficiency of flare EUV emission in removing atmospheric mass over a range of planetary parameters (e.g.…”