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

Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b
Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b
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 ϵ 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 h 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.
0004-6361
Chadney, Joshua
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Koskinen, Tommi
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Galand, Marina
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Unruh, Yvonne
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Sanz-Forcada, Jorge
ad1239b8-b31b-418b-a96c-23b44922cfc4
Chadney, Joshua
4209e45f-6092-4c9d-b46b-4b598269a0f3
Koskinen, Tommi
fd4b7af2-f84b-46ba-9dee-223eb741cf05
Galand, Marina
083b2513-57ab-4b08-88e4-ba759f290c98
Unruh, Yvonne
c8e151aa-e9d2-43e1-bcde-c12a25f31368
Sanz-Forcada, Jorge
ad1239b8-b31b-418b-a96c-23b44922cfc4

Chadney, Joshua, Koskinen, Tommi, Galand, Marina, Unruh, Yvonne and Sanz-Forcada, Jorge (2017) Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b. Astronomy & Astrophysics, 608. (doi:10.1051/0004-6361/201731129).

Record type: Article

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 ϵ 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 h 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.

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1710.08365 - Accepted Manuscript
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Accepted/In Press date: 22 October 2017
e-pub ahead of print date: 8 December 2017
Published date: December 2017

Identifiers

Local EPrints ID: 416405
URI: https://eprints.soton.ac.uk/id/eprint/416405
ISSN: 0004-6361
PURE UUID: 6c957ad3-b009-4b15-a080-17849f600152
ORCID for Joshua Chadney: ORCID iD orcid.org/0000-0002-5174-2114

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Date deposited: 15 Dec 2017 17:30
Last modified: 14 Mar 2019 01:31

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Contributors

Author: Joshua Chadney ORCID iD
Author: Tommi Koskinen
Author: Marina Galand
Author: Yvonne Unruh
Author: Jorge Sanz-Forcada

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