Ionization of Extrasolar Giant Planet Atmospheres

Koskinen, Tommi; Cho, James Y-K.; Achilleos, N.; Aylward, A. D.

doi:10.1088/0004-637x/722/1/178

Cited by 53 publications

(57 citation statements)

References 31 publications

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“…The eddy diffusion coefficient is not well known in exoplanetary atmospheres, but its value is estimated to be about 10 2 m 2 s −1 at Jupiter (Yelle et al 1996). K zz may well be higher in closeorbiting EGPs than at Jupiter (Koskinen et al 2010), in which case the homopause could be located at a lower pressure than we assumed, leading to the destruction of the IR coolant H + 3 through reactions with hydrocarbons, for instance. There are currently no constraints on K zz in EGPs however, a situation that might be improved by the detection of IR emissions from the H + 3 ion (see Sect.…”

Section: Ionospheric Modelmentioning

confidence: 76%

“…Photo-ionisation (or primary ionisation) by stellar photons has been calculated in past EGP models Article published by EDP Sciences A87, page 1 of 14 A&A 587, A87 (2016) (e.g. Yelle 2004;García Muñoz 2007;Koskinen et al 2010Koskinen et al , 2013a. However, to our knowledge, a full description of secondary ionisation, by photo-electrons and their secondaries, has not been included in EGP models.…”

Section: Introductionmentioning

confidence: 99%

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EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

Chadney

Galand

Koskinen

et al. 2016

A&A

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The composition and structure of the upper atmospheres of extrasolar giant planets (EGPs) are affected by the high-energy spectrum of their host stars from soft X-rays to the extreme ultraviolet (EUV). This emission depends on the activity level of the star, which is primarily determined by its age. In this study, we focus upon EGPs orbiting K-and M-dwarf stars of different ages -Eridani, AD Leonis, AU Microscopii -and the Sun. X-ray and EUV (XUV) spectra for these stars are constructed using a coronal model. These spectra are used to drive both a thermospheric model and an ionospheric model, providing densities of neutral and ion species. Ionisation -as a result of stellar radiation deposition -is included through photo-ionisation and electron-impact processes. The former is calculated by solving the Lambert-Beer law, while the latter is calculated from a supra-thermal electron transport model. We find that EGP ionospheres at all orbital distances considered (0.1−1 AU) and around all stars selected are dominated by the long-lived H + ion. In addition, planets with upper atmospheres where H 2 is not substantially dissociated (at large orbital distances) have a layer in which H + 3 is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H + 3 undergo significant diurnal variations, with the maximum value being driven by the stellar X-ray flux. In contrast, densities of longer-lived H + show very little day/night variability and the magnitude is driven by the level of stellar EUV flux. The H + 3 peak in EGPs with upper atmospheres where H 2 is dissociated (orbiting close to their star) under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (e.g. hydrocarbons, water). The inclusion of secondary ionisation processes produces significantly enhanced ion and electron densities at altitudes below the main EUV ionisation peak, as compared to models that do not include electron-impact ionisation. We estimate infrared emissions from H + 3 , and while, in an H/H 2 /He atmosphere, these are larger from planets orbiting close to more active stars, they still appear too low to be detected with current observatories.

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Section: Ionospheric Modelmentioning

confidence: 76%

Section: Introductionmentioning

confidence: 99%

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

Chadney

Galand

Koskinen

et al. 2016

A&A

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“…Perhaps one of the major uncertainties when discussing X‐ray flows is the role of composition, since X‐ray heating and cooling are done by metals. Although calculations of atmospheric composition are beginning to be performed (Yelle ; García Muñoz ; Koskinen, Aylward & Miller ; Koskinen et al ), as a starting point we have assumed the metals have abundance ratios comparable to solar values, and included a basic metallicity scaling derived from previous work on evaporating flows (Ercolano & Clarke ). Thus our models are unlikely to be appropriate for considering very exotic, metal‐rich atmospheres or where the compositions are significantly different from solar.…”

Section: Discussionmentioning

confidence: 99%

Planetary evaporation by UV and X-ray radiation: basic hydrodynamics

Owen

Jackson

2012

Monthly Notices of the Royal Astronomical Society

387

454

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We consider the evaporation of close‐in planets by the star's intrinsic extreme‐ultraviolet (EUV) and X‐ray radiation. We calculate evaporation rates by solving the hydrodynamical problem for planetary evaporation including heating from both X‐ray and EUV radiation. We show that most close‐in planets (a < 0.1 au) are evaporating hydrodynamically, with the evaporation occurring in two distinct regimes: X‐ray driven, in which the X‐ray heated flow contains a sonic point, and EUV driven, in which the X‐ray region is entirely sub‐sonic. The mass‐loss rates scale as LX/a2 for X‐ray driven evaporation, and as Φ*1/2/a for EUV driven evaporation at early times, with mass‐loss rates of the order of 1010–1014 g s−1. No exact scaling exists for the mass‐loss rate with planet mass and planet radius; however, in general evaporation proceeds more rapidly for planets with lower densities and higher masses. Furthermore, we find that in general the transition from X‐ray driven to EUV driven evaporation occurs at lower X‐ray luminosities for planets closer to their parent stars and for planets with lower densities. Coupling our evaporation models to the evolution of the high‐energy radiation – which falls with time – we are able to follow the evolution of evaporating planets. We find that most planets start off evaporating in the X‐ray driven regime, but switch to EUV driven once the X‐ray luminosity falls below a critical value. The evolution models suggest that while ‘hot Jupiters’ are evaporating, they are not evaporating at a rate sufficient to remove the entire gaseous envelope on Gyr time‐scales. However, we do find that close in Neptune mass planets are more susceptible to complete evaporation of their envelopes. Thus we conclude that planetary evaporation is more important for lower mass planets, particularly those in the ‘hot Neptune’/‘super Earth’ regime.

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“…Within the blue end of the EUV observed by EUVE, stellar emission is mostly coronal, whereas at longer, ISM-absorbed wavelengths that cannot be observed from Earth, EUV emission is predominantly from the chromosphere and transition-region, similar to FUV emission . It is the ISM-absorbed portion of the EUV where ionization cross sections of H, neutral He, and H 2 peak, and absorption by these same species in the upper atmosphere of a planet is what powers thermal escape (Lammer et al 2003;Murray-Clay et al 2009;Koskinen et al 2010). Given that the intent of the fiducial flare is to provide input useful to modeling of planetary atmospheres, we consider it reasonable, in lieu of output of detailed models of stellar atmospheres, to approximate the contribution of the EUV to flares using observations of FUV transition-region emission.…”

Section: Euvmentioning

confidence: 99%

The MUSCLES Treasury Survey. V. FUV Flares on Active and Inactive M Dwarfs* † ‡

Loyd

Youngblood

Schneider

et al. 2018

ApJ

134

187

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M dwarf stars are known for their vigorous flaring. This flaring could impact the climate of orbiting planets, making it important to characterize M dwarf flares at the short wavelengths that drive atmospheric chemistry and escape. We conducted a far-ultraviolet flare survey of 6 M dwarfs from the recent MUSCLES (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) observations, as well as 4 highly-active M dwarfs with archival data. When comparing absolute flare energies, we found the active-M-star flares to be about 10× more energetic than inactive-M-star flares. However, when flare energies were normalized by the star's quiescent flux, the active and inactive samples exhibited identical flare distributions, with a power-law index of -0.76 +0.1 −0.09 (cumulative distribution). The rate and distribution of flares are such that they could dominate the FUV energy budget of M dwarfs, assuming the same distribution holds to flares as energetic as those cataloged by Kepler and ground-based surveys. We used the observed events to create an idealized model flare with realistic spectral and temporal energy budgets to be used in photochemical simulations of exoplanet atmospheres. Applied to our own simulation of direct photolysis by photons alone (no particles), we find Corresponding author: R. O. Parke Loyd robert.loyd@colorado.edu * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. † The scientific results reported in this article are based in part on observations made by the Chandra X-ray Observatory. ‡ Based in part on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA arXiv:1809.07322v2 [astro-ph.SR] 25 Sep 2018 2 the most energetic observed flares have little effect on an Earth-like atmosphere, photolyzing ∼0.01% of the total O 3 column. The observations were too limited temporally (73 h cumulative exposure) to catch rare, highly energetic flares. Those that the power-law fit predicts occur monthly would photolyze ∼1% of the O 3 column and those it predicts occur yearly would photolyze the full O 3 column. Whether such energetic flares occur at the rate predicted is an open question.

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Ionization of Extrasolar Giant Planet Atmospheres

Cited by 53 publications

References 31 publications

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

Planetary evaporation by UV and X-ray radiation: basic hydrodynamics

The MUSCLES Treasury Survey. V. FUV Flares on Active and Inactive M Dwarfs* † ‡

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