The escape of heavy atoms from the ionosphere of HD209458b. I. A photochemical–dynamical model of the thermosphere

By T. Koskinen, M. Harris, R. Yelle, P. Lavvas. 2012

198 citations recorded by the source corpus.

The detections of atomic hydrogen, heavy atoms and ions surrounding the extrasolar giant planet (EGP) HD209458b constrain the composition, temperature and density profiles in its upper atmosphere. Thus the observations provide guidance for models that have so far predicted a range of possible conditions. We present the first hydrodynamic escape model for the upper atmosphere that includes all of the detected species in order to explain their presence at high altitudes, and to further constrain the temperature and velocity profiles. This model calculates the stellar heating rates based on recent estimates of photoelectron heating efficiencies, and includes the photochemistry of heavy atoms and ions in addition to hydrogen and helium. The composition at the lower boundary of the escape model is constrained by a full photochemical model of the lower atmosphere. We confirm that molecules dissociate near the 1 μbar level, and find that complex molecular chemistry does not need to be included above this level. We also confirm that diffusive separation of the detected species does not occur because the heavy atoms and ions collide frequently with the rapidly escaping H and H+. This means that the abundance of the heavy atoms and ions in the thermosphere simply depends on the elemental abundances and ionization rates. We show that, as expected, H and O remain mostly neutral up to at least 3Rp, whereas both C and Si are mostly ionized at significantly lower altitudes. We also explore the temperature and velocity profiles, and find that the outflow speed and the temperature gradients depend strongly on the assumed heating efficiencies. Our models predict an upper limit of 8000 K for the mean (pressure averaged) temperature below 3Rp, with a typical value of 7000 K based on the average solar XUV flux at 0.047 AU. We use these temperature limits and the observations to evaluate the role of stellar energy in heating the upper atmosphere.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/0f8fe18b1cde014d5084fc32a919c5dcfb2bd2b1

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. C. Mordasini
3. Th. Henning
4. C. Lovis
5. S. Udry
6. R. van Boekel
7. F. Pepe
8. Paul Mollière

Citations in the corpus

• The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B (2017, background): “. 2003a; Lecavelier et al. 2004; Yelle 2004; Tian et al. 2005; Erkaev et al. 2007; Garcia-Munoz 2007; Koskinen et al. 2009; Lammer et al. 2009; Murray-Clay et al. 2009; Tian 2009; Erkaev et al. 2013; Koskinen et al. 2013; Lammer et al. 2013; Owen and Wu 2013; Koskinen et al. 2014; Lammer et al. 2014”
• Spectrally resolved detection of sodium in the atmosphere of HD 189733b with the HARPS spectrograph (2015, background): “Our results suggest that a part of the sodium absorption occurs in the (lower) thermosphere, where heating by the stellar X/EUV photons occurs (Lammer et al. 2003; Yelle 2004; Vidal-Madjar et al. 2011; Koskinen et al. 2013).”
• LYα TRANSIT SPECTROSCOPY AND THE NEUTRAL HYDROGEN TAIL OF THE HOT NEPTUNE GJ 436b (2014, background): “Koskinen et al. (2013) model the H and H+ density in the atmosphere of HD209458b as a function of planetary radius.”
• Extremely Irradiated Hot Jupiters: Non-oxide Inversions, H− Opacity, and Thermal Dissociation of Molecules (2018, background): “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 2004, 2006; Murray-Clay et al. 2009; Koskinen et al. 2013; Chadney et al. 2017).”
• THE IMPRINT OF EXOPLANET FORMATION HISTORY ON OBSERVABLE PRESENT-DAY SPECTRA OF HOT JUPITERS (2016, background): “…fractionation during the escape process (Hunten et al. 1987) which could modify the atmospheric composition should not be important, especially because the escape fluxes of hydrogen are expected to be sufficiently high in hot Jupiters in order to drag along heavy species (Koskinen et al. 2013).”