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

Abstract

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 * Corresponding author. Faxby 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 3 R p , 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 8,000 K for the mean (pressure averaged) temperature below 3 R p , with a typical value of 7,000 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.

Thermal escape from extrasolar giant planets

“…This is because heating at high altitudes is no longer balanced by conduction-instead, it is balanced by 'adiabatic' cooling that is associated with the expansion and escape of the atmosphere. In this sense, the model at 0.2 AU is qualitatively similar to HD209458b where the same behaviour has been predicted by several previous models [8,9,11,33]. Based on these changes in the location of the exobase and the energy balance, we argue that the transition to 'hydrodynamic' escape occurs near 0.3 AU.…”

“…For this purpose, we used the one-dimensional escape model of Koskinen et al [8,9]. We fixed the lower boundary temperature of this model to a value consistent with the T-P profiles above.…”

“…These observations indicate that close-in EGPs such as HD209458b are surrounded by a hot thermosphere composed of atomic hydrogen that extends to several planetary radii and provide the required line of [20,21] and their application to close-in EGPs [8,9]. We then proceed to generalize the results to a sample of known EGPs to make specific predictions about the nature of their upper atmospheres.…”

Heating efficiency in hydrogen-dominated upper atmospheres

“…One year later, Leitzinger et al (2011) assumed η hν = 25% for mass-loss studies from CoRoT-7b and Kepler-10b, Ehrenreich & Désert (2011) studied the thermal mass-loss evolution of close-in exoplanets by assuming η hν values of 1%, 15%, and 100%, and Jackson et al (2012) investigated the X-ray heating contribution and assumed for the XUV heating efficiency η hν a lower value of 25% and the energylimited approach of 100%. Koskinen et al (2013) studied the escape of heavy atoms from the ionosphere of HD 209458b with a photochemical-dynamical thermosphere model for various η hν values of 10%, 30%, 50%, 80%, and 100%.…”

Extreme hydrodynamic atmospheric loss near the critical thermal escape regime

“…Studies related to hydrodynamic escape and evolution of planetary atmospheres (e.g., Sekiya et al 1980aSekiya et al , 1980bSekiya et al , 1981Watson et al 1981;Kasting & Pollack 1983;Yelle 2004;Tian et al 2005aTian et al , 2005bMurry-Clay et al 2009;Koskinen et al 2013;Erkaev et al 2013Erkaev et al , 2014Lammer et al 2013a indicate that the heating of the upper atmosphere caused by high XUV radiation and the related hydrodynamic expansion of the bulk atmosphere is important for the escape of light gases such as hydrogen from early planetary atmospheres. The hydrodynamic outflow of the atmospheric particles is somewhat similar to that of the solar wind described by the well known isothermic model of Parker (1964aParker ( , 1964b.…”