The escape of heavy atoms from the ionosphere of HD209458b. II. Interpretation of the observations

Abstract

Transits in the H I 1216Å (Lyman α), O I 1334Å, C II 1335Å, and Si III 1206.5Å lines constrain the properties of the upper atmosphere ofHD209458b. In addition to probing the temperature and density profiles in the thermosphere, they have implications for the properties of the lower atmosphere. Fits to the observations with a simple empirical model and a direct comparison with a more complex hydrodynamic model constrain the mean temperature and ionization state of the atmosphere, and imply that the optical depth of the extended thermosphere of the planet in the atomic resonance lines is significant. In particular, it is sufficient to explain the observed transit depths in the H I 1216Å line. The detection of O at high altitudes implies that the minimum mass loss rate from the planet is approximately 6 × 10 6 kg s −1 . The mass loss rate based on our model is higher than this and implies that diffusive separation is prevented for neutral species with a mass lower than about 130 amu by the escape of H. Heavy ions are transported to the upper atmosphere by Coulomb collisions with H + and their presence does not provide as strong constraints on the mass loss rate as the detection of heavy neutral atoms. Models of the upper atmosphere with solar composition and heating based on the average solar X-ray and EUV flux agree broadly with the observations but tend to underestimate the transit depths in the O I, C II, and Si III lines. This suggests that the temperature and/or elemental abundances in the thermosphere may be higher than expected from such models. Observations of the escaping atmosphere can potentially be used to constrain the strength of the planetary magnetic field. We find that a magnetic moment of m 0.04 m J , where m J is the Jovian magnetic moment, allows the ions to escape globally rather than only along open field lines. The detection of Si 2+ in the thermosphere indicates that clouds of forsterite and enstatite do not form in the lower atmosphere. This has implications for the temperature and dynamics of the atmosphere that also affect the interpretation of transit and secondary eclipse observations in the visible and infrared wavelengths.The detection of H, O, C + , and Si 2+ in the upper atmosphere of HD209458b 2 (Vidal-Madjar et al., 2003, 2004 Linsky et al., 2010), and the tentative de-3 tection of H in the upper atmosphere of HD189733b (Lecavelier des Etangs 4 et al., 2010) and Mg + in the upper atmosphere of WASP-12b (Fossati et al., 5 2010) are among the most exciting recent discoveries related to the atmo-6 spheres of extrasolar giant planets (EGPs). The observations demonstrate 7 that the the upper atmospheres of close-in EGPs such as HD209458b differ 8 significantly from the thermospheres of the giant planets in the solar system.9They are much hotter, they extend to several planetary radii and instead of 10 molecular hydrogen, they are primarily composed of atoms and atomic ions. 11The detection of heavy atoms and ions such as O, C + , Si 2+ , and Mg + 12 implies that the atmospheres of close...

The mineral clouds on HD 209458b and HD 189733b

“…in a case of an outward temperature increase. Koskinen et al (2013) conclude that the detection of Si 2+ indicates that clouds involving Si do not form in the atmosphere of HD 209 458b.…”

Thermal escape from extrasolar giant planets

“…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.…”

Spectroscopy of planetary atmospheres in our Galaxy

“…Sophisticated models for the chemistry, photo-ionisation and aeronomy of hot Jupiters were developed by Yelle (2004), Koskinen et al (2007Koskinen et al ( , 2012 and Garcia Munoz (2007). Tian et al (2005) and Murray-Clay et al (2009) Trammell et al (2011), and Adams (2011) included the planetary magnetic field geometry, where gas escapes through open field lines.…”

“…Thermochemical calculations indicate that SiO should be the dominant silicon-bearing gas on HD209458b (Visscher et al 2010). According to Koskinen et al (2012), though the detection of Si 2+ in the upper atmosphere by Linsky et al (2010) implies that the formation of silicon clouds in the lower atmosphere is suppressed.…”