* Astronomy

Title: Re-visit of HST FUV observations of hot-Jupiter system HD 209458: No Si III detection and the need for COS transit observations
Author: G. E. Ballester, L. Ben-Jaffel

The discovery of OI atoms and CII ions in the upper atmosphere of HD 209458b, made with the Hubble Space Telescope Imaging Spectrograph (STIS) using the G140L grating, showed that these heavy species fill an area comparable to the planet's Roche lobe. The derived ~10% transit absorption depths require super-thermal processes and/or supersolar abundances. From subsequent Cosmic Origins Spectrograph (COS) observations, CII absorption was reported with tentative velocity signatures, and absorption by SiIII ions was also claimed in disagreement with a negative STIS G140L detection. Here, we revisit the COS dataset showing a severe limitation in the published results from having contrasted the in-transit spectrum against a stellar spectrum averaged from separate observations, at planetary phases 0.27, 0.72, and 0.49. We find variable stellar SiIII and CII emissions that were significantly depressed not only during transit but also at phase 0.27 compared to phases 0.72 and 0.49. Their respective off-transit 7.5 and 3.1% flux variations are large compared to their reported 8.2±1.4% and 7.8±1.3% transit absorptions. Significant variations also appear in the stellar line shapes, questioning reported velocity signatures. We furthermore present archive STIS G140M transit data consistent with no SiIII absorption, with a negative result of 1.7±18.7 including ~15% variability. Silicon may still be present at lower ionisation states, in parallel with the recent detection of extended magnesium, as MgI atoms. In this frame, the firm detection of OI and CII implying solar or supersolar abundances contradicts the recent inference of potential x20-125 subsolar metallicity for HD 209458b.

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Title: New Analysis Indicates No Thermal Inversion in the Atmosphere of HD 209458b
Author: Hannah Diamond-Lowe, Kevin B. Stevenson, Jacob L. Bean, Michael R. Line, Jonathan J. Fortney

An important focus of exoplanet research is the determination of the atmospheric temperature structure of strongly irradiated gas giant planets, or hot Jupiters. HD 209458b is the prototypical exoplanet for atmospheric thermal inversions, but this assertion does not take into account recently obtained data or newer data reduction techniques. We re-examine this claim by investigating all publicly available Spitzer Space Telescope secondary-eclipse photometric data of HD 209458b and performing a self-consistent analysis. We employ data reduction techniques that minimize stellar centroid variations, apply sophisticated models to known Spitzer systematics, and account for time-correlated noise in the data. We derive new secondary-eclipse depths of 0.119 ± 0.007%, 0.123 ± 0.006%, 0.134 ± 0.035%, and 0.215 ± 0.008% in the 3.6, 4.5, 5.8, and 8.0 micron bandpasses, respectively. We feed these results into a Bayesian atmospheric retrieval analysis and determine that it is unnecessary to invoke a thermal inversion to explain our secondary-eclipse depths. The data are well-fitted by a temperature model that decreases monotonically between pressure levels of 1 and 0.01 bars. We conclude that there is no evidence for a thermal inversion in the atmosphere of HD 209458b.

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Title: The impact of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b
Authors: M. Agundez, O. Venot, N. Iro, F. Selsis, F. Hersant, E. Hebrard, M. Dobrijevic

We investigate the effects of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b. We use a simplified dynamical model and a robust chemical network, as opposed to previous studies which have used a three dimensional circulation model coupled to a simple chemical kinetics scheme. The temperature structure and distribution of the main atmospheric constituents are calculated in the limit of an atmosphere that rotates as a solid body with an equatorial rotation rate of 1 km/s. Such motion mimics a uniform zonal wind which resembles the equatorial superrotation structure found by three dimensional circulation models. The uneven heating of this tidally locked planet causes, even in the presence of such a strong zonal wind, large temperature contrasts between the dayside and nightside, of up to 800 K. This would result in important longitudinal variations of some molecular abundances if the atmosphere were at chemical equilibrium. The zonal wind, however, acts as a powerful disequilibrium process. We identify the existence of a pressure level of transition between two regimes, which may be located between 100 and 0.1 mbar depending on the molecule. Below this transition layer, chemical equilibrium holds, while above it, the zonal wind tends to homogenise the chemical composition of the atmosphere, bringing molecular abundances in the limb and nightside regions close to chemical equilibrium values characteristic of the dayside, i.e. producing an horizontal quenching effect in the abundances. Reasoning based on timescales arguments indicates that horizontal and vertical mixing are likely to compete in HD 209458b's atmosphere, producing a complex distribution where molecular abundances are quenched horizontally to dayside values and vertically to chemical equilibrium values characteristic of deep layers.

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Title: Spitzer/MIPS 24 micron Observations of HD 209458b: 2.5 transits, 3 eclipses, and a Phase Curve Corrupted by Instrumental Sensitivity Variations
Authors: Ian J. M. Crossfield, Heather Knutson, Jonathan Fortney, Adam Showman, Nicolas B. Cowan, Drake Deming

We report the results of an analysis of all Spitzer/MIPS 24 micron observations of HD 209458b, one of the touchstone objects in the study of irradiated giant planet atmospheres. Altogether we analyse 2.5 transits, 3 eclipses, and a 58-hour near-continuous observation designed to detect the planet's thermal phase curve. The results of our analysis are: (1) A mean transit depth of 1.484% ± 0.035%, consistent with previous measurements and showing no evidence of variability in transit depth at the 3% level. (2) A mean eclipse depth of 0.332% ± 0.026%, somewhat higher than that previously reported for this system; this new value brings observations into better agreement with models. The dayside flux shows no evidence of variability at the 12% level. (3) Eclipses in the system occur 32 s ± 129 s earlier than would be expected from a circular orbit, which constrains the orbital quantity (e cos omega) to be 0.00004 ± 0.00033. This result is fully consistent with a circular orbit and sets an upper limit of 140 m/s (3 sigma) on any eccentricity-induced velocity offset during transit. The phase curve observations (including one of the transits) exhibit an anomalous trend similar to the detector ramp seen in previous Spitzer/IRAC observations; by modelling this ramp we recover the system parameters. The photometry which follows the ramp and transit exhibits a gradual, ~0.2% decrease in flux, similar to that seen in pre-launch calibration data. The large uncertainties associated with this poorly-understood, likely instrumental effect prevent us from usefully constraining the planet's thermal phase curve. Our observations highlight the need for a thorough understanding of detector-related instrumental effects on long time scales when making the high-precision mid-infrared measurements planned for future missions such as EChO, SPICA, and JWST.

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