Infrared Observations during the Secondary Eclipse of HD 209458b. I. 3.6 Micron Occultation Spectroscopy Using the Very Large Telescope

Abstract

We search for an infrared signature of the transiting extrasolar planet HD 209458b during secondary eclipse. Our method, which we call 'occultation spectroscopy,' searches for the disappearance and reappearance of weak spectral features due to the exoplanet as it passes behind the star and later reappears. We argue that at the longest infrared wavelengths, this technique becomes preferable to conventional 'transit spectroscopy'. We observed the system in the wing -2of the strong ν 3 band of methane near 3.6 µm during two secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution of 3300. Our analysis, which utilizes a model template spectrum, achieves sufficient precision to expect detection of the spectral structure predicted by an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium model for the exoplanet atmosphere. However, our observations show no evidence for the presence of this spectrum from the exoplanet, with the statistical significance of the non-detection depending on the timing of the secondary eclipse, which depends on the assumed value for the orbital eccentricity. Our results reject certain specific models of the atmosphere of HD 209458b as inconsistent with our observations at the 3σ level, given assumptions about the stellar and planetary parameters.

On the Indirect Detection of Sodium in the Atmosphere of the Planetary Companion to HD 209458

“…The reasons for these large differences are unclear, but they could be due to differing opacities, stellar flux, or planet surface gravity. Our T-P profile is closer to those for HD 209458b discussed by Seager & Sasselov (2000) and Richardson et al (2003). These differences are significant, and we are led to very different conclusions concerning the physical processes of importance for the Na D transit depth.…”

Dynamics and Disequilibrium Carbon Chemistry in Hot Jupiter Atmospheres, with Application to HD 209458b

“…If unexpected opacity sources (although not necessarily clouds) play a strong role in the formation of spectral lines near 2 m, an upper limit to the CO abundance cannot be made with high confidence based on these observations. We note CH 4 has not been seen on this planet either, despite attempts to detect its signature (Richardson et al 2003b(Richardson et al , 2003a.…”

“…The relative concentrations of CO and CH 4 at the photosphere are therefore diagnostic of local conditions in the 1Y10 bar region of the atmosphere, which is deeper than observations can directly probe. Neither species has yet been detected in this planet's atmosphere (Richardson et al 2003b(Richardson et al , 2003aDeming et al 2005a). Our results here suggest that CH 4 depletion (not depletion of CO) is the more likely situation; even in chemical equilibrium, CH 4 is likely to be depleted at the limb.…”

Transit Photometry of the Core‐dominated Planet HD 149026b

“…Of all the previously known transiting systems, only HD 209458 (Charbonneau et al 2000;Henry et al 2000) and TrES-1 (Alonso et al 2004;Sozzetti et al 2004) have parent stars brighter than V ¼ 12, and therefore only they are amenable to a number of fascinating measurements requiring a very high signal-to-noise ratio. Among these studies are (1) the search for satellites and rings ( Brown et al 2001), (2) the search for period variations due to additional companions ( Wittenmyer et al 2005), (3) the detection of (or upper limits on) atmospheric absorption features in transmission Brown et al 2002;Deming et al 2005a), (4) the characterization of the exosphere ( Bundy & Marcy 2000;Moutou et al 2001Moutou et al , 2003Vidal-Madjar et al 2003Winn et al 2004;Narita et al 2005), (5) the measurement of the angle between the sky-projected orbit normal and stellar rotation axis (Queloz et al 2000;Winn et al 2005), (6) the search for spectroscopic features near the times of secondary eclipse (Richardson et al 2003a(Richardson et al , 2003b, and (7) the direct detection of thermal emission from the planet (Charbonneau et al 2005;Deming et al 2005b). Charbonneau (2004) reviewed these techniques and related investigations.…”