Thermal Emission of WASP-14 b Revealed with Three Spitzer Eclipses


An outline of the paper: Thermal Emission of WASP-14b Revealed with Three Spitzer Eclipses by Blecic et al.

WASP-14 b is one of the heaviest known transiting planets with a mass of 7.3 +/- 0.5 MJ.

Blecic et al. measured three secondary eclipses; one each at 3.6, 4.5 and 8.0 microns with the Spitzer space telescope. They then interpreted the data in terms of the atmospheric chemical composition and thermal structure using models assuming local thermodynamic equilibrium and using 1D radiative transfer (Madhusudhan & Seager 2009, 2010). The models include opacity sources from H2O, CO, CH4, CO2, TiO, VO and H2-H2 collisions.

The 3.6 and 4.5 micron brightness temperatures are consistent with a planetary blackbody spectrum at T~2200 K, but the 8 micron flux shows a brightness temperature of 1637 +/- 120 K. The fact that the flux is instead lower implies that the molecular features of water and methane in this wavelength range are in absorption rather than emission, and suggests that the planet lacks a stratosphere and thermal inversion.

Model fits to the data also show that the planet has low day-night energy redistribution, as the dayside of the planet is much brighter than its predicted equilibrium temperature assuming uniform distribution (Teq=1866 K). For a zero albedo, ~0.25 of the stellar energy incident on the dayside can be redistributed. For higher albedos, this value will be lower.

Observations and model spectra for the dayside emission of WASP-14b from Blecic et al. The green model has molecular abundances in chemical equilibrium assuming solar elemental abundances, and the red model has 10 times lower CO and 6 times higher H2O. Green and red circles show model values integrated over the Spitzer bandpasses. Inset are corresponding model temperature-pressure profiles.

Only marginal constraints can be placed on the composition, but the observations are consistent with compositions no different than a factor of 10 from thermochemical equilibrum using solar abundances.

These results are of great interest because WASP-14 b, although highly irradiated, still does not have a stratospheric inversion. This and several other examples are now beginning to show that hot Jupiters may fall into two neat categories, as suggested by Fortney et al., 2008, with the hotter ones subject to a temperature inversion near the photosphere.

Feature Image: From the paper.

Further Reading:

Madhusudhan & Seager (2009), ApJ, 707, 24 – “A Temperature and Abundance Retrieval Method for Exoplanet Atmospheres” – this paper describes the models that the authors of the reviewed paper use to fit the data.

Madhusudhan & Seager (2010), ApJ, 725, 261 – “On the Inference of Thermal Inversions in Hot Jupiter Atmospheres” – this paper talks about degeneracies between thermal inversions and molecular abundances for several hot Jupiters.

About Author

I am undertaking an observational astronomy PhD focusing on the characterisation of planetary atmospheres, working with Dr. David Sing and Prof. Frédéric Pont. I am specialising in using transmission spectroscopy, applied to the hot Jupiter HD 189733b with HST. I am interested in obtaining a more complete picture for the few well-studied exoplanets, comparing observations in different regimes of planetary atmospheres, from secondary eclipse and transmission observations at different wavelengths, both from the ground and from space. (More)