Re-Evaluating WASP-12b

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An outline of the paper: Re-Evaluating WASP-12b: Strong Emission at 2.315 micron, Deeper Occultations, and an Isothermal Atmosphere, by Crossfield et al.

WASP-12b is an unusually hot planet even for a hot Jupiter. It is one of the first exoplanets to be studied with large wavelength coverage in transmission, with preliminary results used by us to predict what the sunset would look like on such a planet. WASP-12b also stands out because previous published observations have indicated that it may have an unusually high C/O ratio (Madhusudhan et al. 2011). Recently, the picture became more confusing when Cowan et al. (2012) measured two full orbits with Spitzer, one at 3.6 microns and one at 4.5 microns and found that the resulting secondary eclipse depths were too different to reconcile with either a solar or a high C/O ratio. This means that the characteristics of WASP-12b pose quite a challenge to our current understanding. A possible explanation was revealed recently: it was announced by Crossfield et al. at the Heidelberg conference that WASP-12 actually has an M dwarf companion star (initially reported by Bergfors et al. 2011,2012), which causes the transit to be diluted and the inferred radius to be smaller than the real radius. It also affects the planetary spectrum because the spectral type of the companion is different to WASP-12.

Firstly, the authors present new narrowband photometry with Subaru at 2.315 microns (FWHM of 27 nm) and find a deeper than expected eclipse depth. The measurement rules out at the 3 σ level the strong absorption feature predicted at this wavelength by high C/O ratio (> 1) models.

Top: narrowband photometry of WASP-12b at 2.315 microns. The model is shown with a red solid line with dotted lines showing the 1 sigma range. Open points are excluded in the analysis due to sky background problems.

It was during these observations that the authors noticed the companion star, which they call Bergfors-6. They followed up the initial discovery with a series of K band observations using IRTF/SpeX to estimate the flux ratio of WASP-12 to Bergfors-6.

Left: Subaru image at 2.315 microns. Right: follow-up image from IRTF/SpeX K band.

The authors also use their i,z and K band photometric measurements to find that Bergfors-6 has a temperature of 3840±70 K with spectroscopic constraints giving a slightly cooler temperature.  Spectral lines are also used to measure the radial velocities, which are 16.5±2.6 km/s and 19.7±1.3 km/s for WASP-12 and Bergfors-6 respectively. This could mean that the two stars are gravitationally bound, but could also be a coincidence. Combining the photometry and spectroscopy implies that Bergfors-6 lies 50 % closer to the Earth than WASP-12. There is still a possibility of Bergfors-6 being bound to WASP-12 if Bergfors-6 is in fact a binary system observed near conjunction, where the fluxes would be greater than the flux from one star.

Finally, all existing data for the planetary atmosphere are corrected for Bergfors-6, with the severity of contamination depending on the aperture size initially used for analysis. The model that now best matches all the current data is a 3000 K black body (an isothermal atmosphere), which is quite unexpected since it is so different from the previously assumed models. The narrowband Subaru measurement fit the blackbody to within 2σ. Alterations to the model (changing the temperature-pressure profile and thermal inversion, changing abundances) to match this point lead to overall worse fits.

 

Bottom axis is wavelength in microns. Solid points are dilution-corrected measurements. Top: the dark spectrum is a model with a thermal inversion. The grey spectrum is this at 100 times higher resolution. Bottom panel: 3000 K blackbody spectrum overplotted onto the data.

 

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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)