A Survey of Alkali Line Features in Exoplanet Atmospheres

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An outline of the paper:  A Survey of Alkali Line Absorption in Exoplanetary Atmospheres  by A. G. Jensen et al.

As exoplanet detections are becoming more and more capable of penetrating the Earth mass regime, the next big front of exoplanet science is the commonplace detection of atmospheric compositions. While HD 209458 b and HD 189733 b have been the subjects of the majority of planetary atmosphere breakthroughs there is increasing effort to characterise the atmospheres of many other hot Jupiters.

Models predict that alkali metals should produce large absorption features in optical observations, predominantly sodium and potassium (Na and K). Encouragingly observations find that Na and K features in a handful of hot Jupiters.

Jensen et al. aim to address this small sample size with a survey of Na and K absorption lines for the hot Jupiters HAT-P-2 b, HD 149026 b, HD 189733 b and HD 209458 b, using the Hobby-Eberly Telescope (HET). They divide the selection of in transit and out of transit spectra to calculate the “transmission” spectrum. In effect they look for changes in the strength of any Na and K features of the system when the planet is in and out of transit. In theory the in transit measurements will show additional absorption features due to the stellar light passing through the upper planetary atmosphere.

Figure 1. Spectra of HD 189733 out of transit, the team looked for changes in the strength of Na and K features during tranist. Ca lines are purely stellar in origin and should not change significantly in strength during transit. They are therefore used for abundance calibration. A comparison star (HD 196867) is shown in black. From Jensen et al. 2011.

The team manage to detect strong Na absorption features in HD 189733 b, and moderate Na in HD 209458 b (both known to have Na features from previous studies), while HAT-P-2 b shows no difference between in and out of transit Na features. Weak/moderate features are seen in HD 149026 b, though they state additional observations are needed to confirm this due to the large uncertainty in the measurement. As for K, no features are seen in HD 209458 b, HD 189733 b and HD 149026 b though this is due to precision limits rather than clearly ruling out. They find HAT-P-2 b to have an emission, rather than absorption, feature in the K region to a 2.1 σ significance. They claim this could be a result of poor signal strength and high uncertainties for the HAT-P-2 b dataset. The team also make reference to a fifth target for which observations are complete, but analysis is still ongoing.

It is very promising that ground based observations can reproduce Na features found by the HST, and though any inferences of the presence of K are poorly supported, this work is definitely a step in the right direction. The puzzle remains of the emission in HAT-P-2 b, though as this seems to be the first atmospheric study of the planet it would be definitely worthwhile for other teams to try to reproduce these results.

Figure 2. Absorption results from the HET survey for sodium, potassium and calcium features (the later two are offset slightly in scale height for clarity). Scale heights are calculated using Jupiter's mean molecular weight and the relevant planetary equilibrium temperature. From Jensen et al. 2011.

Feature Image: NASAESA, and G. Bacon (STScI), found here.

Further Reading:

Sodium Absorption from the Exoplanetary Atmosphere of HD 189733 b Detected in the Optical Transmission Spectrum Redfield et al. 2008 (sodium in HD 189733 b)

Detection of an Extrasolar Planet Atmosphere Charbonneau et al. 2002 (sodium in HD 209458 b)

Gran Telescopio Canarias OSIRIS transiting exoplanet atmospheric survey: detection of potassium in XO-2b from narrowband spectrophotometry
Sing et al. 2011 (potassium in XO-2b)

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About Author

I am a PhD student in the Exeter astrophysics department. I studied as an undergraduate in Durham university. I specialise in statistical properties of radial velocity datasets and theoretical atmospheric line features. (More)