X-Ray Transits of the Exoplanet HD189733Ab

Hot Jupiter type exoplanets are common, with HD 189733Ab being a prototypical, and the only observed, example of an eclipsing transit which is accessible in the X-ray regime. X-rays allow us to probe the upper atmosphere of the planet more efficiently than the optical emission. Using a series of Chandra observations of the star during planetary transit, Poppenhaeger et al. (2013, ApJ, 773, 62) found that the transit in X-ray was three times deeper than in the optical band, signifying that the planet’s atmospheric scale height was around seventy percent larger than the planetary radius estimated from optical observations. We use these observations of HD 189733Ab to help us describe a physically meaningful model based on the parameters, sizes, and chemical composition of the planetary atmosphere, additionally considering systematic effects such as stellar chemical composition, star spots, and limb brightening. We use a geometric model to project light rays, emitted from the stellar corona, through a planetary atmosphere onto a two dimensional observer plane. This then allows us to construct model light curves for the transit. We use this numerical model to constrain the physical parameters of the system by applying it to the Chandra dataset. We confirm the previously published interpretation that an extended atmosphere is necessary to account for the observed depth of the transit in the data. This modeling method can also be applied to monitoring observations of transiting systems that would be observed with missions like SEEJ, NExtUP, and ARCUS.

We acknowledge support from the University of Southampton course number PHYS6013, Chandra contact NAS8-03060, and NASA research grants AR9-20004X and G07-18011X.