Hydrogen and Sodium Absorption in the Optical Transmission Spectrum of WASP-12b

Abstract

We have obtained $> 10$ hours of medium resolution ($R \sim 15000$) spectroscopic exposures on the transiting exoplanet host star WASP-12, including $\sim2$ hours while its planet, WASP-12b, is in transit, with the Hobby-Eberly Telescope (HET). The out-of-transit and in-transit spectra are coadded into master out-of-transit and in-transit spectra, from which we create a master transmission spectrum. Strong, statistically significant absorption features are seen in the transmission spectrum at H$\alpha$ and \ion{Na}{1} (the Na D doublet). There is the suggestion of pre- and post-transit absorption in both H$\alpha$ and \ion{Na}{1} when the transmission spectrum is examined as a function of phase. The timing of the pre-transit absorption is roughly consistent with previous results for metal absorption in WASP-12b, and the level of the \ion{Na}{1} absorption is consistent with a previous tentative detection. No absorption is seen in the control line of \ion{Ca}{1} at $\lambda$6122. We discuss in particular whether or not the WASP-12b H$\alpha$ absorption signal is of circumplanetary origin---an interpretation that is bolstered by the pre- and post-transit evidence---which would make it one of only a small number of detections of circumplanetary H$\alpha$ absorption in an exoplanet to date, the most well-studied being HD 189733b. We further discuss the notable differences between the HD 189733 and WASP-12 systems, and the implications for a physical understanding of the origin of the absorption.