Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias

Abstract

We report observations of HD 80606 using the 10.4-m Gran Telescopio Canarias\n(GTC) and the OSIRIS tunable filter imager. We acquired very-high-precision,\nnarrow-band photometry in four bandpasses around the K I absorption feature\nduring the January 2010 transit of HD 80606b and during out-of-transit\nobservations conducted in January and April of 2010. We obtained differential\nphotometric precisions of \\sim 2.08e-4 for the in-transit flux ratio measured\nat 769.91-nm, which probes the K I line core. We find no significant difference\nin the in-transit flux ratio between observations at 768.76 and 769.91 nm. Yet,\nwe find a difference of \\sim 8.09 \\pm 2.88e-4 between these observations and\nobservations at a longer wavelength that probes the K I wing (777.36 nm). While\nthe presence of red noise in the transit data has a non-negligible effect on\nthe uncertainties in the flux ratio, the 777.36-769.91 nm colour during transit\nshows no effects from red noise and also indicates a significant colour change,\nwith a mean value of \\sim 8.99\\pm0.62e-4. This large change in the colour is\nequivalent to a \\sim 4.2% change in the apparent planetary radius with\nwavelength, which is much larger than the atmospheric scale height. This\nimplies the observations probed the atmosphere at very low pressures as well as\na dramatic change in the pressure at which the slant optical depth reaches\nunity between \\sim770 and 777 nm. We hypothesize that the excess absorption may\nbe due to K I in a high-speed wind being driven from the exoplanet's exosphere.\nWe discuss the viability of this and alternative interpretations, including\nstellar limb darkening, starspots, and effects from Earth's atmosphere. We\nstrongly encourage follow-up observations of HD 80606b to confirm the signal\nmeasured here. Finally, we discuss the future prospects for exoplanet\ncharacterization using tunable filter spectrophotometry.\n