Search for Potassium in XO-2 b and HD 80606 b

GTC OSIRIS Transiting Exoplanet Atmospheric Survey: Detection of potassium in XO-2b from spectrophotometry
http://arxiv.org/abs/1008.4795

Abstract wrote:

We present Gran Telescopio Canarias (GTC) optical transit narrow-band photometry of the hot-Jupiter exoplanet XO-2b using the OSIRIS instrument. This unique instrument has the capabilities to deliver high cadence narrow-band photometric lightcurves, allowing us to probe the atmospheric composition of hot Jupiters from the ground. The observations were taken during three transit events which cover four wavelengths at spectral resolutions near 500, necessary for observing atmospheric features, and have near-photon limited sub-mmag precisions. Precision narrow-band photometry on a large aperture telescope allows for atmospheric transmission spectral features to be observed for exoplanets around much fainter stars than those of the well studied targets HD209458b and HD189733b, providing access to the majority of known transiting planets. For XO-2b, we measure planet-to-star radius contrasts of R_pl/R_star=0.10508+/-0.00052 at 6792 Ang, 0.10640+/-0.00058 at 7582 Ang, and 0.10686+/-0.00060 at 7664.9 Ang, and 0.10362+/-0.00051 at 8839 Ang. These measurements reveal significant spectral features at two wavelengths, with an absorption level of 0.067+/-0.016% at 7664.9 Ang due to atmospheric potassium in the line core (a 4.1-sigma significance level), and an absorption level of 0.058+/-0.016% at 7582 Ang, (a 3.6-sigma significance level). When comparing our measurements to hot-Jupiter atmospheric models, we find good agreement with models which are dominated in the optical by alkali metals. This is the first evidence for potassium in an extrasolar planet, an element that has long been theorized along with sodium to be a dominant source of opacity at optical wavelengths for hot Jupiters.

Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias
http://arxiv.org/abs/1008.4800

Abstract wrote:

We report observations of HD 80606 using the 10.4-m Gran Telescopio Canarias (GTC) and the OSIRIS tunable filter imager. We acquired very-high-precision, narrow-band photometry in four bandpasses around the K I absorption feature during the January 2010 transit of HD 80606b and during out-of-transit observations conducted in April 2010. We obtained differential photometric precisions as small as ~ 2.9 x 10^(-5). We find no significant difference between observations at 768.76 and 769.91 nm, which probe the K I line core. Yet, we observe significant differences [3.08 +/- 0.53 x 10^(-4) and 7.00 +/- 0.40 x 10^(-4)] between these observations and observations at two longer wavelengths that probe the K I wing (773.66 and 777.36 nm). The large change in the apparent planetary radius with wavelength (~3.6%) is much larger than the atmospheric scale height. This implies the observations probed the atmosphere at low pressures as well as a dramatic change in the pressure at which the slant optical depth reaches unity between ~770 and 777 nm. We hypothesize that the excess absorption may be due to K I in a high-speed wind being driven from the exoplanet's exosphere. We discuss the viability of this and alternative interpretations. Finally, we discuss the future prospects for exoplanet characterization using tunable filter spectrophotometry.