INFRARED TRANSMISSION SPECTROSCOPY OF THE EXOPLANETS HD 209458b AND XO-1b USING THE WIDE FIELD CAMERA-3 ON THE<i>HUBBLE SPACE TELESCOPE</i>

Abstract

Exoplanetary transmission spectroscopy in the near-infrared using\nHubble/NICMOS is currently ambiguous because different observational groups\nclaim different results from the same data, depending on their analysis\nmethodologies. Spatial scanning with Hubble/WFC3 provides an opportunity to\nresolve this ambiguity. We here report WFC3 spectroscopy of the giant planets\nHD209458b and XO-1b in transit, using spatial scanning mode for maximum\nphoton-collecting efficiency. We introduce an analysis technique that derives\nthe exoplanetary transmission spectrum without the necessity of explicitly\ndecorrelating instrumental effects, and achieves nearly photon-limited\nprecision even at the high flux levels collected in spatial scan mode. Our\nerrors are within 6-percent (XO-1) and 26-percent (HD209458b) of the\nphoton-limit at a spectral resolving power of 70, and are better than\n0.01-percent per spectral channel. Both planets exhibit water absorption of\napproximately 200 ppm at the water peak near 1.38 microns. Our result for XO-1b\ncontradicts the much larger absorption derived from NICMOS spectroscopy. The\nweak water absorption we measure for HD209458b is reminiscent of the weakness\nof sodium absorption in the first transmission spectroscopy of an exoplanet\natmosphere by Charbonneau et al. (2002). Model atmospheres having\nuniformly-distributed extra opacity of 0.012 cm^2 per gram account\napproximately for both our water measurement and the sodium absorption in this\nplanet. Our results for HD209458b support the picture advocated by Pont et al.\n(2013) in which weak molecular absorptions are superposed on a transmission\nspectrum that is dominated by continuous opacity due to haze and/or dust.\nHowever, the extra opacity needed for HD209458b is grayer than for HD189733b,\nwith a weaker Rayleigh component.\n