Weaker water absorption at XO-1b and HD 209458b

Infrared Transmission Spectroscopy of the Exoplanets HD209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope
http://arxiv.org/abs/1302.1141

Quote :

Exoplanetary transmission spectroscopy in the near-infrared using Hubble/NICMOS is currently ambiguous because different observational groups claim different results from the same data, depending on their analysis methodologies. Spatial scanning with Hubble/WFC3 provides an opportunity to resolve this ambiguity. We here report WFC3 spectroscopy of the giant planets HD209458b and XO-1b in transit, using spatial scanning mode for maximum photon-collecting efficiency. Our new analysis technique achieves nearly photon-limited precision even at the high flux levels collected in spatial scan mode. We evaluate our precision using two independent error analyses to measure possible red noise. Both methods indicate that our errors are within 6% (XO-1) and 26% (HD209458b) of the photon-limit at a spectral resolving power of 70. These high flux level data allow us to attain a precision better than 0.01% per spectral channel. For XO-1b we achieve an average error in transit depth of 96 ppm, and for HD209458b our average error is 36 ppm per spectral channel over our analyzed range from 1.11-1.65 microns. We detect water absorption in both planets, but not other molecules. The absorptions we measure are much more subtle than claimed by some previous observers. Both planets exhibit absorption of approximately 200 ppm at the water peak near 1.38 microns. Our result for XO-1b contradicts the much larger absorption derived from NICMOS spectroscopy. The weak water absorption we measure for HD209458b is reminiscent of the weakness of sodium absorption in the first transmission spectroscopy of an exoplanet atmosphere by Charbonneau et al.(2002). Model atmospheres having uniformly-distributed extra opacity of 0.012 cm^2/g account approximately for both our water measurement and the sodium absorption in this planet.