Volume 505, Number 2, October II 2009
|Page(s)||891 - 899|
|Section||Planets and planetary systems|
|Published online||03 August 2009|
Transit spectrophotometry of the exoplanet HD 189733b
I. Searching for water but finding haze with HST NICMOS
UPMC Univ Paris 06, CNRS, Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France e-mail: email@example.com
2 Lunar and Planetary Laboratory, University of Arizona, Sonett Space Science Building, Tucson, AZ 85721-0063, USA
3 Center of Excellence in Information Systems, Tennessee State University, 3500 John A. Merritt Blvd., Box 9501, Nashville, TN 37209, USA
Accepted: 27 July 2009
We present Hubble Space Telescope near-infrared transit photometry of the nearby hot-Jupiter HD 189733b. The observations were taken with the NICMOS instrument during five transits, with three transits executed with a narrowband filter at 1.87 μm and two performed with a narrowband filter at 1.66 μm. Our observing strategy using narrowband filters is insensitive to the usual HST intra-orbit and orbit-to-orbit measurement of systematic errors, allowing us to accurately and robustly measure the near-IR wavelength dependance of the planetary radius. Our measurements fail to reproduce the previously claimed detection of an absorption signature of atmospheric H2O below 2 μm at a 5σ confidence level. We measure a planet-to-star radius contrast of at 1.66 μm and a contrast of at 1.87 μm. Both of our near-IR planetary radii values are in excellent agreement with the levels expected from Rayleigh scattering by sub-micron haze particles, observed at optical wavelengths, indicating that upper-atmospheric haze still dominates the near-IR transmission spectra over the absorption from gaseous molecular species at least below 2 μm.
Key words: planetary systems / stars: individual: HD 189733 / techniques: photometric / binaries: eclipsing
© ESO, 2009
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