Detection of carbon monoxide in the high-resolution day-side spectrum of the exoplanet HD 189733b^⋆

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: R.J.de.Kok@sron.nl
² Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
³ Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
⁴ Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada

Received: 28 February 2013
Accepted: 12 April 2013

Abstract

Context. After many attempts over more than a decade, high-resolution spectroscopy has recently delivered its first detections of molecular absorption in exoplanet atmospheres, both in transmission and thermal emission spectra. Targeting the combined signal from individual lines in molecular bands, these measurements use variations in the planet radial velocity to separate the planet signal from telluric and stellar contaminants.

Aims. We apply high-resolution spectroscopy to probe molecular absorption in the day-side spectrum of the bright transiting hot Jupiter HD 189733b.

Methods. We observed HD 189733b with the CRIRES high-resolution near-infrared spectograph on the Very Large Telescope during three nights, targeting possible absorption from carbon monoxide, water vapour, methane, and carbon dioxide, at 2.0 and 2.3 μm.

Results. We detect a 5-σ absorption signal from CO at a contrast level of ~4.5 × 10^-4 with respect to the stellar continuum, revealing the planet orbital radial velocity at 154⁺⁴_-3 km s^-1. This allows us to solve for the planet and stellar mass in a similar way as for stellar eclipsing binaries, resulting in 0.846^+0.068_-0.049M⊙ and M_p = 1.162^+0.058_-0.039 M^Jup. No significant absorption is detected from H₂O, CO₂, or CH₄ and we determine upper limits on their line contrasts.

Conclusions. The detection of CO in the day-side spectrum of HD 189733b can be made consistent with the haze layer proposed to explain the optical to near-infrared transmission spectrum if the layer is optically thin at the normal incidence angles probed by our observations, or if the CO abundance is high enough for the CO absorption to originate from above the haze. Our non-detection of CO₂ at 2.0 μm is not inconsistent with the deep CO₂ absorption from low-resolution NICMOS secondary eclipse data in the same wavelength range. If genuine, the absorption would be so strong that it blanks out any planet light completely in this wavelength range, leaving no high-resolution signal to be measured.