Limits on the 2.2 microns contrast ratio of the close orbiting planet HD 189733b

TL;DR

This study used high-resolution near-infrared spectroscopy to search for atmospheric signatures of the exoplanet HD 189733b, setting upper limits on its contrast ratio and challenging existing atmospheric models.

Contribution

First application of spectral deconvolution and tomographic techniques to constrain the near-infrared contrast ratio of HD 189733b.

Findings

No planetary signature detected at the targeted contrast ratio.

The upper limit on the contrast ratio is lower than some model predictions.

Results suggest atmospheric models may overestimate the planet's near-infrared flux.

Abstract

We obtained 238 spectra of the close orbiting extrasolar giant planet HD 189733b with resolution R ~ 15,000 during one night of observations with the near infrared spectrograph, NIRSPEC, at the Keck II Telescope. We have searched for planetary absorption signatures in the 2.0 - 2.4 micron region where H_2O and CO are expected to be the dominant atmospheric opacities. We employ a phase dependent orbital model and tomographic techniques to search for the planetary absorption signatures in the combined stellar and planetary spectra. Because potential absorption signatures are hidden in the noise of each single exposure, we use a model list of lines to apply a spectral deconvolution. The resulting mean profile possesses a S/N ratio that is 20 times greater than that found in individual lines. Our spectral timeseries thus yields spectral signatures with a mean S/N = 2720. We are unable to detect a planetary signature at a contrast ratio of log_10(F_p/F_*) = -3.40, with 63.8 per cent confidence. Our findings are not consistent with model predictions which nevertheless give a good fit to mid-infrared observations of HD 189733. The 1-sigma result is a factor of 1.7 times less than the predicted 2.185 micron planet/star flux ratio of log_10(F_p/F_*) ~ -3.16.