Non-Detection of L-band Line Emission from the Exo-Planet HD189733b

TL;DR

This study conducted high-resolution spectroscopic observations of exoplanet HD189733b to verify previous claims of non-LTE emission at 3.25 microns, but found no such emission, suggesting the earlier detection was likely due to telluric water contamination.

Contribution

The paper provides the first high-resolution spectroscopic test of non-LTE emission claims from HD189733b, challenging prior low-resolution results and highlighting the importance of telluric correction.

Findings

No line emission detected at high spectral resolution.

Disparity with previous low-resolution detection suggests non-LTE emission is unlikely.

Imperfect telluric water correction may explain earlier results.

Abstract

We attempt to confirm bright non-LTE emission from the exoplanet HD189733b at 3.25 microns, as recently reported by Swain et al. (2010) based on observations at low spectral resolving power (R ~ 30). Non-LTE emission lines from gas in an exoplanet atmosphere will not be significantly broadened by collisions, so the measured emission intensity per resolution element must be substantially brighter when observed at high spectral resolving power. We observed the planet before, during, and after a secondary eclipse event at a resolving power R = 27,000 using the NIRSPEC spectrometer on the Keck II telescope. Our spectra cover a spectral window near the peak found by Swain et al., and we compare emission cases that could account for the magnitude and wavelength dependence of the Swain et al. result with our final spectral residuals. To model the expected line emission, we use a general non-equilibrium formulation to synthesize emission features from all plausible molecules that emit in this spectral region. In every case, we detect no line emission to a high degree of confidence. After considering possible explanations for the Swain et al. results and the disparity with our own data, we conclude that an astrophysical source for the putative non-LTE emission is unlikely. We note that the wavelength dependence of the signal seen by Swain et al. closely matches the 2nu2 band of water vapor at 300K, and we suggest that an imperfect correction for telluric water is the source of the feature claimed by Swain et al.