Using the chromatic Rossiter-McLaughlin effect to probe the broadband signature in the optical transmission spectrum of HD 189733b

TL;DR

This study demonstrates that the chromatic Rossiter-McLaughlin effect can effectively measure broadband transmission features, such as Rayleigh scattering, in exoplanet atmospheres using ground-based high-dispersion spectroscopy.

Contribution

It introduces the use of the chromatic RM effect to probe broadband spectral features, expanding the capabilities of ground-based transmission spectroscopy.

Findings

Measured the Rayleigh-scattering slope in HD 189733b's spectrum at 2.5σ significance.

Estimated atmospheric temperature of 2300 ± 900 K consistent with previous results.

Showed ground-based high-dispersion spectroscopy can probe broad-band features in exoplanet atmospheres.

Abstract

Transmission spectroscopy is a powerful technique for probing exoplanetary atmospheres. A successful ground-based observational method uses a differential technique based on high-dispersion spectroscopy, but that only preserves narrow features in transmission spectra. Here we use the chromatic Rossiter-McLaughlin (RM) effect to measure the Rayleigh-scattering slope in the transmission spectrum of HD 189733b with the aim to show that it can be effectively used to measure broadband transmission features. The amplitude of the RM effects depends on the effective size of the planet, and in the case of an atmospheric contribution therefore depends on the observed wavelength. We analysed archival HARPS data of three transits of HD 189733b, covering a wavelength range of 400 to 700 nm. We measured the slope in the transmission spectrum of HD 189733b at a $2.5\sigma$ significance. Assuming it is due to Rayleigh scattering and not caused by stellar activity, it would correspond to an atmospheric temperature, as set by the scale height, of $T = 2300 \pm 900 \mathrm{K}$, well in line with previously obtained results. This shows that ground-based high-dispersion spectral observations can be used to probe broad-band features in the transmission spectra of extrasolar planets, by using the chromatic RM effect. This method will be particularly interesting in conjunction with the new echelle spectrograph ESPRESSO, which currently is under construction for ESOs Very Large Telescope and will provide a gain in signal-to-noise ratio of about a factor 4 compared to HARPS. This will be of great value because of the limited and uncertain future of the Hubble Space Telescope and because the future James Webb Space Telescope will not cover this wavelength regime.