Seeing above the Clouds with High Resolution Spectroscopy

Abstract

In the last decade ground based high resolution Doppler spectroscopy (HRS) has detected numerous species in transiting and non-transiting hot Jupiters, and is ideally placed for atmospheric characterisation of warm Neptunes and super Earths. Many of these cooler and smaller exoplanets have shown cloudy atmospheres from low resolution near infrared observations, making constraints on chemical species difficult. We investigate how HRS can improve on these given its sensitivity to spectral line cores which probe higher altitudes above the clouds. We model transmission spectra for the warm Neptune GJ~3470~b and determine the detectability of H$_2$O with the CARMENES, GIANO and SPIRou spectrographs. We also model a grid of spectra for another warm Neptune, GJ~436~b, over a range of cloud-top pressure and H$_2$O abundance. We show H$_2$O is detectable for both planets with modest observational time and that the high H$_2$O abundance-high cloud deck degeneracy is broken with HRS. However, meaningful constraints on abundance and cloud-top pressure are only possible in the high metallicity scenario. We also show that detections of CH$_4$ and NH$_3$ are possible from cloudy models of GJ~436~b. Lastly, we show how the presence of the Earth's transmission spectrum hinders the detection of H$_2$O for the most cloudy scenarios given that telluric absorption overlaps with the strongest H$_2$O features. The constraints possible with HRS on the molecular species can be used for compositional analysis and to study the chemical diversity of such planets in the future.