Diagnosing aerosols in extrasolar giant planets with cross-correlation function of water bands

¹ Observatoire astronomique de l'Université de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
e-mail: lorenzo.pino@unige.ch
² Dipartimento di Fisica e Astronomia “Galileo Galilei”, Université di Padova, Vicolo dell'Osservatorio 3, Padova 35122, Italy
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Center for Space and Habitability, University of Bern, Sidlerstrasse 5, 3012, Bern, Switzerland
⁵ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122, Padova, Italy

Received: 9 March 2018
Accepted: 16 July 2018

Abstract

Transmission spectroscopy with ground-based, high-resolution instruments provides key insight into the composition of exoplanetary atmospheres. Molecules such as water and carbon monoxide have been unambiguously identified in hot gas giants through cross-correlation techniques. A maximum in the cross-correlation function (CCF) is found when the molecular absorption lines in a binary mask or model template match those contained in the planet. Here, we demonstrate how the CCF method can be used to diagnose broadband spectroscopic features such as scattering by aerosols in high-resolution transit spectra. The idea consists in exploiting the presence of multiple water bands from the optical to the near-infrared. We have produced a set of models of a typical hot Jupiter spanning various conditions of temperature and aerosol coverage. We demonstrate that comparing the CCFs of individual water bands for the models constrains the presence and the properties of the aerosol layers. The contrast difference between the CCFs of two bands can reach ~100 ppm, which could be readily detectable with current or upcoming high-resolution stabilized spectrographs spanning a wide spectral range, such as ESPRESSO, CARMENES, HARPS-N+GIANO, HARPS+NIRPS, SPIRou, or CRIRES+.