Random sampling technique for ultra-fast computations of molecular opacities for exoplanet atmospheres

Abstract

Context. Opacities of molecules in exoplanet atmospheres rely on increasingly detailed line-lists for these molecules. The line lists available today contain for many species up to several billions of lines. Computation of the spectral line profile created by pressure and temperature broadening, the Voigt profile, of all of these lines is becoming a computational challenge. Aims. We aim to create a method to compute the Voigt profile in a way that automatically focusses the computation time into the strongest lines, while still maintaining the continuum contribution of the high number of weaker lines. Methods. Here, we outline a statistical line sampling technique that samples the Voigt profile quickly and with high accuracy. The number of samples is adjusted to the strength of the line and the local spectral line density. This automatically provides high accuracy line shapes for strong lines or lines that are spectrally isolated. The line sampling technique automatically preserves the integrated line opacity for all lines, thereby also providing the continuum opacity created by the large number of weak lines at very low computational cost. Results. The line sampling technique is tested for accuracy when computing line spectra and correlated-k tables. Extremely fast computations (∼ 3.5 · 10 5 lines per second per core on a standard current day desktop computer) with high accuracy (≤ 1% almost everywhere) are obtained. A detailed recipe on how to perform the computations is given.

The ExoMolOP database: Cross sections andk-tables for molecules of interest in high-temperature exoplanet atmospheres

“…The efficient and accurate numerical computation of such a profile has been the subject of a number of publications (see, e.g. Hedges & Madhusudhan 2016;Min 2017;Schreier 2017;Grimm & Heng 2015;Yurchenko et al 2018a;Amundsen et al 2014). The coefficients γ 0 and n L in Eq.…”

“…One of the methods used in this work, for example, is based on the use of Gauss Legendre polynomials (see Section 4 for details of the opacities produced for individual retrieval codes). K-tables are produced using a method of opacity sampling which enables low-resolution computations while still taking strong opacity fluctuations at high resolution into account; see, for example, Min (2017). The assumption made for the k-distribution method that the k-coefficients at each Guassian quadrature point are correlated, breaks down for inhomogeneous atmospheres (Lee et al 2019)…”

ARCiS framework for exoplanet atmospheres

“…To compute the spectra we have developed a radiative transfer tool for simulating exoplanet spectra. This code uses molecular line lists from the ExoMol project and the HITEMP and HITRAN databases to compute the molecular opacities with the method by Min (2017). A validation of this ARCiS module is given in App.…”

High Accuracy Molecular Line Lists for Studies of Exoplanets and Other Hot Atmospheres

“…Besides ExoMol, the TheoRets project (Rey et al, 2016) and NASA Ames (eg., Huang et al, 2017) provide comprehensive, computed line lists for a variety of molecules while the MoLLIST project of Bernath (2020) attempts to address this problem from a more experimental standpoint. These line lists, which can be very large, are often pre-processed into cross sections representing opacities for ease of use in forward models and spectroscopic retrievals (Freedman et al, 2014;Min, 2017;Allard et al, 2000;Goyal et al, 2018;Yurchenko et al, 2018a;Tennyson and Yurchenko, 2018;Chubb et al, 2021;Gharib-Nezhad et al, 2021;Grimm et al, 2021).…”