petitRADTRANS

Abstract

We present the easy-to-use, publicly available, Python package petitRADTRANS, built for the spectral characterization of exoplanet atmospheres. The code is fast, accurate, and versatile; it can calculate both transmission and emission spectra within a few seconds at low resolution (λ/Δλ = 1000; correlated-k method) and high resolution (λ/Δλ = 106; line-by-line method), using only a few lines of input instruction. The somewhat slower, correlated-k method is used at low resolution because it is more accurate than methods such as opacity sampling. Clouds can be included and treated using wavelength-dependent power law opacities, or by using optical constants of real condensates, specifying either the cloud particle size, or the atmospheric mixing and particle settling strength. Opacities of amorphous or crystalline, spherical or irregularly-shaped cloud particles are available. The line opacity database spans temperatures between 80 and 3000 K, allowing to model fluxes of objects such as terrestrial planets, super-Earths, Neptunes, or hot Jupiters, if their atmospheres are hydrogen-dominated. Higher temperature points and species will be added in the future, allowing to also model the class of ultra hot-Jupiters, with equilibrium temperatures Teq ≳ 2000 K. Radiative transfer results were tested by cross-verifying the low- and high-resolution implementation of petitRADTRANS, and benchmarked with the petitCODE, which itself is also benchmarked to the ATMO and Exo-REM codes. We successfully carried out test retrievals of synthetic JWST emission and transmission spectra (for the hot Jupiter TrES-4b, which has a Teq of ∼1800 K).

Detection of Fe I and Fe II in the atmosphere of MASCARA-2b using a cross-correlation method

“…Transmission spectra models of Fe i and Fe ii used in our analysis were calculated using petitRADTRANS (Mollière et al 2019), which allows us to create high-resolution models of atoms and molecules at the typical temperatures of exoplanet atmospheres. To calculate the model for MASCARA-2b, we assumed a surface gravity (log g) of 3.0 (corresponding to a mass of about 1.2M J ), considering that the actual mass of the planet is unknown.…”

Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST

“…One method of studying the atmospheric constituents of an exoplanet is by employing an atmospheric retrieval (Madhusudhan & Seager 2009;Line et al 2013;Waldmann et al 2015a,b;Blecic et al 2017;Cubillos et al 2017;Mollière et al 2019;Benneke et al 2019). Atmospheric retrieval is the act of obtaining the physical and chemical characteristics of an atmosphere based on observations.…”

Atmospheric compositions and observability of nitrogen-dominated ultra-short-period super-Earths

“…Our synthetic emission and transmission spectra are computed using the open source radiative transfer code peti-tRADTRANS 9 (Mollière et al 2019). We use the low resolution mode of λ/∆λ = 1000, which follows the correlated-k approximation.…”

“…We take as line opacities: H 2 , H 2 O, HCN, C 2 H 2 , C 2 H 4 , CO, CO 2 , CH 4 , CN, NH 3 , OH, CH, NH and NO. Respective sources of the opacities can be found in the Table 2 of Mollière et al (2019), and the code documentation page 10 , where petitRADTRANS is described at length. The reference list for added opacities, which were not originally part of petitRADTRANS, of C 2 H 4 , CH, NO, NH and CN can be found in Table 1.…”