HELIOSK: An Ultrafast, Opensource Opacity Calculator for Radiative Transfer
Abstract
We present an ultrafast opacity calculator that we name HELIOSK. It takes a line list as an input, computes the shape of each spectral line, and provides an option for grouping an enormous number of lines into a manageable number of bins. We implement a combination of Algorithm 916 and GaussHermite quadrature to compute the Voigt profile, write the code in CUDA, and optimize the computation for graphics processing units (GPUs). We restate the theory of the kdistribution method and use it to reduce ∼ {10}^{5}10^{8} lines to ∼1010^{4} wavenumber bins, which may then be used for radiative transfer, atmospheric retrieval and general circulation models. The choice of linewing cutoff for the Voigt profile is a significant source of error and affects the value of the computed flux by ∼ 10%. This is an outstanding physical (rather than computational) problem, due to our incomplete knowledge of pressure broadening of spectral lines in the far line wings. We emphasize that this problem remains regardless of whether one performs linebyline calculations or uses the kdistribution method and affects all calculations of exoplanetary atmospheres requiring the use of wavelengthdependent opacities. We elucidate the correlatedk approximation and demonstrate that it applies equally to inhomogeneous atmospheres with a single atomic/molecular species or homogeneous atmospheres with multiple species. Using a NVIDIA K20 GPU, HELIOSK is capable of computing an opacity function with ∼ {10}^{5} spectral lines in ∼1 s and is publicly available as part of the Exoclimes Simulation Platform (www.exoclime.org).
 Publication:

The Astrophysical Journal
 Pub Date:
 August 2015
 DOI:
 10.1088/0004637X/808/2/182
 arXiv:
 arXiv:1503.03806
 Bibcode:
 2015ApJ...808..182G
 Keywords:

 methods: numerical;
 planets and satellites: atmospheres;
 radiative transfer;
 Astrophysics  Earth and Planetary Astrophysics;
 Physics  Atmospheric and Oceanic Physics
 EPrint:
 Accepted by ApJ. 8 pages, 5 figures