13739

HELIOS-K: An Ultrafast, Open-source Opacity Calculator for Radiative Transfer

Simon L. Grimm, Kevin Heng
University of Zurich, Institute for Computational Science, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland
arXiv:1503.03806 [astro-ph.EP], (12 Mar 2015)

@article{grimm2015heliosk,

   title={HELIOS-K: An Ultrafast, Open-source Opacity Calculator for Radiative Transfer},

   author={Grimm, Simon L. and Heng, Kevin},

   year={2015},

   month={mar},

   archivePrefix={"arXiv"},

   primaryClass={astro-ph.EP}

}

We present an ultrafast opacity calculator for application to exoplanetary atmospheres, which we name HELIOS-K. It takes a line list as an input, computes the shape of each spectral line (e.g., a Voigt profile) 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 Gauss-Hermite quadrature to compute the Voigt profile, write the code in CUDA and optimise the computation for graphics processing units (GPUs). We use the k-distribution method to reduce $sim 10^5$ to $10^8$ lines to $sim 10$ to $10^4$ wavenumber bins, which may then be used for radiative transfer, atmospheric retrieval and general circulation models. We demonstrate that the resampling of the k-distribution function, within each bin, is an insignificant source of error across a broad range of wavenumbers and column masses. By contrast, the choice of line-wing cutoff for the Voigt profile is a significant source of error and affects the value of the computed flux by $sim 10%$. 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 line-by-line calculations or uses the k-distribution method and affects all calculations of exoplanetary atmospheres requiring the use of wavelength-dependent opacities. We provide a checklist for reviewing radiative transfer and retrieval studies that require computations of the opacity function. Using a NVIDIA K20 GPU, HELIOS-K is capable of computing an opacity function with $sim 10^5$ spectral lines in $sim 1$ second and is publicly available as part of the Exoclimes Simulation Platform (ESP; www.exoclime.org).
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