HELIOS-K 2.0 Opacity Calculator and Open-source Opacity Database for Exoplanetary Atmospheres

Computing and using opacities is a key part of modeling and interpreting data of exoplanetary atmospheres. Since the underlying spectroscopic line lists are constantly expanding and currently include up to ∼10¹⁰-10¹¹ transition lines, the opacity calculator codes need to become more powerful. Here we present major upgrades to the HELIOS-K GPU-accelerated opacity calculator and describe the necessary steps to process large line lists within a reasonable amount of time. Besides performance improvements, we include more capabilities and present a toolbox for handling different atomic and molecular data sets, from downloading and preprocessing the data to performing the opacity calculations in a user-friendly way. HELIOS-K supports line lists from ExoMol, HITRAN, HITEMP, NIST, Kurucz, and VALD3. By matching the resolution of 0.1 cm^-1 and cutting length of 25 cm^-1 used by the ExoCross code for timing performance (251 s excluding data read-in time), HELIOS-K can process the ExoMol BT2 water line list in 12.5 s. Using a resolution of 0.01 cm^-1, it takes 45 s, equivalent to about 10⁷ lines s^-1. As a wavenumber resolution of 0.01 cm^-1 suffices for most exoplanetary atmosphere spectroscopic calculations, we adopt this resolution in calculating opacity functions for several hundred atomic and molecular species and make them freely available on the open-access DACE database. For the opacity calculations of the database, we use a cutting length of 100 cm^-1 for molecules and no cutting length for atoms. Our opacities are available for downloading from https://dace.unige.ch/opacityDatabase and may be visualized using https://dace.unige.ch/opacity.