Theoretical hot methane line lists up to t = 2000 k for astrophysical applications

Abstract

The paper describes the construction of complete sets of hot methane lines based on accurate ab initio potential and dipole moment surfaces and extensive first-principle calculations. Four line lists spanning the [0-5000] cm -1 infrared region were built at T = 500, 1000, 1500, and 2000 K. For each of these four temperatures, we have constructed two versions of line lists: a version for high-resolution applications containing strong and medium lines and a full version appropriate for low-resolution opacity calculations. A comparison with available empirical databases is discussed in detail for both cold and hot bands giving a very good agreement for line positions, typically <0.1-0.5 cm-1 and 5% for intensities of strong lines. Together with numerical tests using various basis sets, this confirms the computational convergence of our results for the most important lines, which is the major issue for theoretical spectra predictions. We showed that transitions with lower state energies up to 14,000 cm-1 could give significant contributions to the methane opacity and have to be systematically taken into account. Our list at 2000 K calculated up to J = 50 contains 11.5 billion transitions for I > 10-29 cm mol-1. These new lists are expected to be quantitatively accurate with respect to the precision of available and currently planned observations of astrophysical objects with improved spectral resolution. © 2014. The American Astronomical Society. All rights reserved.