Low-temperature gas opacity. ÆSOPUS: a versatile and quick computational tool

We introduce a new tool – AESOPUS: Accurate Equation of State and OPacity Utility Software – for computing the equation of state and the Rosseland mean (RM) opacities of matter in the ideal gas phase. Results are given as a function of one pair of state variables, (i.e. temperature T in the range 3.2 ≤ log(T ) ≤ 4.5, and parameter R = ρ/(T/10 6 K) 3 in the range −8 ≤ log(R) ≤ 1), and arbitrary chemical mixture. The chemistry is presently solved for about 800 species, consisting of almost 300 atomic and 500 molecular species. The gas opacities account for many continuum and discrete sources, including atomic opacities, molecular absorption bands, and collision-induced absorption. Several tests made on AESOPUS have proved that the new opacity tool is accurate in the results, flexible in the management of the input prescriptions, and agile in terms of computational time requirement. Purpose of this work is to greatly expand the public availability of Rosseland mean opacity data in the low-temperature regime. We set up a web-interface (http://stev.oapd.inaf.it/aesopus) which enables the user to compute and shortly retrieve RM opacity tables according to his/her specific needs, allowing a full degree of freedom in specifying the chemical composition of the gas. As discussed in the paper, useful applications may regard, for instance, RM opacities of gas mixtures with i) scaled-solar abundances of metals, choosing among various solar mixture compilations available in the literature; ii) varying CNO abundances, suitable for evolutionary models of red and asymptotic giant branch stars and massive stars in the Wolf-Rayet stages; iii) various degrees of enhancement in α-elements, and C-N, O-Na, and Mg-Al abundance anti-correlations, necessary to properly describe the properties of stars in early-type galaxies and Galactic globular clusters; iv) zero-metal abundances appropriate for studies of gas opacity in primordial conditions.