Fast spectral line calculations with the escape probability method and tests with synthetic observations of interstellar clouds

Abstract

Radiative transfer effects need to be taken into account when analysing spectral line observations. When the data are not sufficient for detailed modelling, simpler methods are needed. The escape probability formalism (EPF) is one such tool. We wish to quantify the model errors in the EPF analysis of interstellar clouds and cores. We introduce PEP, a parallel program for calculating fast EPF parameters quickly. We model full radiative transfer to generate synthetic observations for various cloud models. These are examined with the PEP program, and their results are compared to the actual beam-averaged kinetic temperatures, column densities, and volume densities. PEP enables the calculations of even millions of parameter combinations in a matter of seconds. However, the simple assumptions of EPF can lead to significant errors. In the tests the errors were typically within a factor of two, but could in some cases reach an order of magnitude. The model errors are thus similar or even larger than the statistical errors caused by the typical observational noise. Due to degeneracies, parameter combinations are better constrained than the individual parameters. The model errors could be reduced by using full radiative transfer modelling. However, in the absence of full knowledge of the source structure, the errors are difficult to quantify. We also present a method for approximate handling of hyperfine structure lines in EPF calculations. Both the observational statistical errors and the model errors need to be considered when estimating the reliability of EPF results. Full radiative transfer modelling is needed to better understand the true uncertainties.