Non-local radiative transfer in strongly inverted masers

Laboratory of Molecular AstrophysicsDepartment of Astrophysics, Centro de Astrobiología (CSIC-INTA), Crta. Torrejón a Ajalvir km4. 28840 Torrejón de Ardoz, Madrid, Spain
e-mail: danielf@cab-inta.csic.es

Received: 12 May 2012
Accepted: 21 December 2012

Abstract

Context. Maser transitions are commonly observed in media exhibiting a large range of densities and temperatures. They can be used to obtain information on the dynamics and physical conditions of the observed regions. To obtain reliable constraints on the physical conditions prevailing in the masing regions, it is necessary to model the excitation mechanisms of the energy levels of the observed molecules.

Aims. We present a numerical method that enables us to obtain self-consistent solutions for both the statistical equilibrium and radiative transfer equations.

Methods. Using the standard maser theory, the method of short characteristics (SC) is extended to obtain the solution of the integro-differential radiative transfer equation, appropriate to the case of intense masing lines.

Results. We have applied our method to the maser lines of the H₂O molecule and we compare these results with the results obtained using a less accurate approach. In the regime of large maser opacities we find large differences in the intensity of the maser lines that could be as high as several orders of magnitude. The comparison between the two methods shows, however, that the effect on the thermal lines is modest. Finally, the effect introduced by rate coefficients on the prediction of H₂O masing lines and opacities is discussed, making use of various sets of rate coefficients involving He, o–H₂, and p–H₂. We find that the masing nature of a line is not affected by the selected collisional rates; however, from one set to the other the modelled line opacities and intensities can vary by up to a factor ~2 and ~10 respectively.