LIME - a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths

TL;DR

The paper introduces LIME, a versatile 3D radiative transfer code for molecular and atomic excitation modeling in millimeter and far-infrared wavelengths, optimized for complex geometries and high-resolution data like ALMA.

Contribution

LIME is a new flexible code that efficiently solves line excitation and radiation transfer in arbitrary 3D geometries using unstructured grids, accommodating various physical models and overlapping lines.

Findings

Efficient convergence in opaque models.

Ability to generate observable images from complex models.

Suitable for high-resolution ALMA data analysis.

Abstract

We present a new code for solving the molecular and atomic excitation and radiation transfer problem in a molecular gas and predicting emergent spectra. This code works in arbitrary three dimensional geometry using unstructured Delaunay latices for the transport of photons. Various physical models can be used as input, ranging from analytical descriptions over tabulated models to SPH simulations. To generate the Delaunay grid we sample the input model randomly, but weigh the sample probability with the molecular density and other parameters, and thereby we obtain an average grid point separation that scales with the local opacity. Our code does photon very efficiently so that the slow convergence of opaque models becomes traceable. When convergence between the level populations, the radiation field, and the point separation has been obtained, the grid is ray-traced to produced images that can readily be compared to observations. Because of the high dynamic range in scales that can be resolved using this type of grid, our code is particularly well suited for modeling of ALMA data. Our code can furthermore deal with overlapping lines of multiple molecular and atomic species.