Space reconstruction of the morphology and kinematics of axisymmetric radio sources

TL;DR

This paper develops a method to reconstruct the 3D morphology and kinematics of axisymmetric radio sources from ALMA observations, using assumptions of rotational invariance and polar coordinates to improve analysis accuracy.

Contribution

It introduces a systematic approach leveraging simulated data and polar coordinates to better interpret ALMA observations of axisymmetric gas envelopes around stars.

Findings

Effective identification of star axis orientation

Distinction between expansion and rotation in gas flows

Application to real star observations demonstrates method usefulness

Abstract

The unprecedented quality of the observations available from the Atacama Large Millimetre/sub-millimetre Array (ALMA) calls for analysis methods making the best of them. Reconstructing in space the morphology and kinematics of radio sources is an underdetermined problem that requires imposing additional constraints for its solution. The hypothesis of rotational invariance about a well-defined star axis, which is a good approximation to the description of the gas envelopes of many evolved stars and protostars, is particularly efficient in this role. In the first part of the article, a systematic use of simulated observations allows for identifying the main problems and for constructing quantities aimed at solving them. In particular the evaluation of the orientation of the star axis in space and the differentiation between expansion along the star axis and rotation about it are given special attention. The use of polar rather than Cartesian sky coordinates is shown to better match the morphology and kinematics of actual stars. The radial dependence of the gas density and temperature and the possible presence of velocity gradients are briefly considered. In the second part, the results obtained in the first part are applied to a few stars taken as examples with the aim of evaluating their usefulness when applied to concrete cases. A third part takes stock of what precedes and formulates some guidelines for modelling the radio emission of axisymmetric radio sources, limited however to the mathematics and geometry of the problem, physics considerations being ignored.