De-projection of radio observations of axi-symmetric expanding circumstellar envelopes

TL;DR

This paper develops methods for de-projecting radio observations of axi-symmetric expanding circumstellar envelopes, enabling better analysis of their physical properties through effective emissivity and wind velocity modeling.

Contribution

It introduces tools to measure and constrain key parameters of the wind velocity distribution from radio observations, simplifying the analysis process.

Findings

Effective emissivity can be calculated given known wind velocity distribution.

Tools developed help constrain the orientation and shape parameters of the envelope.

Case studies demonstrate practical application with ALMA and NOEMA data.

Abstract

The problem of de-projection of radio line observations of axi-symmetric expanding circumstellar envelopes is studied with the aim of easing their analysis in terms of physics models. The arguments developed rest on the remark that, in principle, when the wind velocity distribution is known, the effective emissivity can be calculated at any point in space. The paper provides a detailed study of how much this is true in practice. The wind velocity distribution assumed to be axi-symmetric and in expansion, is described by four parameters: the angles defining the orientation of the symmetry axis, an overall velocity scale and a parameter measuring the elongation (prolateness) of the distribution. Tools are developed that allow for measuring, or at least constraining, each of the four parameters. The use of effective emissivity as relevant quantity, rather than temperature and density being considered separately, implies important assumptions and simplifications meaning that the approach being considered here is only a preliminary to, and by no means a replacement for, a physics analysis accounting for radiative transfer and hydrodynamics arguments. While most considerations are developed using simulated observations as examples, two case studies (EP Aqr, observed with ALMA, and RS Cnc, with NOEMA) are presented that illustrate their usefulness in practical cases.