Transverse Wind Velocity Recorded in Spiral-Shell Pattern

TL;DR

This paper challenges the common assumption that observed spiral-shell patterns directly indicate wind expansion speed, showing that transverse wind velocities vary significantly with direction and depend on the circumstellar morphology.

Contribution

It revisits hydrodynamic models to analyze transverse wind velocities in spiral-shell patterns, highlighting the importance of velocity anisotropy in interpreting observations.

Findings

Transverse wind velocity varies up to half the average wind speed.

Directional dependence of wind velocity reflects circumstellar morphology.

Kinematic information is crucial for modeling and interpreting observations.

Abstract

The propagation speed of a circumstellar pattern revealed in the plane of the sky is often assumed to represent the expansion speed of the wind matter ejected from a post-main-sequence star at the center. We point out that the often-adopted isotropic wind assumption and the binary hypothesis as the underlying origin for the circumstellar pattern in the shape of multilayered shells are, however, mutually incompatible. We revisit the hydrodynamic models for spiral-shell patterns induced by the orbital motion of a hypothesized binary, of which one star is losing mass at a high rate. The distributions of transverse wind velocities as a function of position angle in the plane of the sky are explored along viewing directions. The variation of the transverse wind velocity is as large as half the average wind velocity over the entire three dimensional domain in the simulated models investigated in this work. The directional dependence of the wind velocity is indicative of the overall morphology of the circumstellar material, implying that kinematic information is an important ingredient in modeling the snapshot monitoring (often in the optical and near-infrared) or the spectral imaging observations for molecular line emissions.