Observing the onset of outflow collimation in a massive protostar

TL;DR

This study observes the evolution of a massive protostar's outflow from isotropic to collimated over 18 years, supporting models of star formation involving episodic, magnetically driven winds shaping outflow morphology.

Contribution

It provides the first direct observational evidence of the transition from isotropic to collimated outflows in a massive protostar, linking maser shell morphology to wind evolution.

Findings

Protostar VLA 2's outflow evolved from circular to elliptical shape.

The outflow's morphology change aligns with a model of episodic, isotropic wind within a toroidal density structure.

Supports the paradigm of magnetically driven, collimated outflows in star formation.

Abstract

The current paradigm of star formation through accretion disks, and magnetohydrodynamically driven gas ejections, predicts the development of collimated outflows, rather than expansion without any preferential direction. We present radio continuum observations of the massive protostar W75N(B)-VLA 2, showing that it is a thermal, collimated ionized wind and that it has evolved in 18 years from a compact source into an elongated one. This is consistent with the evolution of the associated expanding water-vapor maser shell, which changed from a nearly circular morphology, tracing an almost isotropic outflow, to an elliptical one outlining collimated motions. We model this behavior in terms of an episodic, short-lived, originally isotropic, ionized wind whose morphology evolves as it moves within a toroidal density stratification.