Stellar physics. Observing the onset of outflow collimation in a massive protostar

Area of Science:

• * Astrophysics
• * Star Formation
• * Radio Astronomy

Background:

• * The standard model of star formation predicts collimated outflows driven by accretion disks and magnetohydrodynamics.
• * Previous observations suggested isotropic outflows from massive protostars, contrasting with theoretical predictions.

Purpose of the Study:

• * To investigate the morphology and evolution of the ionized wind from the massive protostar W75N(B)-VLA 2.
• * To reconcile observational evidence with theoretical models of star formation and protostellar outflows.

Main Methods:

• * Radio continuum observations using the Very Large Array (VLA).
• * Analysis of morphological changes in the protostar and its associated water-vapor maser shell over an 18-year period.
• * Modeling of the ionized wind's evolution within a toroidal density structure.

Main Results:

• * W75N(B)-VLA 2 displays a thermal, collimated ionized wind.
• * The source has evolved from a compact to an elongated structure in 18 years.
• * The water-vapor maser shell transitioned from a nearly circular to an elliptical shape, indicating a shift from isotropic to collimated motion.

Conclusions:

• * The observed evolution supports models of episodic, short-lived ionized winds.
• * The wind's morphology changes as it propagates through a toroidal density stratification.
• * Findings challenge purely isotropic outflow models for massive protostars.