A rotating fast bipolar wind and disk system around the B[e]-type star MWC 922

Area of Science:

• Astronomy and Astrophysics
• Stellar Evolution
• Interstellar Medium

Background:

• The Red Square Nebula (MWC 922) is a B[e]-type star system with an obscured central region.
• Previous observations suggested the presence of an ionized region, but its structure and kinematics were unresolved.
• The distance to MWC 922 is uncertain, typically estimated between 1.7-3 kpc.

Purpose of the Study:

• To investigate the structure and kinematics of the compact ionized region at the center of the Red Square Nebula.
• To resolve the spatio-kinematic properties of the central disk and any associated outflows.
• To understand the interplay between disk rotation and the launching of bipolar winds.

Main Methods:

• Interferometric observations using the Atacama Large Millimeter Array (ALMA) at 1 and 3 mm wavelengths.
• Imaging of free-free continuum and various hydrogen recombination line emissions (H30α, H39α, H51ε, H55γ, H63δ).
• Modeling of observational data using the radiative transfer code MORELI.

Main Results:

• Unprecedented angular resolution revealed a compact, nearly edge-on rotating disk (<0.14 arcsec diameter) around a central mass of ~10-18 M☉.
• Detected a fast bipolar ejection (~100 km s⁻¹) orthogonal to the disk, with a launching radius <30-51 au.
• Observed a slow wind (<15 km s⁻¹) potentially lifting off the disk; both winds show rotation similar to the disk.

Conclusions:

• The study provides the first empirical proof of rotation in a high-velocity bipolar wind, strongly suggesting a disk-mediated launching mechanism.
• The fast wind's origin at very small radii (<30-51 au) points to active launching, possibly by a disk around a compact companion.
• MWC 922 is likely a post-main sequence star in a mass-exchanging binary system, serving as a key laboratory for studying disk-wind interactions.