An expanding ring of the hypercompact HII region W49N:A2

TL;DR

This study uses high-resolution ALMA data to analyze the structure and dynamics of an ionized ring in the hypercompact HII region W49N:A2, revealing its expansion, possible rotation, and implications for massive star formation.

Contribution

It provides the first detailed observation of an expanding ionized ring in a hypercompact HII region, suggesting it is a remnant of the accretion disk during massive star formation.

Findings

The ionized ring has a radius of ~700 au and is expanding at 13.2 km/s.

The ring shows a hint of rotation at 2.7 km/s, less than Keplerian velocity.

The data suggests the ring is a remnant of the accretion disk that fed the star.

Abstract

We present 250~GHz continuum and H29alpha line data toward W49N:A2, a hypercompact HII region ionized by an O9 star. The data obtained with ALMA at a resolution of ~0"05 (600 au) confirmed the presence of an ionized ring with a radius of~700 au inclined by ~50degree (0degree for pole-on). It has a width of ~1000 au and is relatively flat with a scale height of less than several hundred au. The tilted ring, or the apparent ellipse, has a prominent velocity difference between its NW and SE ridges along the minor axis, suggesting that it is expanding in the equatorial plane at a velocity of 13.2 km/s. The ring also shows a hint of rotation at 2.7 km/s, which is significantly (2.5sigma) smaller than the Kepler velocity of 5.2 km/s at its radius around the 20 M$_sun star. This can be interpreted that the ring gas has been transported from the radius of ~170 au by conserving its original specific angular momentum that it had there. The ionized ring may thus be a remnant of the accretion disk that fed the O9 star, whose radiation or magnetic activities became so strong that the disk accretion was reversed due to the intense thermal or magneto-hydrodynamic pressure around the star. The data has revealed a rare example of how a massive star terminates its accretion at the end of its formation, transforming a hypercompact HII region into an ultracompact HII region.