2023PASJ...75..225M

We present 250 GHz continuum and H29α line data toward W 49 N:A2, a hypercompact H II region ionized by an O9 star. The data obtained with ALMA at a resolution of ∼0${_{.}^{\prime\prime}}$05 (600 au) confirmed the presence of an ionized ring with a radius of ∼700 au inclined by ∼50° (0° 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^–1. The ring also shows a hint of rotation at 2.7 km s^–1, which is significantly (2.5 σ) smaller than the Kepler velocity of 5.2 km s^–1 at its radius around the 20 M_⊙ star. This can be interpreted as 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, the radiation or magnetic activities of which 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 H II region into an ultracompact H II region.