Clustered Formation of Massive Stars within an Ionized Rotating Disk

Abstract

We present Atacama Large Millimeter/submillimeter Array observations with a 800 au resolution and radiative-transfer modeling of the inner part (r ≈ 6000 au) of the ionized accretion flow around a compact star cluster in formation at the center of the luminous ultracompact H ii region G10.6-0.4. We modeled the flow with an ionized Keplerian disk with and without radial motions in its outer part, or with an external Ulrich envelope. The Markov Chain Monte Carlo fits to the data give total stellar masses M ⋆ from 120 to 200 M ⊙, with much smaller ionized-gas masses M ion-gas = 0.2–0.25 M ⊙. The stellar mass is distributed within the gravitational radius R g ≈ 1000 to 1500 au, where the ionized gas is bound. The viewing inclination angle from the face-on orientation is i = 49°–56°. Radial motions at radii r > R g converge to v r,0 ≈ 8.7 km s−1, or about the speed of sound of ionized gas, indicating that this gas is marginally unbound at most. From additional constraints on the ionizing-photon rate and far-IR luminosity of the region, we conclude that the stellar cluster consists of a few massive stars with M star = 32–60 M ⊙, or one star in this range of masses accompanied by a population of lower-mass stars. Any active accretion of ionized gas onto the massive (proto)stars is residual. The inferred cluster density is very large, comparable to that reported at similar scales in the Galactic center. Stellar interactions are likely to occur within the next million years.