Isolated Massive Star Formation in G28.20-0.05

We report high-resolution 1.3~mm continuum and molecular line observations of the massive protostar G28.20-0.05 with ALMA. The continuum image reveals a ring-like structure with 2,000~au radius, similar to morphology seen in archival 1.3~cm VLA observations. Based on its spectral index and associated H$30\alpha$ emission, this structure mainly traces ionised gas. However, there is evidence for $\sim30$~M$_{\odot}$ of dusty gas near the main mm continuum peak on one side of the ring, as well as in adjacent regions within 3,000~au. A virial analysis on scales of $\sim$2,000~au from hot core line emission yields a dynamical mass of $\sim80\:M_\odot$. A strong velocity gradient in the H$30\alpha$ emission is evidence for a rotating, ionized disk wind, which drives a larger-scale molecular outflow. An infrared SED analysis indicates a current protostellar mass of $m_*\sim40\:M_\odot$ forming from a core with initial mass $M_c\sim300\:M_\odot$ in a clump with mass surface density of $\Sigma_{\rm cl}\sim 0.8\:{\rm g\:cm}^{-2}$. Thus the SED and other properties of the system can be understood in the context of core accretion models. Structure-finding analysis on the larger-scale continuum image indicates G28.20-0.05 is forming in a relatively isolated environment, with no other concentrated sources, i.e., protostellar cores, above $\sim 1\:M_\odot$ found from $\sim$0.1 to 0.4~pc around the source. This implies that a massive star can form in relative isolation and the dearth of other protostellar companions within the $\sim1$~pc environs is a strong constraint on massive star formation theories that predict the presence of a surrounding protocluster.