Isolated Massive Star Formation in G28.20-0.05

Abstract

We report high-resolution 1.3 mm continuum and molecular line observations of the massive protostar G28.20-0.05 with Atacama Large Millimeter/submillimeter Array. The continuum image reveals a ring-like structure with 2000 au radius, similar to morphology seen in archival 1.3 cm Very Large Array observations. Based on its spectral index and associated H30 α emission, this structure mainly traces ionized gas. However, there is evidence for ∼30 M ⊙ of dusty gas near the main millimeter continuum peak on one side of the ring, as well as in adjacent regions within 3000 au. A virial analysis on scales of ∼2000 au from hot core line emission yields a dynamical mass of ∼80 M ⊙ . A strong velocity gradient in the H30 α emission is evidence for a rotating, ionized disk wind, which drives a larger-scale molecular outflow. An infrared spectral energy distribution (SED) analysis indicates a current protostellar mass of m * ∼ 40 M ⊙ forming from a core with initial mass M c ∼ 300 M ⊙ in a clump with mass surface density of Σ cl ∼ 0.8 g cm −2 . Thus the SED and other properties of the system can be understood in the context of core accretion models. A 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 ∼1 M ⊙ found from ∼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 ∼1 pc environs is a strong constraint on massive star formation theories that predict the presence of a surrounding protocluster.