A Collimated Jet and an Infalling-Rotating Disk in G192.16−3.84 Traced by H2O Maser Emission

Abstract

We report on $\mathrm{H}_2 \mathrm{O}$ masers associated with the massive-star forming region G192.16$-$3.84 observed with the new Japanese VLBI network at three epochs spanned for two months, which have revealed the three-dimensional kinematical structure of the whole $\mathrm{H}_2 \mathrm{O}$ maser region in G192.16$-$3.84, containing two young stellar objects separated by ${\sim 1200 \,\mathrm{AU}}$. The maser spatio-kinematical structure has well persisted since previous observations, in which the masers are expected to be associated with a highly collimated bipolar jet and an infalling-rotating disk in the northern and southern clusters of $\mathrm{H}_2 \mathrm{O}$ maser features, respectively. We estimated a jet expansion speed of ${\sim 100 \,\mathrm{km} \,\mathrm{s}^{-1}}$ and re-estimated the dynamical age of the whole jet to be $5.6 \times 10^{4} \,\mathrm{yr}$. We investigated the spatial distribution of the Doppler velocities during the previous and present observations and the relative proper motions of $\mathrm{H}_2 \mathrm{O}$ maser features in the southern cluster, as well as the relative bulk motion between the two maser clusters. They are well explained by a model of an infalling-rotating disk with a radius of $\sim 1000 \,\mathrm{AU}$ and a central stellar mass of 5–10${M}_{\odot}$, rather than by a model of a bipolar jet perpendicular to the observed CO outflow. Based on the derived $\mathrm{H}_2 \mathrm{O}$ maser spatio-kinematical parameters, we discuss the formation mechanism of the massive young stellar objects and the outflow development in G192.16$-$3.84.