Asymmetry in the protostellar system HOPS 198: Evidence for the evolution of outflow opening angle driven by density of the surrounding core

Abstract

Protostellar outflows are thought to be responsible for the low star formation efficiency of protostellar cores. However, whether outflows can disperse a significant fraction of the gas in the core depends on the outflow opening angle. It is established that the outflow opening angle increases during the early stages of the protostellar evolution, but the underlying mechanism is poorly understood. Observations of HOPS 198, a Class 0 protostar in the Orion A molecular cloud, provide insights into this question. HOPS 198 exhibits a strong east-west asymmetry in its outflow and its core. The opening angle of the eastern lobe ($\sim80^{\circ}$) is more than twice wider than that of the western lobe ($\sim30^{\circ}$), while the surface density of the west side of the core is $1.5-2.8$ times higher than the east side. Using an analytical model in which the molecular outflow morphology is shaped by interactions between the wide-angle protostellar wind ($\gtrsim 80^{\circ}$) and surrounding material in the core, we find that the difference in opening angle for the two lobes can be explained by the difference in core density on the two sides. This result supports the hypothesis that the evolution of the outflow opening angle is driven by the evolution in the density of the protostellar core.