Origin of the Shell Structure in the Primary Outflow from IRAS 15398−3359

Abstract IRAS 15398−3359, a Class 0 protostar in the Lupus I star-forming region, is associated with three generations of outflows. The primary outflow, i.e., the most recent one, shows an internal structure named the “shell structure” in the near-infrared emission map. The shell structure is also seen in the emission lines of CO, H 2 CO, and other species. We find a similar structure in an underexpanded jet produced in aerodynamics and other engineering applications. A high-pressure gas ejected through a nozzle expands to form a supersonic flow. When the pressure of the ejected gas becomes lower than that of the ambient gas, the jet is compressed to form a shock wave. The shock-heated gas expands again to form substructures along the jet. We examine the similarity between the primary outflow of IRAS 15398−3359 and the industrial underexpanded jet and the possibility that the shell structure of the former is due to repeated expansion and compression in the direction perpendicular to the jet propagation.