Protostellar Outflows: From Simulations to Synthetic Observations

Author: Taishi Ushirogi

Jets and outflows are a universal outcome of star formation. Young protostars, from low- to high-mass regimes, eject collimated jets and molecular outflows that drive shocks such as Herbig–Haro objects. By removing mass and angular momentum, these outflows are thought to regulate how efficiently stars form inside molecular clouds.

Star formation often proceeds in clustered environments rather than in isolation. In such regions, multiple protostars launch outflows into the same surrounding gas. Observations of regions such as IC 1396N (e.g. Beltrán et al. 2009) and BHR 71 (e.g. Zapata et al. 2018) suggest that neighbouring outflows can interact or collide, leading to enhanced shock-excited emission, increased velocity dispersion, or changes in outflow orientation. However, projection effects and limited spatial resolution make it difficult to unambiguously identify these interactions observationally.

In this work, we use numerical simulations with the FLASH code to study these processes. High-resolution, three-dimensional simulations follow the collapse of turbulent molecular cloud cores, the formation of multiple protostars, and the launch of protostellar jets in a clustered environment. By combining these simulations with synthetic molecular line observations, we aim to identify observable signatures of outflow–outflow interactions and to understand how geometry and line-of-sight effects shape what we observe.

Our simulations show that protostellar jets carve pronounced bipolar cavities into the surrounding molecular cloud. As shown in Figure 1, hot atomic and ionised gas trace the jet interior and strong shock regions in the immediate vicinity of the protostar, while cooler molecular gas outlines cavity walls and bow shocks. This clear spatial segregation reflects the different physical conditions within the outflow and highlights the importance of feedback in shaping the local cloud structure.

When the same outflow is analysed through synthetic CO (J=1-0) observations, its appearance depends strongly on the viewing geometry (Figure 2). Integrated intensity, velocity field, and velocity dispersion maps reveal asymmetric structures that can change dramatically with line of sight. In particular, one outflow lobe may appear significantly fainter or even absent in certain projections, despite being physically present in the simulation. Regions of enhanced velocity dispersion are closely linked to shocked gas and internal working surfaces within the outflow.

These results demonstrate that some commonly observed features—such as monopolar outflows or asymmetric velocity structures—do not necessarily require intrinsic asymmetries in the launching mechanism. Instead, they can arise naturally from outflow dynamics and projection effects in clustered environments.

In the next step, we will extend this analysis to additional protostars within the same clustered simulation to investigate how frequently outflow–outflow interactions occur and under which conditions they leave observable signatures. We also plan to include higher CO transitions and isotopologues (e.g. 13CO and C18O) to better constrain excitation conditions and optical depth effects, and to compare the synthetic observations more directly with current ALMA data.