Effect of Nonuniform Parallel Magnetic Field on the Kelvin–Helmholtz Instability in the Orion Nebula

Abstract The surface rippled structures of Orion molecular cloud have been attributed to the Kelvin–Helmholtz (KH) instability, and the influence of magnetic field and pressure on the formation and evolution of these structures is one of the important issues that has been considered frequently in the dynamics of clouds. However, the effects of magnetic field inhomogeneity and compressibility variations are still underexplored. Here, the two-dimensional radiation magnetohydrodynamic simulations are performed to study the evolution of the KH instability in the Orion Nebula in the presence of a nonuniform magnetic field parallel to the flow direction. The results indicate that the rippled structures in the near Trapezium Cluster region are more pronounced. Also, the sound Mach number increases with decreasing gas pressure, enhancing the system compressibility and weakening the KH instability. And the position of the slow rarefaction layer, where the gas pressure is relatively weak, corresponds to that of the strong compression layer of the magnetic flux tube. Finally, it is found that the strong magnetic field gradient with small magnetic shear thickness between the H II region and the rippled region has a stabilizing effect on instability, which is a result of the increased magnetic field strength within the shear layer. Based on these results, it is preliminarily discussed that the magnetic field in the Orion Nebula is the key feedback process to regulate or stimulate the formation of stars, and the compressibility also affects the efficiency of the star formation.