研究成果藝廊

開啟圖片

Effects of temperature and magnetization on molecular outflows: the structure of the base of outflows at 1000 yr is shown with different physical conditions. Panels (a) to (c) shows the Alfvénic Mach number from 6, 30, to 90 [decreasing magnetic field strength]; and (1) to (3) shows the wind temperature from 10 K, 400 K, to 2000 K [increasing wind temperature]. These corresponds to different plasma beta parameter (β_plasma ≣ 8πa²ρ/B²) from 0.001 to 46. The central axial jet is less collimated and the shell structure is widened when β_plasma increases. In these cases, the poloidal magnetic field in the toroid is not included.

The kinematics and morphologies of molecular outflows from low-mass young stars are explored by magnetohydrodynamic simulations in the context of the unified wind model (Shang et al. 2006) which has provided a natural explanation of high-velocity jet and low-velocity shell features. In this work, we investigate how these characteristics are affected by the underlying physics of temperature and magnetic field strength. Such knowledge may help us understand the rich morphologies and kinematics of molecular outflows. We study the problem of a warm wind running into a cold ambient toroid by using a tracer field that keeps track of the wind material. While an isothermal equation of state is adopted, the effective temperature is determined locally based on the wind mass fraction. In the unified wind model, the density of the wind is cylindrically stratified and highly concentrated toward the outflow axis. Our simulations show that for a sufficiently magnetized wind, the jet identity can be well maintained even at high temperatures. However, for a high-temperature wind with low magnetization, the thermal pressure of the wind gas can drive material away from the axis, making the jet less collimated as it propagates. We also study the role of the poloidal magnetic field of the toroid. It is shown that the wind-ambient interface becomes more resistant to corrugation when the poloidal field is present, and the poloidal field that bunches up within the toroid prevent the swept-up material from being compressed into a thin layer. This suggests that the ambient poloidal field may play a role in producing a smoother and thicker swept-up shell structure in the molecular outflow.