Using Smoothed Particle Radiation Magnetohydrodynamics to Explore How Protostars are Formed

Abstract

The properties which define a molecular cloud core — the evolutionary phase bridging between a molecular cloud and a protostar — are extensive. These properties include the initial density profile; velocity field; and magnetic field strength and geometry (and the alignment of this with other fields). These properties have a major effect on the nature of the protostar or protostars ultimately produced when the core collapses.

We present a series of calculations using smoothed particle radiation magnetohydrodynamics of the collapse of a molecular cloud core to the first hydrostatic core phase. Before this, we describe and analyse our numerical method, including exploring historical difficulties and the limits of the stability.

We explore the role of the geometry of the magnetic field, and showing that the nature of any outflows produced from a first hydrostatic core is closely related to the inclination angle of the field. We continue this analysis into the role of the field strength and geometry. We find that highly misaligned fields do not form bipolar outflows and discuss the cause of this, and additionally find that the angular momentum transport in weak field calculations is insufficient to prevent fragmentation and the formation of binary systems. When an outflow is formed, even in the most idealised initial conditions, the velocity is never |v_z| > 10 km · s^{−1}.

We consider next the role of turbulent and rotational kinetic energy, and find that transonic turbulence can prevent the formation of an outflow unless a critical ratio of rotation to turbulent energy is exceeded. Even so, we observe that outflows produced in non–laminar calculations are slower (|v_z| ∼ 1 km · s^{−1}) than those from laminar configurations. We then show that a Bonnor–Ebert density profile can produce a stable binary system with a helical outflow without the prolific fragmentation seen in fast rotating uniform density distributions.