High‐ and Low‐Velocity Magnetized Outflows in the Star Formation Process in a Gravitationally Collapsing Cloud

Abstract

The driving mechanisms of low- and high-velocity outflows in star formation processes are studied using three-dimensional resistive MHD simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate cloud evolution from the molecular cloud core (nc = 104 cm −3) to the stellar core (nc = 1022 cm −3), where nc denotes the central density. In the collapsing cloud core, we found two distinct flows: low-velocity flows (~5 km s−1) with a wide opening angle, driven from the adiabatic core when the central density exceeds nc 1012 cm −3; and high-velocity flows (~30 km s−1) with good collimation, driven from the protostar when the central density exceeds nc 1021 cm −3. High-velocity flows are enclosed by low-velocity flows after protostar formation. The difference in the degree of collimation between the two flows is caused by the strength of the magnetic field and configuration of the magnetic field lines. The magnetic field around an adiabatic core is strong and has an hourglass configuration; therefore, flows from the adiabatic core are driven mainly by the magnetocentrifugal mechanism and guided by the hourglass-like field lines. In contrast, the magnetic field around the protostar is weak and has a straight configuration owing to ohmic dissipation in the high-density gas region. Therefore, flows from the protostar are driven mainly by the magnetic pressure gradient force and guided by straight field lines. Differing depth of the gravitational potential between the adiabatic core and the protostar causes the difference of flow speed. Low-velocity flows may correspond to the observed molecular outflows, while high-velocity flows may correspond to the observed optical jets. We suggest that the protostellar outflow and the jet are driven by different cores, rather than the outflow being entrained by the jet.