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 starformation processes are studied using three-dimensional resistive MHDsimulations. Starting with a Bonnor-Ebert isothermal cloud rotating in auniform magnetic field, we calculate cloud evolution from the molecularcloud core (nc=104 cm-3) to the stellarcore (nc=1022 cm-3), wherenc 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 thecentral density exceeds nc>~1012cm-3; and high-velocity flows (~30 km s-1) withgood collimation, driven from the protostar when the central densityexceeds nc>~1021 cm-3. High-velocityflows are enclosed by low-velocity flows after protostar formation. Thedifference in the degree of collimation between the two flows is causedby the strength of the magnetic field and configuration of the magneticfield lines. The magnetic field around an adiabatic core is strong andhas an hourglass configuration; therefore, flows from the adiabatic coreare driven mainly by the magnetocentrifugal mechanism and guided by thehourglass-like field lines. In contrast, the magnetic field around theprotostar is weak and has a straight configuration owing to ohmicdissipation in the high-density gas region. Therefore, flows from theprotostar are driven mainly by the magnetic pressure gradient force andguided by straight field lines. Differing depth of the gravitationalpotential between the adiabatic core and the protostar causes thedifference of flow speed. Low-velocity flows may correspond to theobserved molecular outflows, while high-velocity flows may correspond tothe observed optical jets. We suggest that the protostellar outflow andthe jet are driven by different cores, rather than the outflow beingentrained by the jet.