The driving mechanism of jets and outflows in star formation process is studied using three-dimensional resistive MHD nested grid simulations .
Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field , we calculated cloud evolution from the molecular cloud core ( n _ { c } = 10 ^ { 4 } { cm } ^ { -3 } , r = 4.6 \times 10 ^ { 4 } AU ) to the stellar core ( n _ { c } = 10 ^ { 22 } { cm } ^ { -3 } , r \sim 1 R _ { \odot } ) , where n _ { c } and r denote the central density and radius of each object , respectively .
In the collapsing cloud core , we found two distinct flows : Low-velocity flows ( \sim 5 { km s ^ { -1 } } ) with a wide opening angle , driven from the adiabatic core when the central density exceeds n _ { c } \gtrsim 10 ^ { 12 } { cm } ^ { -3 } , and high-velocity flows ( \sim 30 { km s ^ { -1 } } ) with good collimation , driven from the protostar when the central density exceeds n _ { c } \gtrsim 10 ^ { 21 } { 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 cause the difference of the flow speed .
Low-velocity flows correspond to the observed molecular outflows , while high-velocity flows correspond to the observed optical jets .
We suggest that the outflow and the jet are driven by different cores , rather than that the outflow being entrained by the jet .