Brown dwarf formation and star formation efficiency are studied using a nested grid simulation that covers five orders of magnitude in spatial scale ( 10 ^ { 4 } – 0.1 AU ) .
Starting with a rotating magnetized compact cloud with a mass of 0.22 \thinspace M _ { \odot } , we follow the cloud evolution until the end of main accretion phase .
Outflow of \sim 5 { km s } ^ { -1 } emerges \sim 100 yr before the protostar formation and does not disappear until the end of the calculation .
The mass accretion rate declines from \sim 10 ^ { -6 } M _ { \odot } { yr } ^ { -1 } to \sim 10 ^ { -8 } – 10 ^ { -12 } M _ { \odot } { yr } ^ { -1 } in a short time ( \sim 10 ^ { 4 } yr ) after the protostar formation .
This is because ( 1 ) a large fraction of mass is ejected from the host cloud by the protostellar outflow and ( 2 ) the gas escapes from the host cloud by the thermal pressure .
At the end of the calculation , 74 \% ( 167 M _ { Jup } ) of the total mass ( 225 M _ { Jup } ) is outflowing from the protostar , in which 34 \% ( 77 M _ { Jup } ) of the total mass is ejected by the protostellar outflow with supersonic velocity and 40 \% ( 90 M _ { Jup } ) escapes with subsonic velocity .
On the other hand , 20 \% ( 45 M _ { Jup } ) is converted into the protostar and 6 \% ( 13 M _ { Jup } ) remains as the circumstellar disk .
Thus , the star formation efficiency is \epsilon = 0.2 .
The resultant protostellar mass is in the mass range of brown dwarfs .
Our results indicate that brown dwarfs can be formed in compact cores in the same manner as hydrogen-burning stars , and the magnetic field and protostellar outflow are essential in determining the star formation efficiency and stellar mass .