FIRST DIRECT SIMULATION OF BROWN DWARF FORMATION IN A COMPACT CLOUD CORE

Brown dwarf formation and star formation efficiency are studied using a nested grid simulation that covers 5 orders of magnitude in spatial scale (10(4)-0.1 AU). Starting with a rotating magnetized compact cloud with a mass of 0.22 M(circle dot) (225 M(Jup)), we follow the cloud evolution until the end of the main accretion phase. An outflow of similar to 5 km s(-1) emerges similar to 100 yr before the protostar formation and does not disappear until the end of the calculation. The mass accretion rate declines from similar to 10(-6) M(circle dot) yr(-1) to similar to 10(-8)-10(-12) M(circle dot) yr(-1) in a short time (similar to 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.