2,422 research outputs found
The Formation of Population III Stars in Gas Accretion Stage: Effects of Magnetic Fields
The formation of Population III stars is investigated using resistive
magnetohydrodynamic simulations. Starting from a magnetized primordial
prestellar cloud, we calculate the cloud evolution several hundreds of years
after first protostar formation, resolving the protostellar radius. When the
natal minihalo field strength is weaker than B \lesssim 10^-13 (n/1 cm^-3)^-2/3
G (n is the hydrogen number density), magnetic effects can be ignored. In this
case, fragmentation occurs frequently and a stellar cluster forms, in which
stellar mergers and mass exchange between protostars contribute to the mass
growth of these protostars. During the early gas accretion phase, the most
massive protostar remains near the cloud centre, whereas some of the less
massive protostars are ejected. The magnetic field significantly affects
Population III star formation when B_amb \gtrsim 10^-12 (n/1 cm^-3)^-2/3 G. In
this case, because the angular momentum around the protostar is effectively
transferred by both magnetic braking and protostellar jets, the gas falls
directly onto the protostar without forming a disk, and only a single massive
star forms. In addition, a massive binary stellar system appears when B_amb
\sim 10^-12 (n/1cm^-3)^-2/3 G. Therefore, the magnetic field determines the end
result of the formation process (cluster, binary or single star) for Population
III stars. Moreover, no persistent circumstellar disk appears around the
protostar regardless of the magnetic field strength, which may influence the
further evolution of Population III stars.Comment: 59 pages, 21 figures, Accepted for publication in MNRAS. For high
resolution figures see
http://jupiter.geo.kyushu-u.ac.jp/machida/arxiv/PopIII
The Circumbinary Outflow: A Protostellar Outflow Driven by a Circumbinary Disk
The protostellar outflows have indispensable role in the formation of single
stars, because they carry off the excess angular momentum from the centre of
the shrinking gas cloud, and permits further collapse to form a star. On the
other hand, a significant fraction of stars is supposed to be born as binaries
with circumbinary disk that are frequently observed. Here, we investigate the
evolution of a magnetized rotating cloud using three-dimensional resistive MHD
nested-grid code, and show that the outflow is driven by the circumbinary disk
and has an important role even in the binary formation. After the adiabatic
core formation in the collapsing cloud core, the magnetic flux is significantly
removed from the centre of the cloud by the Ohmic dissipation. Since this
removal makes the magnetic braking ineffective, the adiabatic core continuously
acquires the angular momentum to induce fragmentation and subsequent binary
formation. The magnetic field accumulates in the circumbinary disk where the
removal and accretion of magnetic field are balanced, and finally drives
circumbinary outflow. This result explains the spectacular morphology of some
specific young stellar objects such as L1551 IRS5. We can infer that most of
the bipolar molecular outflows observed by low density tracers (i.e., CO) would
correspond to circumbinary or circum-multiple outflows found in this report,
since most of the young stellar objects are supposed to be binaries or
multiples.Comment: 11 pages, 3 figures, Submitted to ApJL. For high resolution figures
see http://www2-tap.scphys.kyoto-u.ac.jp/~machidam/astro-ph/Circumbinary.pd
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 five orders of magnitude in spatial scale
(10^4 - 0.1AU). Starting with a rotating magnetized compact cloud with a mass
of 0.22 M_sun, we follow the cloud evolution until the end of main accretion
phase. Outflow of about 5 km/s emerges about 100 yr before the protostar
formation and does not disappear until the end of the calculation. The mass
accretion rate declines from 10^-6 M_sun/yr to 10^-8 - 10^-12 M_sun/yr in a
short time (about 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.Comment: 13 pages, 3 figures. Accepted for publication in ApJL. For high
resolution figures, see
http://www2-tap.scphys.kyoto-u.ac.jp/~machidam/astro-ph/BD.pd
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