16,697 research outputs found
Mechanism of Magnetic Flux Loss in Molecular Clouds
We investigate the detailed processes working in the drift of magnetic fields
in molecular clouds. To the frictional force, whereby the magnetic force is
transmitted to neutral molecules, ions contribute more than half only at cloud
densities , and charged grains contribute more
than 90% at . Thus grains play a decisive role
in the process of magnetic flux loss. Approximating the flux loss time by
a power law , where is the mean field strength in
the cloud, we find , characteristic to ambipolar diffusion,
only at . At higher densities,
decreases steeply with , and finally at , where magnetic fields
effectively decouple from the gas, is attained, reminiscent of
Ohmic dissipation, though flux loss occurs about 10 times faster than by Ohmic
dissipation. Ohmic dissipation is dominant only at . While ions and electrons drift in the direction of
magnetic force at all densities, grains of opposite charges drift in opposite
directions at high densities, where grains are major contributors to the
frictional force. Although magnetic flux loss occurs significantly faster than
by Ohmic dissipation even at very high densities as , the process going on at high densities is quite different from ambipolar
diffusion in which particles of opposite charges are supposed to drift as one
unit.Comment: 34 pages including 9 postscript figures, LaTex, accepted by
Astrophysical Journal (vol.573, No.1, July 1, 2002
Protostar Formation in Magnetic Molecular Clouds beyond Ion Detachment: I. Formulation of the Problem and Method of Solution
We formulate the problem of the formation of magnetically supercritical cores
in magnetically subcritical parent molecular clouds, and the subsequent
collapse of the cores to high densities, past the detachment of ions from
magnetic field lines and into the opaque regime. We employ the six-fluid MHD
equations, accounting for the effects of grains (negative, positive and
neutral) including their inelastic collisions with other species. We do not
assume that the magnetic flux is frozen in any of the charged species. We
derive a generalized Ohm's law that explicitly distinguishes between flux
advection (and the associated process of ambipolar diffusion) and Ohmic
dissipation, in order to assess the contribution of each mechanism to the
increase of the mass-to-flux ratio of the central parts of a collapsing core
and possibly to the resolution of the magnetic flux problem of star formation.
We show how our formulation is related to and can be transformed into the
traditional, directional formulation of the generalized Ohm's law, and we
derive formulae for the perpendicular, parallel and Hall conductivities
entering the latter, which include, for the first time, the effect of inelastic
collisions between grains. In addition, we present a general (valid in any
geometry) solution for the velocities of charged species as functions of the
velocity of the neutrals and of the effective flux velocity (which can in turn
be calculated from the dynamics of the system and Faraday's law). The last two
sets of formulae can be adapted for use in any general non-ideal MHD code to
study phenomena beyond star formation in magnetic clouds. The results,
including a detailed parameter study, are presented in two accompanying papers.Comment: 17 pages, emulateapj; accepted for publication in the Astrophysical
Journa
Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation
We study numerically the ambipolar diffusion-driven evolution of
non-rotating, magnetically subcritical, disk-like molecular clouds, assuming
axisymmetry. Previous similar studies have concentrated on the formation of
single magnetically supercritical cores at the cloud center, which collapse to
form isolated stars. We show that, for a cloud with many Jeans masses and a
relatively flat mass distribution near the center, a magnetically supercritical
ring is produced instead. The supercritical ring contains a mass well above the
Jeans limit. It is expected to break up, through both gravitational and
possibly magnetic interchange instabilities, into a number of supercritical
dense cores, whose dynamic collapse may give rise to a burst of star formation.
Non-axisymmetric calculations are needed to follow in detail the expected ring
fragmentation into multiple cores and the subsequent core evolution.
Implications of our results on multiple star formation in general and the
northwestern cluster of protostars in the Serpens molecular cloud core in
particular are discussed.Comment: 25 pages, 4 figures, to appear in Ap
Approximate black hole binary spacetime via asymptotic matching
We construct a fully analytic, general relativistic, nonspinning black hole
binary spacetime that approximately solves the vacuum Einstein equations
everywhere in space and time for black holes sufficiently well separated. The
metric is constructed by asymptotically matching perturbed Schwarzschild
metrics near each black hole to a two-body post-Newtonian metric far from them,
and a two-body post-Minkowskian metric farther still. Asymptotic matching is
done without linearizing about a particular time slice, and thus it is valid
dynamically and for all times, provided the binary is sufficiently well
separated. This approximate global metric can be used for long dynamical
evolutions of relativistic magnetohydrodynamical, circumbinary disks around
inspiraling supermassive black holes to study a variety of phenomena.Comment: 17 pages, 8 figures, 1 table. Appendix added to match published
versio
Formation and Collapse of Nonaxisymmetric Protostellar Cores in Planar Magnetic Interstellar Clouds: Formulation of the Problem and Linear Analysis
We formulate the problem of the formation and collapse of nonaxisymmetric
protostellar cores in weakly ionized, self-gravitating, magnetic molecular
clouds. In our formulation, molecular clouds are approximated as isothermal,
thin (but with finite thickness) sheets. We present the governing dynamical
equations for the multifluid system of neutral gas and ions, including
ambipolar diffusion, and also a self-consistent treatment of thermal pressure,
gravitational, and magnetic (pressure and tension) forces. The dimensionless
free parameters characterizing model clouds are discussed. The response of
cloud models to linear perturbations is also examined, with particular emphasis
on length and time scales for the growth of gravitational instability in
magnetically subcritical and supercritical clouds. We investigate their
dependence on a cloud's initial mass-to-magnetic-flux ratio (normalized to the
critical value for collapse), the dimensionless initial neutral-ion collision
time, and also the relative external pressure exerted on a model cloud. Among
our results, we find that nearly-critical model clouds have significantly
larger characteristic instability lengthscales than do more distinctly sub- or
supercritical models. Another result is that the effect of a greater external
pressure is to reduce the critical lengthscale for instability. Numerical
simulations showing the evolution of model clouds during the linear regime of
evolution are also presented, and compared to the results of the dispersion
analysis. They are found to be in agreement with the dispersion results, and
confirm the dependence of the characteristic length and time scales on
parameters such as the initial mass-to-flux ratio and relative external
pressure.Comment: 30 pages, 7 figures Accepted by Ap
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
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