19,255 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
Nonlinear Criterion for the Stability of Molecular Clouds
Dynamically significant magnetic fields are routinely observed in molecular
clouds, with mass-to-flux ratio lambda = (2 pi sqrt{G}) (Sigma/B) ~ 1 (here
Sigma is the total column density and B is the field strength). It is widely
believed that ``subcritical'' clouds with lambda < 1 cannot collapse, based on
virial arguments by Mestel and Spitzer and a linear stability analysis by
Nakano and Nakamura. Here we confirm, using high resolution numerical models
that begin with a strongly supersonic velocity dispersion, that this criterion
is a fully nonlinear stability condition. All the high-resolution models with
lambda <= 0.95 form ``Spitzer sheets'' but collapse no further. All models with
lambda >= 1.02 collapse to the maximum numerically resolvable density. We also
investigate other factors determining the collapse time for supercritical
models. We show that there is a strong stochastic element in the collapse time:
models that differ only in details of their initial conditions can have
collapse times that vary by as much as a factor of 3. The collapse time cannot
be determined from just the velocity dispersion; it depends also on its
distribution. Finally, we discuss the astrophysical implications of our
results.Comment: 11 pages, 5 figure
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
Planet formation around stars of various masses: The snow line and the frequency of giant planets
We use a semi-analytic circumstellar disk model that considers movement of
the snow line through evolution of accretion and the central star to
investigate how gas giant frequency changes with stellar mass. The snow line
distance changes weakly with stellar mass; thus giant planets form over a wide
range of spectral types. The probability that a given star has at least one gas
giant increases linearly with stellar mass from 0.4 M_sun to 3 M_sun. Stars
more massive than 3 M_sun evolve quickly to the main-sequence, which pushes the
snow line to 10-15 AU before protoplanets form and limits the range of disk
masses that form giant planet cores. If the frequency of gas giants around
solar-mass stars is 6%, we predict occurrence rates of 1% for 0.4 M_sun stars
and 10% for 1.5 M_sun stars. This result is largely insensitive to our assumed
model parameters. Finally, the movement of the snow line as stars >2.5 M_sun
move to the main-sequence may allow the ocean planets suggested by Leger et.
al. to form without migration.Comment: Accepted to ApJ. 12 pages of emulateap
How is chiral symmetry restored at finite density?
Taking into account pseudoscalar as well as scalar condensates, we reexamine
the chiral restoration path on the chiral manifold. We shall see both
condensates coherently produce a density wave at a certain density, which
delays chiral restoration as density or temperature is increased.Comment: 4 pages, 2 figures; proc. of QM0
Small Structures via Thermal Instability of Partially Ionized Plasma. I. Condensation Mode
(Shortened) Thermal instability of partially ionized plasma is investigated
by linear perturbation analysis. According to the previous studies under the
one fluid approach, the thermal instability is suppressed due to the magnetic
pressure. However, the previous studies did not precisely consider the effect
of the ion-neutral friction, since they did not treat the flow as two fluid
which is composed of ions and neutrals. Then, we revisit the effect of the
ion-neutral friction of the two fluid to the growth of the thermal instability.
According to our study, (1) The instability which is characterized by the mean
molecular weight of neutrals is suppressed via the ion-neutral friction only
when the magnetic field and the friction are sufficiently strong. The
suppression owing to the friction occurs even along the field line. If the
magnetic field and the friction are not so strong, the instability is not
stabilized. (2) The effect of the friction and the magnetic field is mainly
reduction of the growth rate of the thermal instability of weakly ionized
plasma. (3) The effect of friction does not affect the critical wavelength
lambdaF for the thermal instability. This yields that lambdaF of the weakly
ionized plasma is not enlarged even when the magnetic field exists. We insist
that the thermal instability of the weakly ionized plasma in the magnetic field
can grow up even at the small length scale where the instability under the
assumption of the one fluid plasma can not grow owing to the stabilization by
the magnetic field. (4) The wavelength of the maximum growth rate of the
instability shifts shortward according to the decrement of the growth rate,
because the friction is effective at rather larger scale. Therefore, smaller
structures are expected to appear than those without the ion-neutral friction.Comment: To appear in Ap
- …