147 research outputs found
Turbulence in the Star-forming Interstellar Medium: Steps toward Constraining Theories with Observations
Increasingly sophisticated observational tools and techniques are now being
developed for probing the nature of interstellar turbulence. At the same time,
theoretical advances in understanding the nature of turbulence and its effects
on the structure of the ISM and on star formation are occurring at a rapid
pace, aided in part by numerical simulations. These increased capabilities on
both fronts open new opportunities for strengthening the links between
observation and theory,and for meaningful comparisons between the two.Comment: 9 pages, 2 figures, Summary of Interstellar Turbulence Sessions at
the Workshop on Magnetic Fields and Star Formation: Theory versus
Observation
Ambipolar Diffusion-Mediated Thermal Fronts in the Neutral ISM
In a thermally bistable medium, cold, dense gas is separated from warm,
rareified gas by thin phase transition layers, or fronts, in which heating,
radiative cooling, thermal conduction, and convection of material are balanced.
We calculate the steady-state structure of such fronts in the presence of
magnetic fields, including the processes of ion-neutral drift and ion-neutral
frictional heating. We find that ambipolar diffusion efficiently transports the
magnetic field across the fronts, leading to a flat magnetic field strength
profile. The thermal profiles of such fronts are not significantly different
from those of unmagnetized fronts. The near uniformity of the magnetic field
strength across a front is consistent with the flat field strength-gas density
relation that is observed in diffuse interstellar gas.Comment: 17 pages, 12 figures, 1 table, accepted for publication in Ap
Fast Reconnection of Weak Magnetic Fields
Fast magnetic reconnection refers to annihilation or topological rearrangement of magnetic fields on a timescale that is independent (or nearly independent) of the plasma resistivity. The resistivity of astrophysical plasmas is so low that reconnection is of little practical interest unless it is fast. Yet, the theory of fast magnetic reconnection is on uncertain ground, as models must avoid the tendency of magnetic fields to pile up at the reconnection layer, slowing down the flow. In this paper it is shown that these problems can be avoided to some extent if the flow is three dimensional. On the other hand, it is shown that in the limited but important case of incompressible stagnation point flows, every flow will amplify most magnetic fields. Although examples of fast magnetic reconnection abound, a weak, disordered magnetic field embedded in stagnation point flow will in general be amplified, and should eventually modify the flow. These results support recent arguments against the operation of turbulent resistivity in highly conducting fluids
Virial theorem analysis of the structure and stability of magnetized clouds
The tensor virial theorem is used to analyze the structure and stability of self-gravitating, magnetized spheroids surrounded by a low-density medium with pressure and magnetic field. Analytical expressions are developed for the effect of a weak field and calculate critical states when the effect of the field is arbitrarily strong, comparing the results with full magnetohydrostatic calculations. This analysis suggests that a magnetic field may prevent gravitational collapse but may also be destabilizing, depending on its degree of concentration within the cloud
The Weak Field Limit of the Magnetorotational Instability
We investigate the behavior of the magneto-rotational instability in the
limit of extremely weak magnetic field, i.e., as the ratio of ion cyclotron
frequency to orbital frequency (X) becomes small. Considered only in terms of
cold two-fluid theory, instability persists to arbitrarily small values of X,
and the maximum growth rate is of order the orbital frequency except for the
range m_e/m_i < |X| < 1, where it can be rather smaller. In this range, field
aligned with rotation (X > 0) produces slower growth than anti-aligned field (X
< 0). The maximum growth rate is generally achieved at smaller and smaller
wavelengths as |X| diminishes. When |X| < m_e/m_i, new unstable
"electromagnetic-rotational" modes appear that do not depend on the equilibrium
magnetic field. Because the most rapidly-growing modes have extremely short
wavelengths when |X| is small, they are often subject to viscous or resistive
damping, which can result in suppressing all but the longest wavelengths, for
which growth is much slower. We find that this sort of damping is likely to
curtail severely the frequently-invoked mechanism for cosmological magnetic
field growth in which a magnetic field seeded by the Biermann battery is then
amplified by the magneto-rotational instability. On the other hand, the small
|X| case may introduce interesting effects in weakly-ionized disks in which
dust grains carry most of the electric charge.Comment: 30 pages, including 4 figures; revised version resubmitted to Ap
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