241 research outputs found
Is the Plasma Within Bubbles and Superbubbles Hot or Cold?
I review what is known about the temperature of the plasma within stellar
wind bubbles and superbubbles. Classical theory suggests that it should be hot,
with characteristic temperatures of order a million degrees. This temperature
should be set by the balance between heating by the internal termination shocks
of the central stellar winds and supernovae, which expand at thousands of km/s,
and cooling by conductive evaporation of cold gas off the shell walls. However,
if the hot interior gas becomes dense enough due to evaporation or ablation off
of interior clouds, it will cool in less than a dynamical time, leading to a
cold interior. The observational evidence appears mixed. On the one hand, X-ray
emission has been observed from both stellar wind bubbles and superbubbles. On
the other hand, no stellar wind bubble or superbubble has yet been observed
emitting at the rate predicted by the classical theory: they are either too
faint or too bright, by up to an order of magnitude. Alternate explanations
have been proposed for the observed emission, including off-center supernova
remnants hitting the shell walls of superbubbles, and residual emission from
highly-ionized gas out of coronal equilibrium. Furthermore, the structures of
post-main sequence stellar wind bubbles, expanding into what are presumably old
stellar wind bubbles, appear in at least some cases to show that the bubble
interior is cold, not hot. (The classical example of this is NGC 6888.) What is
the actual state of bubble and superbubble interiors?Comment: 7 pages, 1 figures, to be published in Astrophysical Plasmas: Codes,
Models and Observations, RMxAA Conf Ser, 2000. Requires rmaa.cl
Turbulent Velocity Structure in Molecular Clouds
We compare velocity structure in the Polaris Flare molecular cloud at scales
ranging from 0.015 pc to 20 pc to simulations of supersonic hydrodynamic and
MHD turbulence computed with the ZEUS MHD code. We use several different
statistical methods to compare models and observations. The Delta-variance
wavelet transform is most sensitive to characteristic scales and scaling laws,
but is limited by a lack of intensity weighting. The scanning-beam
size-linewidth relation is more robust with respect to noisy data. Obtaining
the global velocity scaling behaviour requires that large-scale trends in the
maps not be removed but treated as part of the turbulent cascade. We compare
the true velocity PDF in our models to velocity centroids and average line
profiles in optically thin lines, and find that the line profiles reflect the
true PDF better unless the map size is comparable to the total line-of-sight
thickness of the cloud. Comparison of line profiles to velocity centroid PDFs
can thus be used to measure the line-of-sight depth of a cloud. The observed
density and velocity structure is consistent with supersonic turbulence with a
driving scale at or above the size of the molecular cloud and dissipative
processes below 0.05 pc. Ambipolar diffusion could explain the dissipation. The
velocity PDFs exclude small-scale driving such as that from stellar outflows as
a dominant process in the observed region. In the models, large-scale driving
is the only process that produces deviations from a Gaussian PDF shape
consistent with observations. Strong magnetic fields impose a clear anisotropy
on the velocity field, reducing the velocity variance in directions
perpendicular to the field. (abridged)Comment: 21 pages, 24 figures, accepted by A&A, with some modifications,
including change of claimed direct detection of dissipation scale to an upper
limi
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