173 research outputs found
Rarefaction Shocks, Shock Errors and Low Order of Accuracy in ZEUS
We show that there are simple one dimensional problems for which the MHD
code, ZEUS, generates significant errors, whereas upwind conservative schemes
perform very well on these problems.Comment: 11 pages, accepted in ApJ Letter
Generation of density inhomogeneities by magnetohydrodynamic waves in two dimensions
Using two dimensional simulations, we study the formation of structures with
a high-density contrast by magnetohydrodynamic waves in regions in which the
ratio of thermal to magnetic pressure is small. The initial state is a uniform
background perturbed by fast-mode wave. Our most significant result is that
dense structures persist for far longer in a two-dimensional simulation than in
the one-dimensional case. Once formed, these structures persist as long as the
fast-mode amplitude remains high.Comment: 6 pages, 7 figures, accepted by MNRA
Modeling Helical Structures in Relativistic Jets
Many jets exhibit twisted helical structures. Where superluminal motions are
detected, jet orientation and pattern/flow speed are considerably constrained.
In this case modeling efforts can place strong limits on conditions in the jet
and in the external environment. This can be done by modeling the spatial
development of helical structures which are sensitively dependent on these
conditions. Along an expanding jet this sensitivity manifests itself in
predictable changes in pattern speed and observed wavelength. In general,
twists of low frequency relative to the local resonant frequency are advected
along the expanding jet into a region in which the twist frequency is high
relative to the local resonant frequency. The wave speed can be very different
in these two frequency regimes. Potential effects include helical twists with a
nearly constant apparent wavelength, an apparent wavelength scaling
approximately with the jet radius for up to two orders of magnitude of jet
expansion, or multiple twist wavelengths with vastly different intrinsic scale
and vastly different wave speeds that give rise to similar observed twist
wavelengths but with very different observed motion. In this paper I illustrate
the basic intrinsic and observed behavior of these structures and show how to
place constraints on jet conditions in superluminal jets using the apparent
structures and motions in the inner 3C 120 jet.Comment: 18 pages, 7 figure
Cosmic ray moderation of the thermal instability
We apply the Hermite-Bieler theorem in the analysis of the effect of cosmic rays on the thermal stability of an initially uniform, static background. The cosmic rays were treated in a fluid approximation and the diffusion coefficient was assumed to be constant in time and space. The inclusion of cosmic rays does not alter the criterion for the thermal stability of a medium subjected to isobaric perturbations. It does alter the criteria for the stability of a medium perturbed by small amplitude sound waves. In the limit of a high background cosmic ray pressure to thermal pressure ratio, the instability in response to high frequency sound waves is suppressed
The Formation of Broad Emission Line Regions in Supernova-QSO Wind Interactions
We show that a cooled region of shocked supernova ejecta forms in a type II
supernova-QSO wind interaction, and has a density, an ionization parameter, and
a column density compatible with those inferred for the high ionization
component of the broad emission line regions in QSOs. The calculations are
based on the assumption that the ejecta flow is described initially by a
similarity solution investigated by Chevalier (1982) and Nadyozhin (1985) and
is spherically symmetric. Heating and cooling appropriate for gas irradiated by
a nearby powerful continuum source is included in our model, together with
reasonable assumptions for the properties of the QSO wind. The model results
are also in agreement with observational correlations and imply reasonable
supernova rates.Comment: 13 pages, 7 figures, to be published in A&
The production of magnetically dominated star-forming regions
We consider the dynamical evolution of an interstellar cloud that is initially in thermal equilibrium in the warm phase and is then subjected to a sudden increase in the pressure of its surroundings. We find that if the initial plasma β of the cloud is of order unity, then there is a considerable period during which the material in the cloud both has a small β and is in the thermally unstable temperature range.
These conditions are not only consistent with observations of star-forming regions but also ideally suited to the production of density inhomogeneities by magnetohydrodynamic waves. The end result should be a cloud whose size and average density are typical of Giant Molecular Clouds (GMCs) and that contains denser regions whose densities are in the range inferred for the translucent clumps in GMCs
Shock-triggered formation of magnetically-dominated clouds
To understand the formation of a magnetically dominated molecular cloud out
of an atomic cloud, we follow the dynamical evolution of the cloud with a
time-dependent axisymmetric magnetohydrodynamic code. A thermally stable warm
atomic cloud is initially in static equilibrium with the surrounding hot
ionised gas. A shock propagating through the hot medium interacts with the
cloud. As a fast-mode shock propagates through the cloud, the gas behind it
becomes thermally unstable. The value of the gas also becomes much
smaller than the initial value of order unity. These conditions are ideal for
magnetohydrodynamic waves to produce high-density clumps embedded in a rarefied
warm medium. A slow-mode shock follows the fast-mode shock. Behind this shock a
dense shell forms, which subsequently fragments. This is a primary region for
the formation of massive stars. Our simulations show that only weak and
moderate-strength shocks can form cold clouds which have properties typical of
giant molecular clouds.Comment: 7 pages, 6 figures, accepted by Astronomy and Astrophysic
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