610 research outputs found
The Thermal Stability of Mass-Loaded Flows
We present a linear stability analysis of a flow undergoing
conductively-driven mass-loading from embedded clouds. We find that
mass-loading damps isobaric and isentropic perturbations, and in this regard is
similar to the effect of thermal conduction, but is much more pronounced where
many embedded clumps exist. The stabilizing influence of mass-loading is
wavelength independent against isobaric (condensing) perturbations, but
wavelength dependent against isentropic (wave-like) perturbations. We derive
equations for the degree of mass-loading needed to stabilize such
perturbations. We have also made 1D numerical simulations of a mass-loaded
radiative shock and demonstrated the damping of the overstability when
mass-loading is rapid enough.Comment: 4 pages, 1 figure, to be published in A&
Dust in Interstellar Clouds, Evolved Stars and Supernovae
Outflows of pre-main-sequence stars drive shocks into molecular material
within 0.01 - 1 pc of the young stars. The shock-heated gas emits infrared,
millimeter and submillimeter lines of many species including. Dust grains are
important charge carriers and play a large role in coupling the magnetic field
and flow of neutral gas. Some effects of the dust on the dynamics of oblique
shocks began to emerge in the 1990s. However, detailed models of these shocks
are required for the calculation of the grain sputtering contribution to gas
phase abundances of species producing observed emissions. We are developing
such models. Some of the molecular species introduced into the gas phase by
sputtering in shocks or by thermally driven desorption in hot cores form on
grain surfaces. Recently laboratory studies have begun to contribute to the
understanding of surface reactions and thermally driven desorption important
for the chemistry of star forming clouds. Dusty plasmas are prevalent in many
evolved stars just as well as in star forming regions. Radiation pressure on
dust plays a significant role in mass loss from some post-main-sequence stars.
The mechanisms leading to the formation of carbonaceous dust in the stellar
outflows are similar to those important for soot formation in flames. However,
nucleation in oxygen-rich outflows is less well understood and remains a
challenging research area. Dust is observed in supernova ejecta that have not
passed through the reverse shocks that develop in the interaction of ejecta
with ambient media. Dust is detected in high redshift galaxies that are
sufficiently young that the only stars that could have produced the dust were
so massive that they became supernovae. Consequently, the issue of the survival
of dust in strong supernova shocks is of considerable interest.Comment: 4 pages, to be published in the proceedings of Fifth International
Conference on Physics of Dusty Plasma
Overview of recent mountain-building events in the Big Bend region, West Texas and Northern Mexico
The terrain of the Big Bend region, as well as locations of many ore bodies, hot springs, and the Rio Grande River, mostly result from two mountain-building events: Basin and Range extension that continues today, but also Rocky Mountain (or âLaramideâ) contraction that ended 50 million years ago. Both events continued for tens of millions of years, producing complex arrangements of folds and faults throughout broad, overlapping zones extending from southern Mexico into Canada. Laramide contraction and Basin and Range extension are caused, at least in part, by changing plate interactions along the western margin of the North American plate. Two compilation maps of the Big Bend region, one showing Laramide structures and one showing Basin and Range structures, summarize current understanding. Panoramic photographs illustrate well-exposed structures in Big Bend National Park. An extensive reference list compiles work on Laramide and Basin and Range structures. Work to date, which includes recent detailed mapping in two small areas, emphasizes several points: a) Laramide structures in the Big Bend region include thick-skinned basement uplifts and coeval thin-skinned thrust belts, b) Map-scale and outcrop-scale folds formed during Basin and Range extension as well as during Laramide contraction, and c) Long-lived fault zones moved repeatedly during Laramide contraction and Basin and Range extension
Mass-loaded spherical accretion flows
We have calculated the evolution of spherical accretion flows undergoing mass-loading from embedded clouds through either conduction or hydrodynamical ablation. We have observed the effect of varying the ratios of the mass-loading timescale and the cooling timescale to the ballistic crossing timescale through the mass-loading region.
We have also varied the ratio of the potential energy of a particle injected into the flow near the outer region of mass-loading to the temperature at which a minimum occurs in the cooling curve. The two types of mass-loading produce qualitatively different types of behaviour in the accretion flow, since mass-loading through conduction requires the ambient gas to be hot, whereas mass ablation from clumps occurs throughout the flow. Higher ratios of injected to accreted mass typically occur with hydrodynamical ablation, in agreement with previous work on wind-blown bubbles and supernova remnants. We find that mass-loading damps the radiative overstability of such flows, in agreement with our earlier work. If the mass-loading is high enough it can stabilize the accretion shock at a constant radius, yielding an almost isothermal subsonic post-shock flow. Such solutions may be relevant to cooling flows onto massive galaxies. Mass-loading can also lead to the formation of isolated shells of high temperature material, separated by gas at cooler temperatures
Geometrical methods in loop calculations and the three-point function
A geometrical way to calculate N-point Feynman diagrams is reviewed. As an
example, the dimensionally-regulated three-point function is considered,
including all orders of its epsilon-expansion. Analytical continuation to other
regions of the kinematical variables is discussed.Comment: 6 pages, LaTeX, 3 eps figures, contribution to proceedings of
ACAT2005 (Zeuthen, May 2005
Gauge and parametrization dependencies of the one-loop counterterms in the Einstein gravity.
The parametrization and gauge dependencies of the one-loop counterterms on
the mass-shell in the Einstein gravity are investigated. The physical meaning
of the loop calculation results on the mass shell and the parametrization
dependence of the renormgroup functions in the nonrenormalizable theories are
discussed.Comment: 14 pages in LATEX (Some references added
Transient evolution of C-type shocks in dusty regions of varying density
Outflows of young stars drive shocks into dusty, molecular regions. Most
models of such shocks assume that they are steady and propagating perpendicular
to the magnetic field. Real shocks often violate both of these assumptions and
the media through which they propagate are inhomogeneous. We use the code
employed previously to produce the first time-dependent simulations of
fast-mode, oblique C-type shocks interacting with density perturbations. We
include a self-consistent calculation of the thermal and ionisation balances
and a fluid treatment of grains. We identify features that develop when a
multifluid shock encounters a density inhomogeneity to investigate whether any
part of the precursor region ever behaves in a quasi-steady fashion. If it does
the shock may be modelled approximately without solving the time-dependent
hydromagnetic equations. Simulations were made for initially steady oblique
C-type shocks encountering density inhomogeneities. For a semi-finite
inhomogeneity with a density larger than the surrounding medium, a transmitted
shock evolves from being J-type to a steady C-type shock on a timescale
comparable to the ion-flow time through it. A sufficiently upstream part of the
precursor of an evolving J-type shock is quasi-steady. The ion-flow timescale
is also relevant for the evolution of a shock moving into a region of
decreasing density. The models for shocks propagating into regions in which the
density increases and then decreases to its initial value cannot be entirely
described in terms of the results obtained for monotonically increasing and
decreasing densities. For the latter model, the long-term evolution to a C-type
shock cannot be approximated by quasi-steady models.Comment: 11 pages, 9 figure
Lorentz and CPT symmetries in commutative and noncommutative spacetime
We investigate the fermionic sector of a given theory, in which massive and
charged Dirac fermions interact with an Abelian gauge field, including a non
standard contribution that violates both Lorentz and CPT symmetries. We offer
an explicit calculation in which the radiative corrections due to the fermions
seem to generate a Chern-Simons-like effective action. Our results are obtained
under the general guidance of dimensional regularization, and they show that
there is no room for Lorentz and CPT violation in both commutative and
noncommutative spacetime.Comment: RevTex4, 7 pages, to be published in J. Phys.
Explicit results for all orders of the epsilon-expansion of certain massive and massless diagrams
An arbitrary term of the epsilon-expansion of dimensionally regulated
off-shell massless one-loop three-point Feynman diagram is expressed in terms
of log-sine integrals related to the polylogarithms. Using magic connection
between these diagrams and two-loop massive vacuum diagrams, the
epsilon-expansion of the latter is also obtained, for arbitrary values of the
masses. The problem of analytic continuation is also discussed.Comment: 8 pages, late
On-shell two-loop three-gluon vertex
The two-loop three-gluon vertex is calculated in an arbitrary covariant
gauge, in the limit when two of the gluons are on the mass shell. The
corresponding two-loop results for the ghost-gluon vertex are also obtained. It
is shown that the results are consistent with the Ward-Slavnov-Taylor
identities.Comment: 34 pages, LaTeX, including 5 figures, uses eps
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