82,471 research outputs found
Two-Fluid MHD Simulations of Converging Hi Flows in the Interstellar Medium. II: Are Molecular Clouds Generated Directly from Warm Neutral Medium?
Formation of interstellar clouds as a consequence of thermal instability is
studied using two-dimensional two-fluid magnetohydrodynamic simulations. We
consider the situation of converging, supersonic flows of warm neutral medium
in the interstellar medium that generate a shocked slab of thermally unstable
gas in which clouds form. We found, as speculated in paper I, that in the
shocked slab magnetic pressure dominates thermal pressure and the thermal
instability grows in the isochorically cooling, thermally unstable slab that
leads formation of HI clouds whose number density is typically n < 100 cm^-3,
even if the angle between magnetic field and converging flows is small. We also
found that even if there is a large dispersion of magnetic field, evolution of
the shocked slab is essentially determined by the angle between the mean
magnetic field and converging flows. Thus, the direct formation of molecular
clouds by piling up warm neutral medium does not seem a typical molecular cloud
formation process, unless the direction of supersonic converging flows is
biased to the orientation of mean magnetic field by some mechanism. However,
when the angle is small, the HI shell generated as a result of converging flows
is massive and possibly evolves into molecular clouds, provided gas in the
massive HI shell is piled up again along the magnetic field line. We expect
that another subsequent shock wave can pile up again the gas of the massive
shell and produce a larger cloud. We thus emphasize the importance of multiple
episodes of converging flows, as a typical formation process of molecular
clouds.Comment: 9 pages, 8 figures, accepted by Ap
Fermi~I particle acceleration in converging flows mediated by magnetic reconnection
Context. Converging flows with strong magnetic fields of different polarity
can accelerate particles through magnetic reconnection. If the particle mean
free path is longer than the reconnection layer is thick, but much shorter than
the entire reconnection structure, the particle will mostly interact with the
incoming flows potentially with a very low escape probability. Aims. We
explore, in general and also in some specific scenarios, the possibility of
particles to be accelerated in a magnetic reconnection layer by interacting
only with the incoming flows. Methods. We characterize converging flows that
undergo magnetic reconnection, and derive analytical estimates for the particle
energy distribution, acceleration rate, and maximum energies achievable in
these flows. We also discuss a scenario, based on jets dominated by magnetic
fields of changing polarity, in which this mechanism may operate. Results. The
proposed acceleration mechanism operates if the reconnection layer is much
thinner than its transversal characteristic size, and the magnetic field has a
disordered component. Synchrotron losses may prevent electrons from entering in
this acceleration regime. The acceleration rate should be faster, and the
energy distribution of particles harder than in standard diffusive shock
acceleration. The interaction of obstacles with the innermost region of jets in
active galactic nuclei and microquasars may be suitable sites for particle
acceleration in converging flows.Comment: 4 pages, 2 figures, Reserch Note, in press, A&A (final version
Dense core formation in supersonic turbulent converging flows
We use numerical hydrodynamic simulations to investigate prestellar core
formation in the dynamic environment of giant molecular clouds, focusing on
planar post-shock layers produced by colliding turbulent flows. A key goal is
to test how core evolution and properties depend on the velocity dispersion in
the parent cloud; our simulation suite consists of 180 models with inflow Mach
numbers Ma=v/c_s=1.1-9. At all Mach numbers, our models show that turbulence
and self-gravity collect gas within post-shock regions into filaments at the
same time as overdense areas within these filaments condense into cores. This
morphology, together with the subsonic velocities we find inside cores, is
similar to observations. We extend previous results showing that core collapse
develops in an ``outside-in'' manner, with density and velocity approaching the
Larson-Penston asymptotic solution. The time for the first core to collapse
varies as 1/sqrt(v), consistent with analytic estimates. Core building takes 10
times as long as core collapse, consistent with observed prestellar core
lifetimes. Core shapes change from oblate to prolate as they evolve. To define
cores, we use isosurfaces of the gravitational potential. We compare to cores
defined using the potential computed from projected surface density, finding
good agreement for core masses and sizes; this offers a new way to identify
cores in observed maps. Cores with masses varying by three orders of magnitude
(0.05 - 50 M_sun) are identified in our simulations. Stability analysis of
post-shock layers predicts that the first core to collapse will have mass M
\propto v^-1/2, and that the minimum mass for cores formed at late times will
have M\propto v^-1. From our simulations, the median mass lies between these
two relations.Comment: Accepted to ApJ. 54 pages, 21 figure
On smooth approximations of rough vector fields and the selection of flows
In this work we deal with the selection problem of flows of an irregular
vector field. We first summarize an example from \cite{CCS} of a vector field
and a smooth approximation for which the sequence
of flows of has subsequences converging to different flows of the
limit vector field . Furthermore, we give some heuristic ideas on the
selection of a subclass of flows in our specific case.Comment: Proceeding of the "XVII International Conference on Hyperbolic
Problems: Theory, Numerics, Applications.
Small scale energy release driven by supergranular flows on the quiet Sun
In this article we present data and modelling for the quiet Sun that strongly suggest a ubiquitous small-scale atmospheric heating mechanism that is driven solely by converging supergranular flows.
A possible energy source for such events is the power transfer to the plasma via the work done on the magnetic field by photospheric convective flows, which exert drag of the footpoints of magnetic structures. In this paper we present evidence of small scale energy release events driven directly by the hydrodynamic forces that act on the magnetic elements in the photosphere, as a result of supergranular scale flows. We show strong spatial and temporal correlation between quiet Sun soft X-ray emission (from <i>Yohkoh</i> and <i>SOHO</i> MDI-derived flux removal events driven by deduced photospheric flows.
We also present a simple model of heating generated by flux submergence, based on particle acceleration by converging magnetic mirrors.
In the near future, high resolution soft X-ray images from XRT on the <i>Hinode</i> satellite will allow definitive, quantitative verification of our results
Breakdown of Burton-Prim-Slichter approach and lateral solute segregation in radially converging flows
A theoretical study is presented of the effect of a radially converging melt
flow, which is directed away from the solidification front, on the radial
solute segregation in simple solidification models. We show that the classical
Burton-Prim-Slichter (BPS) solution describing the effect of a diverging flow
on the solute incorporation into the solidifying material breaks down for the
flows converging along the solidification front. The breakdown is caused by a
divergence of the integral defining the effective boundary layer thickness
which is the basic concept of the BPS theory. Although such a divergence can
formally be avoided by restricting the axial extension of the melt to a layer
of finite height, radially uniform solute distributions are possible only for
weak melt flows with an axial velocity away from the solidification front
comparable to the growth rate. There is a critical melt velocity for each
growth rate at which the solution passes through a singularity and becomes
physically inconsistent for stronger melt flows. To resolve these
inconsistencies we consider a solidification front presented by a disk of
finite radius subject to a strong converging melt flow and obtain an
analytic solution showing that the radial solute concentration depends on the
radius as and close to the rim and
at large distances from it. The logarithmic increase of concentration is
limited in the vicinity of the symmetry axis by the diffusion becoming
effective at a distance comparable to the characteristic thickness of the
solute boundary layer. The converging flow causes a solute pile-up forming a
logarithmic concentration peak at the symmetry axis which might be an
undesirable feature for crystal growth processes.Comment: 15 pages, 5 figure
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