162 research outputs found
From the warm magnetized atomic medium to molecular clouds
{It has recently been proposed that giant molecular complexes form at the
sites where streams of diffuse warm atomic gas collide at transonic
velocities.} {We study the global statistics of molecular clouds formed by
large scale colliding flows of warm neutral atomic interstellar gas under ideal
MHD conditions. The flows deliver material as well as kinetic energy and
trigger thermal instability leading eventually to gravitational collapse.} {We
perform adaptive mesh refinement MHD simulations which, for the first time in
this context, treat self-consistently cooling and self-gravity.} {The clouds
formed in the simulations develop a highly inhomogeneous density and
temperature structure, with cold dense filaments and clumps condensing from
converging flows of warm atomic gas. In the clouds, the column density
probability density distribution (PDF) peaks at \sim 2 \times 10^{21} \psc
and decays rapidly at higher values; the magnetic intensity correlates weakly
with density from to 10^4 \pcc, and then varies roughly as
for higher densities.} {The global statistical properties of such
molecular clouds are reasonably consistent with observational determinations.
Our numerical simulations suggest that molecular clouds formed by the
moderately supersonic collision of warm atomic gas streams.}Comment: submitted to A&
Clump morphology and evolution in MHD simulations of molecular cloud formation
Abridged: We study the properties of clumps formed in three-dimensional
weakly magnetized magneto-hydrodynamic simulations of converging flows in the
thermally bistable, warm neutral medium (WNM). We find that: (1) Similarly to
the situation in the classical two-phase medium, cold, dense clumps form
through dynamically-triggered thermal instability in the compressed layer
between the convergent flows, and are often characterised by a sharp density
jump at their boundaries though not always. (2) However, the clumps are bounded
by phase-transition fronts rather than by contact discontinuities, and thus
they grow in size and mass mainly by accretion of WNM material through their
boundaries. (3) The clump boundaries generally consist of thin layers of
thermally unstable gas, but these layers are often widened by the turbulence,
and penetrate deep into the clumps. (4) The clumps are approximately in both
ram and thermal pressure balance with their surroundings, a condition which
causes their internal Mach numbers to be comparable to the bulk Mach number of
the colliding WNM flows. (5) The clumps typically have mean temperatures 20 < T
< 50 K, corresponding to the wide range of densities they contain (20 < n <
5000 pcc) under a nearly-isothermal equation of state. (6) The turbulent ram
pressure fluctuations of the WNM induce density fluctuations that then serve as
seeds for local gravitational collapse within the clumps. (7) The velocity and
magnetic fields tend to be aligned with each other within the clumps, although
both are significantly fluctuating, suggesting that the velocity tends to
stretch and align the magnetic field with it. (8) The typical mean field
strength in the clumps is a few times larger than that in the WNM. (9) The
magnetic field strength has a mean value of B ~ 6 mu G ...Comment: substantially revised version, accepted by MNRAS, 13 pages, 14
figures, high resolution version:
http://www.ita.uni-heidelberg.de/~banerjee/publications/MC_Formation_Paper2.pd
Density probability distribution in one-dimensional polytropic gas dynamics
We discuss the generation and statistics of the density fluctuations in
highly compressible polytropic turbulence, based on a simple model and
one-dimensional numerical simulations. Observing that density structures tend
to form in a hierarchical manner, we assume that density fluctuations follow a
random multiplicative process. When the polytropic exponent is equal
to unity, the local Mach number is independent of the density, and our
assumption leads us to expect that the probability density function (PDF) of
the density field is a lognormal. This isothermal case is found to be singular,
with a dispersion which scales like the square turbulent Mach
number , where and is the fluid density.
This leads to much higher fluctuations than those due to shock jump relations.
Extrapolating the model to the case , we find that, as the
Mach number becomes large, the density PDF is expected to asymptotically
approach a power-law regime, at high densities when , and at low
densities when . This effect can be traced back to the fact that the
pressure term in the momentum equation varies exponentially with , thus
opposing the growth of fluctuations on one side of the PDF, while being
negligible on the other side. This also causes the dispersion to
grow more slowly than when . In view of these
results, we suggest that Burgers flow is a singular case not approached by the
high- limit, with a PDF that develops power laws on both sides.Comment: 9 pages + 12 postscript figures. Submitted to Phys. Rev.
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