5,384 research outputs found
On the structure of the turbulent interstellar atomic hydrogen. I- Physical characteristics
{We study in some details the statistical properties of the turbulent 2-phase
interstellar atomic gas.{We present high resolution bidimensional numerical
simulations of the interstellar atomic hydrogen which describe it over 3 to 4
orders of magnitude in spatial scales.}{The simulations produce naturally small
scale structures having either large or small column density. It is tempting to
propose that the former are connected to the tiny small scale structures
observed in the ISM. We compute the mass spectrum of CNM structures and find
that , which is remarkably similar to the
mass spectrum inferred for the CO clumps. We propose a theoretical explanation
based on a formalism inspired from the Press & Schecter (1974) approach and
used the fact that the turbulence within WNM is subsonic. This theory predicts
in 2D and in
3D. We compute the velocity and the density power-spectra and conclude that,
although the latter is rather flat, as observed in supersonic isothermal
simulations, the former follows the Kolmogorov prediction and is dominated by
its solenoidal component. This is due to the bistable nature of the flow which
produces large density fluctuations even when the rms Mach number (of WNM) is
not large. We also find that, whereas the energy at large scales is mainly in
the WNM, at smaller scales, it is dominated by the kinetic energy of the CNM
fragments.}Comment: Accepted for publication in A&
A radiation-hydrodynamics scheme valid from the transport to the diffusion limit
We present in this paper the numerical treatment of the coupling between
hydrodynamics and radiative transfer. The fluid is modeled by classical
conservation laws (mass, momentum and energy) and the radiation by the grey
moment system. The scheme introduced is able to compute accurate
numerical solution over a broad class of regimes from the transport to the
diffusive limits. We propose an asymptotic preserving modification of the HLLE
scheme in order to treat correctly the diffusion limit. Several numerical
results are presented, which show that this approach is robust and have the
correct behavior in both the diffusive and free-streaming limits. In the last
numerical example we test this approach on a complex physical case by
considering the collapse of a gas cloud leading to a proto-stellar structure
which, among other features, exhibits very steep opacity gradients.Comment: 29 pages, submitted to Journal of Computational physic
3D simulations of pillars formation around HII regions: the importance of shock curvature
Radiative feedback from massive stars is a key process to understand how HII
regions may enhance or inhibit star formation in pillars and globules at the
interface with molecular clouds. We aim to contribute to model the interactions
between ionization and gas clouds to better understand the processes at work.
We study in detail the impact of modulations on the cloud-HII region interface
and density modulations inside the cloud. We run three-dimensional
hydrodynamical simulations based on Euler equations coupled with gravity using
the HERACLES code. We implement a method to solve ionization/recombination
equations and we take into account typical heating and cooling processes at
work in the interstellar medium and due to ionization/recombination physics. UV
radiation creates a dense shell compressed between an ionization front and a
shock ahead. Interface modulations produce a curved shock that collapses on
itself leading to stable growing pillar-like structures. The narrower the
initial interface modulation, the longer the resulting pillar. We interpret
pillars resulting from density modulations in terms of the ability of these
density modula- tions to curve the shock ahead the ionization front. The shock
curvature is a key process to understand the formation of structures at the
edge of HII regions. Interface and density modulations at the edge of the cloud
have a direct impact on the morphology of the dense shell during its formation.
Deeper in the cloud, structures have less influence due to the high densities
reached by the shell during its expansion.Comment: Accepted by A&A 03/11/201
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&
Three-nucleon mechanisms in photoreactions
The C reaction has been measured for
E=150-800 MeV in the first study of this reaction in a target
heavier than He. The experimental data are compared to a microscopic many
body calculation. The model, which predicts that the largest contribution to
the reaction arises from final state interactions following an initial pion
production process, overestimates the measured cross sections and there are
strong indications that the overestimate arises in this two-step process. The
selection of suitable kinematic conditions strongly suppresses this two-step
contribution leaving cross sections in which up to half the yield is predicted
to arise from the absorption of the photon on three interacting nucleons and
which agree with the model. The results indicate measurements on
nuclei may be a valuable tool for obtaining information on the nuclear
three-body interaction.Comment: 5 pages, 3 figure
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