391 research outputs found
Synthetic Observations of Carbon Lines of Turbulent Flows in Diffuse Multiphase Interstellar Medium
We examine observational characteristics of multi-phase turbulent flows in
the diffuse interstellar medium (ISM) using a synthetic radiation field of
atomic and molecular lines. We consider the multi-phase ISM which is formed by
thermal instability under the irradiation of UV photons with moderate visual
extinction . Radiation field maps of C, C, and CO line
emissions were generated by calculating the non-local thermodynamic equilibrium
(nonLTE) level populations from the results of high resolution hydrodynamic
simulations of diffuse ISM models. By analyzing synthetic radiation field of
carbon lines of [\ion{C}{2}] 158 m, [\ion{C}{1}] (809 GHz),
(492 GHz), and CO rotational transitions, we found a high ratio
between the lines of high- and low-excitation energies in the diffuse
multi-phase interstellar medium. This shows that simultaneous observations of
the lines of warm- and cold-gas tracers will be useful in examining the thermal
structure, and hence the origin of diffuse interstellar clouds.Comment: 16 pages, 10 figures : accepted for publication in ApJ. PDF version
with high resolution figures is available
(http://yso.mtk.nao.ac.jp/~ymasako/paper/ms_hires.pdf
The pressure distribution in thermally bistable turbulent flows
We present a systematic numerical study of the effect of turbulent velocity
fluctuations on the thermal pressure distribution in thermally bistable flows.
The simulations employ a random turbulent driving generated in Fourier space
rather than star-like heating. The turbulent fluctuations are characterized by
their rms Mach number M and the energy injection wavenumber, k_for. Our results
are consistent with the picture that as either of these parameters is
increased, the local ratio of turbulent crossing time to cooling time
decreases, causing transient structures in which the effective behavior is
intermediate between the thermal-equilibrium and adiabatic regimes. As a
result, the effective polytropic exponent gamma_ef ranges between ~0.2 to ~1.1.
The fraction of high-density zones with P>10^4 Kcm^-3 increases from roughly
0.1% at k_for=2 and M=0.5 to roughly 70% for k_for=16 and M=1.25. A preliminary
comparison with the pressure measurements of Jenkins (2004) favors our case
with M=0.5 and k_for=2. In all cases, the dynamic range of the pressure summed
over the entire density range, typically spans 3-4 orders of magnitude. The
total pressure histogram widens as the Mach number is increased, and develops
near-power-law tails at high (resp.low) pressures when gamma_ef<~ 0.5 (resp.
gamma_ef>~ 1), which occurs at k_for=2 (resp.k_for=16) in our simulations. The
opposite side of the pressure histogram decays rapidly, in an approx. lognormal
form. Our results show that turbulent advection alone can generate large
pressure scatters, with power-law high-P tails for large-scale driving, and
provide validation for approaches attempting to derive the shape of the
pressure histogram through a change of variable from the known form of the
density histogram, such as that performed by MacLow et al.(2004).Comment: to be published in Ap
Two-dimensional AMR simulations of colliding flows
Colliding flows are a commonly used scenario for the formation of molecular
clouds in numerical simulations. Due to the thermal instability of the warm
neutral medium, turbulence is produced by cooling. We carry out a
two-dimensional numerical study of such colliding flows in order to test
whether statistical properties inferred from adaptive mesh refinement (AMR)
simulations are robust with respect to the applied refinement criteria. We
compare probability density functions of various quantities as well as the
clump statistics and fractal dimension of the density fields in AMR simulations
to a static-grid simulation. The static grid with 2048^2 cells matches the
resolution of the most refined subgrids in the AMR simulations. The density
statistics is reproduced fairly well by AMR. Refinement criteria based on the
cooling time or the turbulence intensity appear to be superior to the standard
technique of refinement by overdensity. Nevertheless, substantial differences
in the flow structure become apparent. In general, it is difficult to separate
numerical effects from genuine physical processes in AMR simulations.Comment: 6 pages, 6 figures, submitted to A&
Protostellar disk formation and transport of angular momentum during magnetized core collapse
Theoretical studies of collapsing clouds have found that even a relatively
weak magnetic field (B) may prevent the formation of disks and their
fragmentation. However, most previous studies have been limited to cases where
B and the rotation axis of the cloud are aligned. We study the transport of
angular momentum, and its effects on disk formation, for non-aligned initial
configurations and a range magnetic intensities. We perform 3D AMR MHD
simulations of magnetically supercritical collapsing dense cores using the code
Ramses. We compute the contributions of the processes transporting angular
momentum (J), in the envelope and the region of the disk. We clearly define
what could be defined as centrifugally supported disks and study their
properties. At variance with earlier analyses, we show that the transport of J
acts less efficiently in collapsing cores with non-aligned rotation axis and B.
Analytically, this result can be understood by taking into account the bending
of field lines occurring during the gravitational collapse. For the transport
of J, we conclude that magnetic braking in the mean direction of B tends to
dominate over both the gravitational and outflow transport of J. We find that
massive disks, containing at least 10% of the initial core mass, can form
during the earliest stages of star formation even for mass-to-flux ratios as
small as 3 to 5 times the critical value. At higher field intensities, the
early formation of massive disks is prevented. Given the ubiquity of Class I
disks, and because the early formation of massive disks can take place at
moderate magnetic intensities, we speculate that for stronger fields, disks
will form later, when most of the envelope will have been accreted. In
addition, we speculate that some observed early massive disks may actually be
outflow cavities, mistaken for disks by projection effects. (Abridged version
of the abstract.)Comment: 23 pages, 23 figures, to be published in A&
Disc formation in turbulent cloud cores: Circumventing the magnetic braking catastrophe
We present collapse simulations of strongly magnetised, 100 M_sun, turbulent
cloud cores. Around the protostars formed during the collapse Keplerian discs
with typical sizes of up to 100 AU build up in contrast to previous simulations
neglecting turbulence. Analysing the condensations in which the discs form, we
show that the magnetic flux loss is not sufficient to explain the build-up of
Keplerian discs. The average magnetic field is strongly inclined to the disc
which might reduce the magnetic braking efficiency. However, the main reason
for the reduced magnetic braking efficiency is the highly disordered magnetic
field in the surroundings of the discs. Furthermore, due to the lack of a
coherently rotating structure in the turbulent environment of the disc no
toroidal magnetic field necessary for angular momentum extraction can build up.
Simultaneously the angular momentum inflow remains high due to local shear
flows created by the turbulent motions. We suggest that the "magnetic braking
catastrophe" is an artefact of the idealised non-turbulent initial conditions
and that turbulence provides a natural mechanism to circumvent this problem.Comment: 4 pages, 2 figures. To appear in the proceedings of 'The Labyrinth of
Star Formation' (18-22 June 2012, Chania, Greece), published by Springe
Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores
Stars and more particularly massive stars, have a drastic impact on galaxy
evolution. Yet the conditions in which they form and collapse are still not
fully understood. In particular, the influence of the magnetic field on the
collapse of massive clumps is relatively unexplored, it is thus of great
relevance in the context of the formation of massive stars to investigate its
impact. We perform high resolution, MHD simulations of the collapse of hundred
solar masses, turbulent and magnetized clouds, using the adaptive mesh
refinement code RAMSES. We compute various quantities such as mass
distribution, magnetic field and angular momentum within the collapsing core
and study the episodic outflows and the fragmentation that occurs during the
collapse. The magnetic field has a drastic impact on the cloud evolution. We
find that magnetic braking is able to substantially reduce the angular momentum
in the inner part of the collapsing cloud. Fast and episodic outflows are being
launched with typical velocities of the order of 3-5 km s although the
highest velocities can be as high as 30-40 km s. The fragmentation in
several objects, is reduced in substantially magnetized clouds with respect to
hydrodynamical ones by a factor of the order of 1.5-2. We conclude that
magnetic fields have a significant impact on the evolution of massive clumps.
In combination with radiation, magnetic fields largely determine the outcome of
massive core collapse. We stress that numerical convergence of MHD collapse is
a challenging issue. In particular, numerical diffusion appears to be important
at high density therefore possibly leading to an over-estimation of the number
of fragments.Comment: accepted for publication in A&
Formation of low-mass stars and brown dwarfs
These lectures attempt to expose the most important ideas, which have been
proposed to explain the formation of stars with particular emphasis on the
formation of brown dwarfs and low-mass stars. We first describe the important
physical processes which trigger the collapse of a self-gravitating piece of
fluid and regulate the star formation rate in molecular clouds. Then we review
the various theories which have been proposed along the years to explain the
origin of the stellar initial mass function paying particular attention to four
models, namely the competitive accretion and the theories based respectively on
stopped accretion, MHD shocks and turbulent dispersion. As it is yet unsettled
whether the brown dwarfs form as low-mass stars, we present the theory of brown
dwarfs based on disk fragmentation stressing all the uncertainties due to the
radiative feedback and magnetic field. Finally, we describe the results of
large scale simulations performed to explain the collapse and fragmentation of
molecular clouds.Comment: proceedings of the Evry Schatzman School on "Low-mass stars and the
transition between stars and brown dwarfs" (Roscoff 2011), to appear in EAS
Publication Series (Eds C.Reyl\'e, C.Charbonnel, & M.Schultheis
Simulated CII observations for SPICA/SAFARI
We investigate the case of CII 158 micron observations for SPICA/SAFARI using
a three-dimensional magnetohydrodynamical (MHD) simulation of the diffuse
interstellar medium (ISM) and the Meudon PDR code. The MHD simulation consists
of two converging flows of warm gas (10,000 K) within a cubic box 50 pc in
length. The interplay of thermal instability, magnetic field and self-gravity
leads to the formation of cold, dense clumps within a warm, turbulent
interclump medium. We sample several clumps along a line of sight through the
simulated cube and use them as input density profiles in the Meudon PDR code.
This allows us to derive intensity predictions for the CII 158 micron line and
provide time estimates for the mapping of a given sky area.Comment: 4 pages, 5 figures, to appear in the proceedings of the workshop "The
Space Infrared Telescope for Cosmology & Astrophysics: Revealing the Origins
of Planets and Galaxies" (July 2009, Oxford, United Kingdom
Gravitational Collapse in Turbulent Molecular Clouds. II. Magnetohydrodynamical Turbulence
Hydrodynamic supersonic turbulence can only prevent local gravitational
collapse if the turbulence is driven on scales smaller than the local Jeans
lengths in the densest regions, a very severe requirement (Paper I). Magnetic
fields have been suggested to support molecular clouds either magnetostatically
or via magnetohydrodynamic (MHD) waves. Whereas the first mechanism would form
sheet-like clouds, the second mechanism not only could exert a pressure onto
the gas counteracting the gravitational forces, but could lead to a transfer of
turbulent kinetic energy down to smaller spatial scales via MHD wave
interactions. This turbulent magnetic cascade might provide sufficient energy
at small scales to halt local collapse.
We test this hypothesis with MHD simulations at resolutions up to 256^3
zones, done with ZEUS-3D. We first derive a resolution criterion for
self-gravitating, magnetized gas: in order to prevent collapse of
magnetostatically supported regions due to numerical diffusion, the minimum
Jeans length must be resolved by four zones. Resolution of MHD waves increases
this requirement to roughly six zones. We then find that magnetic fields cannot
prevent local collapse unless they provide magnetostatic support. Weaker
magnetic fields do somewhat delay collapse and cause it to occur more uniformly
across the supported region in comparison to the hydrodynamical case. However,
they still cannot prevent local collapse for much longer than a global
free-fall time.Comment: 32 pages, 14 figures, accepted by Ap
- âŠ