148 research outputs found
Super-Sonic Turbulence in the Perseus Molecular Cloud
We compare the statistical properties of J=1-0 13CO spectra observed in the
Perseus Molecular Cloud with synthetic J=1-0 13CO spectra, computed solving the
non-LTE radiative transfer problem for a model cloud obtained as solutions of
the three dimensional magneto-hydrodynamic (MHD) equations. The model cloud is
a randomly forced super-Alfvenic and highly super-sonic turbulent isothermal
flow.
The purpose of the present work is to test if idealized turbulent flows,
without self-gravity, stellar radiation, stellar outflows, or any other effect
of star formation, are inconsistent or not with statistical properties of star
forming molecular clouds.
We present several statistical results that demonstrate remarkable similarity
between real data and the synthetic cloud. Statistical properties of molecular
clouds like Perseus are appropriately described by random super-sonic and
super-Alfvenic MHD flows. Although the description of gravity and stellar
radiation are essential to understand the formation of single protostars and
the effects of star formation in the cloud dynamics, the overall description of
the cloud and of the initial conditions for star formation can apparently be
provided on intermediate scales without accounting for gravity, stellar
radiation, and a detailed modeling of stellar outflows.
We also show that the relation between equivalent line width and integrated
antenna temperature indicates the presence of a relatively strong magnetic
field in the core B1, in agreement with Zeeman splitting measurements.Comment: 20 pages, 8 figures included, ApJ (in press
Can we trace very cold dust from its emission alone ?
Context. Dust is a good tracer of cold dark clouds but its column density is
difficult to quantify. Aims. We want to check whether the far-infrared and
submillimeter high-resolution data from Herschel SPIRE and PACS cameras
combined with ground-based telescope bolometers allow us to retrieve the whole
dust content of cold dark clouds. Methods. We compare far-infrared and
submillimeter emission across L183 to the 8 m absorption map from Spitzer
data and fit modified blackbody functions towards three different positions.
Results. We find that none of the Herschel SPIRE channels follow the cold dust
profile seen in absorption. Even the ground-based submillimeter telescope
observations, although more closely following the absorption profile, cannot
help to characterize the cold dust without external information such as the
dust column density itself. The difference in dust opacity can reach up to a
factor of 3 in prestellar cores of high extinction. Conclusions. In dark
clouds, the amount of very cold dust cannot be measured from its emission
alone. In particular, studies of dark clouds based only on Herschel data can
miss a large fraction of the dust content. This has an impact on core and
filament density profiles, masse and stability estimates.Comment: Letter to A&A (accepted for publication). must be viewed with ACROBAT
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SOC program for dust continuum radiative transfer
Context. Thermal dust emission carries information on physical conditions and dust properties in many astronomical sources. Because observations represent a sum of emission along the line of sight, their interpretation often requires radiative transfer (RT) modelling. Aims. We describe a new RT program, SOC, for computations of dust emission, and examine its performance in simulations of interstellar clouds with external and internal heating. Methods. SOC implements the Monte Carlo RT method as a parallel program for shared-memory computers. It can be used to study dust extinction, scattering, and emission. We tested SOC with realistic cloud models and examined the convergence and noise of the dust-temperature estimates and of the resulting surface-brightness maps. Results. SOC has been demonstrated to produce accurate estimates for dust scattering and for thermal dust emission. It performs well with both CPUs and GPUs, the latter providing a speed-up of processing time by up to an order of magnitude. In the test cases, accelerated lambda iterations (ALIs) improved the convergence rates but was also sensitive to Monte Carlo noise. Run-time refinement of the hierarchical-grid models did not help in reducing the run times required for a given accuracy of solution. The use of a reference field, without ALI, works more robustly, and also allows the run time to be optimised if the number of photon packages is increased only as the iterations progress. Conclusions. The use of GPUs in RT computations should be investigated further.Peer reviewe
Molecular cloud cores with high deuterium fractions : nobeyama mapping survey
Publisher Copyright: © 2021. The American Astronomical Society.We present the results of on-the-fly mapping observations of 44 fields containing 107 SCUBA-2 cores in the emission lines of molecules N2H+, HC3N, and CCS at 82-94 GHz using the Nobeyama 45 m telescope. This study aimed at investigating the physical properties of cores that show high deuterium fractions and might be close to the onset of star formation. We found that the distributions of the N2H+ and HC3N line emissions are approximately similar to the distribution of the 850 mu m dust continuum emission, whereas the CCS line emission is often undetected or is distributed in a clumpy structure surrounding the peak position of the 850 mu m dust continuum emission. Occasionally (12%), we observe CCS emission, which is an early-type gas tracer toward the young stellar object, probably due to local high excitation. Evolution toward star formation does not immediately affect the nonthermal velocity dispersion.Peer reviewe
The Power Spectrum of Supersonic Turbulence in Perseus
We test a method of estimating the power spectrum of turbulence in molecular
clouds based on the comparison of power spectra of integrated intensity maps
and single-velocity-channel maps, suggested by Lazarian and Pogosyan. We use
synthetic 13CO data from non-LTE radiative transfer calculations based on
density and velocity fields of a simulation of supersonic hydrodynamic
turbulence. We find that the method yields the correct power spectrum with good
accuracy. We then apply the method to the Five College Radio Astronomy
Observatory 13CO map of the Perseus region, from the COMPLETE website. We find
a power law power spectrum with slope beta=1.81+-0.10. The values of beta as a
function of velocity resolution are also confirmed using the lower resolution
map of the same region obtained with the AT&T Bell Laboratories antenna.
Because of its small uncertainty, this result provides a useful constraint for
numerical codes used to simulate molecular cloud turbulence.Comment: 4 pages, 3 figures. ApJ Letters, in pres
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