1,752 research outputs found

### Template matching method for the analysis of interstellar cloud structure

The structure of interstellar medium can be characterised at large scales in
terms of its global statistics (e.g. power spectra) and at small scales by the
properties of individual cores. Interest has been increasing in structures at
intermediate scales, resulting in a number of methods being developed for the
analysis of filamentary structures. We describe the application of the generic
template-matching (TM) method to the analysis of maps. Our aim is to show that
it provides a fast and still relatively robust way to identify elongated
structures or other image features. We present the implementation of a TM
algorithm for map analysis. The results are compared against rolling Hough
transform (RHT), one of the methods previously used to identify filamentary
structures. We illustrate the method by applying it to Herschel surface
brightness data. The performance of the TM method is found to be comparable to
that of RHT but TM appears to be more robust regarding the input parameters,
for example, those related to the selected spatial scales. Small modifications
of TM enable one to target structures at different size and intensity levels.
In addition to elongated features, we demonstrate the possibility of using TM
to also identify other types of structures. The TM method is a viable tool for
data quality control, exploratory data analysis, and even quantitative analysis
of structures in image data.Comment: 12 pages, accepted to A&

### The degeneracy between the dust colour temperature and the spectral index. The problem of multiple chi^2 minima

Because of the Herschel and Planck satellite missions, there is strong
interest in the interpretation the sub-millimetre dust spectra from
interstellar clouds. Much work has been done to understand the dependence
between the spectral index beta_Obs and the colour temperature T_C that is
partly caused by the noise. The (T_C, beta_Obs) confidence regions are
elongated, banana-shaped structures. We studied under which conditions these
exhibit anomalous, strongly non-Gaussian behaviour that could affect the
interpretation of the observed (T_C, beta_Obs) relations. We examined modified
black body spectra and spectra calculated from radiative transfer models of
filamentary clouds at wavelengths 100um-850um. We performed modified black body
fits and examined the structure of the chi^2(T_, beta_Obs) function. We show
cases where, when the signal-to-noise ratio is low, the chi^2 has multiple
local minima in the (T_C, beta_Obs) plane. A small change in the weighting of
the data points can cause the solution to jump to completely different values.
In particular, noise can lead to the appearance of a separate population of
solutions with low colour temperatures and high spectral indices. The anomalies
are caused by the noise but the tendency to show multiple chi^2 minima depends
on the model and the wavelengths analysed. Deviations from the assumed single
modified black body spectrum are not important. The presence of local minima
implies that the results obtained from the chi^2 minimisation depend on the
starting point of the optimisation and may correspond to non-global minima. The
(T_C,beta_Obs) distributions may be contaminated by a few solutions with
unrealistically low colour temperatures and high spectral indices. Proper
weighting must be applied to avoid the determination of the beta_Obs(T_C)
relation to be unduly affected by these measurements.Comment: 11 pages, accepted to A&

### Profiles of interstellar cloud filaments. Observational effects in synthetic sub-millimetre observations

Sub-millimetre observations suggest that the filaments of interstellar clouds
have rather uniform widths and can be described with the so-called Plummer
profiles. The shapes of the filament profiles are linked to their physical
state. Before drawing conclusions on the observed column density profiles, we
must evaluate the observational uncertainties. We want to estimate the bias
that could result from radiative transfer effects or from variations of submm
dust emissivity. We use cloud models obtained with magnetohydrodynamic
simulations and carry out radiative transfer calculations to produce maps of
sub-millimetre emission. Column densities are estimated based on the synthetic
observations. For selected filaments, the estimated profiles are compared to
those derived from the original column density. Possible effects from spatial
variations of dust properties are examined. With instrumental noise typical of
the Herschel observations, the parameters derived for nearby clouds are correct
to within a few percent. The radiative transfer effects have only a minor
effect on the results. If the signal-to-noise ratio is degraded by a factor of
four, the errors become significant and for half of the examined filaments the
values cannot be constrained. The errors increase in proportion to the cloud
distance. Assuming the resolution of Herschel instruments, the model filaments
are barely resolved at a distance of ~400 pc and the errors in the parameters
of the Plummer function are several tens of per cent. The Plummer parameters,
in particular the power-law exponent p, are sensitive to noise but can be
determined with good accuracy using Herschel data. One must be cautious about
possible line-of-sight confusion. In our models, a large fraction of the
filaments seen in the column density maps are not continuous structures in
three dimensions.Comment: 12 pages, 14 figures, accepted to A&

### A Corona Australis cloud filament seen in NIR scattered light II: Comparison with sub-millimeter data

We study a northern part of the Corona Australis molecular cloud that
consists of a filament and a dense sub-millimetre core inside the filament. Our
aim is to measure dust temperature and sub-mm emissivity within the region. We
also look for confirmation that near-infrared (NIR) surface brightness can be
used to study the structure of even very dense clouds. We extend our previous
NIR mapping south of the filament. The dust colour temperatures are estimated
using Spitzer 160um and APEX/Laboca 870um maps. The column densities derived
based on the reddening of background stars, NIR surface brightness, and thermal
sub-mm dust emission are compared. A three dimensional toy model of the
filament is used to study the effect of anisotropic illumination on
near-infrared surface brightness and the reliability of dust temperature
determination. Relative to visual extinction, the estimated emissivity at 870um
is kappa(870) = (1.3 +- 0.4) x 10^{-5} 1/mag. This is similar to the values
found in diffuse medium. A significant increase in the sub-millimetre
emissivity seems to be excluded. In spite of saturation, NIR surface brightness
was able to accurately pinpoint, and better than measurements of the colour
excesses of background stars, the exact location of the column density maximum.
Both near- and far-infrared data show that the intensity of the radiation field
is higher south of the filament.Comment: 9 pages, 9 figures, accepted to A&

### Efficient Monte Carlo methods for continuum radiative transfer

We discuss the efficiency of Monte Carlo methods in solving continuum
radiative transfer problems. The sampling of the radiation field and
convergence of dust temperature calculations in the case of optically thick
clouds are both studied. For spherically symmetric clouds we find that the
computational cost of Monte Carlo simulations can be reduced, in some cases by
orders of magnitude, with simple importance weighting schemes. This is
particularly true for models consisting of cells of different sizes for which
the run times would otherwise be determined by the size of the smallest cell.
We present a new idea of extending importance weighting to scattered photons.
This is found to be useful in calculations of scattered flux and could be
important for three-dimensional models when observed intensity is needed only
for one general direction of observations. Convergence of dust temperature
calculations is studied for models with optical depths 10-10000. We examine
acceleration methods where radiative interactions inside a cell or between
neighbouring cells are treated explicitly. In optically thick clouds with
strong self-coupling between dust temperatures the run times can be reduced by
more than one order of magnitude. The use of a reference field was also
examined. This eliminates the need for repeating simulation of constant sources
(e.g., background radiation) after the first iteration and significantly
reduces sampling errors. The applicability of the methods for three-dimensional
models is discussed.Comment: submitted to A&A, 19 page

### Estimation of high-resolution dust column density maps: Empirical model fits

Sub-millimetre dust emission is an important tracer of density N of dense
interstellar clouds. One has to combine surface brightness information at
different spatial resolutions, and specific methods are needed to derive N at a
resolution higher than the lowest resolution of the observations. Some methods
have been discussed in the literature, including a method (in the following,
method B) that constructs the N estimate in stages, where the smallest spatial
scales being derived only use the shortest wavelength maps. We propose simple
model fitting as a flexible way to estimate high-resolution column density
maps. Our goal is to evaluate the accuracy of this procedure and to determine
whether it is a viable alternative for making these maps. The new method
consists of model maps of column density (or intensity at a reference
wavelength) and colour temperature. The model is fitted using Markov chain
Monte Carlo (MCMC) methods, comparing model predictions with observations at
their native resolution. We analyse simulated surface brightness maps and
compare its accuracy with method B and the results that would be obtained using
high-resolution observations without noise. The new method is able to produce
reliable column density estimates at a resolution significantly higher than the
lowest resolution of the input maps. Compared to method B, it is relatively
resilient against the effects of noise. The method is computationally more
demanding, but is feasible even in the analysis of large Herschel maps. The
proposed empirical modelling method E is demonstrated to be a good alternative
for calculating high-resolution column density maps, even with considerable
super-resolution. Both methods E and B include the potential for further
improvements, e.g., in the form of better a priori constraints.Comment: Accepted to A&

### On the stability of non-isothermal Bonnor-Ebert spheres. II. The effect of gas temperature on the stability

Aims. We investigate the stability of non-isothermal Bonnor-Ebert spheres
with a model that includes a self-consistent calculation of the gas
temperature. This way we can discard the assumption of equality between the
dust and gas temperatures, and study the stability as the gas temperature
changes with chemical evolution of the gas.
Methods. We use a gas-grain chemical model including a time-dependent
treatment of depletion onto grain surfaces, which strongly influences the gas
temperature as the main coolant, CO, depletes from the gas. Dust and gas
temperatures are solved with radiative transfer. For comparison with previous
work, we assume that the cores are deeply embedded in a larger external
structure, corresponding to visual extinction $A_{\rm V}^{\rm ext}=10$ mag.
Results. We find that the critical non-dimensional radius $\xi_1$ derived
here is similar to our previous work where we assumed $T_{\rm dust}=T_{\rm
gas}$; the $\xi_1$ values lie below the isothermal critical value
$\xi_0\sim6.45$, but the difference is less than 10%. Chemical evolution does
not affect notably the stability condition of low-mass cores (<0.75 $M_\odot$).
For higher masses the decrease of cooling owing to CO depletion causes
substantial temporal changes in the temperature and density profiles of the
cores. In the mass range 1-2 $M_\odot$ , $\xi_1$ decreases with chemical
evolution, whereas above 3 $M_\odot$ , $\xi_1$ instead increases. We also find
that decreasing $A_{\rm V}^{\rm ext}$ increases the gas temperature especially
when the gas is chemically old, causing $\xi_1$ to increase with respect to
models with higher $A_{\rm V}^{\rm ext}$. The derived $\xi_1$ values are close
to $\xi_0$. The density contrast between the core center and edge varies
between 8 to 16 depending on core mass and the chemical age of the gas,
compared to the constant value $\sim$ 14.1 for the isothermal BES.Comment: 7 pages, 5 figures; accepted for publication in A&A; abstract
(heavily) abridged for arXi

### Radiative transfer on hierarchial grids

We present new methods for radiative transfer on hierarchial grids. We
develop a new method for calculating the scattered flux that employs the grid
structure to speed up the computation. We describe a novel subiteration
algorithm that can be used to accelerate calculations with strong dust
temperature self-coupling. We compute two test models, a molecular cloud and a
circumstellar disc, and compare the accuracy and speed of the new algorithms
against existing methods. An adaptive model of the molecular cloud with less
than 8 % of the cells in the uniform grid produced results in good agreement
with the full resolution model. The relative RMS error of the surface
brightness <4 % at all wavelengths, and in regions of high column density the
relative RMS error was only 10^{-4}. Computation with the adaptive model was
faster by a factor of ~5. The new method for calculating the scattered flux is
faster by a factor of ~4 in large models with a deep hierarchy structure, when
images of the scattered light are computed towards several observing
directions. The efficiency of the subiteration algorithm is highly dependent on
the details of the model. In the circumstellar disc test the speed-up was a
factor of two, but much larger gains are possible. The algorithm is expected to
be most beneficial in models where a large number of small, dense regions are
embedded in an environment with a lower mean density.Comment: Accepted to A&A; 13 pages, 8 figures; (v2: minor typos corrected

- â€¦