Modelling the dust emission from dense interstellar clouds: Disentangling the effects of radiative transfer and dust properties

Astronomy and Astrophysics (Impact Factor: 4.38). 02/2012; 542. DOI: 10.1051/0004-6361/201118420
Source: arXiv


With Planck and Herschel, we now have the spectral coverage and angular
resolution required to observe dense and cold molecular clouds. As these clouds
are optically thick at short wavelength but optically thin at long wavelength,
it is tricky to conclude anything about dust properties without a proper
treatment of the radiative transfer (RT). Our aim is to disentangle the effects
of RT and of dust properties on the variations in the dust emission to provide
observers with keys to analyse the emission arising from dense clouds. We model
cylindrical clouds, illuminated by the ISRF, and carry out full RT
calculations. Dust temperatures are solved using DustEM for amorphous carbons
and silicates, grains coated with carbon mantles, and mixed aggregates of
carbon and silicate. We allow variations of the grain optical properties with
wavelength and temperature. We determine observed colour temperatures, T, and
emissivity spectral indices, beta, by fitting the dust emission with modified
blackbodies, to compare our models with observations. RT effects can neither
explain the low T nor the increased submm emissivity measured at the centre of
dense clouds, nor the observed beta-T anti-correlation. Adding noise to the
modelled data, we show that it is not likely to be the unique explanation for
the beta-T anti-correlation observed in starless clouds. It may be explained by
intrinsic variations in the grain optical properties with temperature. As for
the increased submm emissivity and the low T, they have to originate in
variations in the grain optical properties, probably caused by their growth to
form porous aggregates. We find it difficult to track back the nature of grains
from the spectral variations in their emission. Finally, the column density is
underestimated when determined with blackbody fitting because of the
discrepancy between T and the true dust temperature at the cloud centre.

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Available from: Nathalie Ysard, Feb 20, 2014
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    ABSTRACT: 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.
    Astronomy and Astrophysics 03/2012; 541(0004-6361). DOI:10.1051/0004-6361/201118596 · 4.38 Impact Factor
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    ABSTRACT: We present an analysis of the dust and gas in Andromeda, using Herschel images sampling the entire far-infrared peak. We fit a modified-blackbody model to ~4000 quasi-independent pixels with spatial resolution of ~140pc and find that a variable dust-emissivity index (beta) is required to fit the data. We find no significant long-wavelength excess above this model suggesting there is no cold dust component. We show that the gas-to-dust ratio varies radially, increasing from ~20 in the center to ~70 in the star-forming ring at 10kpc, consistent with the metallicity gradient. In the 10kpc ring the average beta is ~1.9, in good agreement with values determined for the Milky Way (MW). However, in contrast to the MW, we find significant radial variations in beta, which increases from 1.9 at 10kpc to ~2.5 at a radius of 3.1kpc and then decreases to 1.7 in the center. The dust temperature is fairly constant in the 10kpc ring (ranging from 17-20K), but increases strongly in the bulge to ~30K. Within 3.1kpc we find the dust temperature is highly correlated with the 3.6 micron flux, suggesting the general stellar population in the bulge is the dominant source of dust heating there. At larger radii, there is a weak correlation between the star formation rate and dust temperature. We find no evidence for 'dark gas' in M31 in contrast to recent results for the MW. Finally, we obtained an estimate of the CO X-factor by minimising the dispersion in the gas-to-dust ratio, obtaining a value of (1.9+/-0.4)x10^20 cm^-2 [K kms^-1]^-1.
    The Astrophysical Journal 04/2012; 756(1). DOI:10.1088/0004-637X/756/1/40 · 5.99 Impact Factor
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    ABSTRACT: With recent Herschel observations, the northern filament of the Corona Australis cloud has now been mapped in a number of bands from 1.2um to 870um. The data set provides a good starting point for the study of the cloud over several orders of magnitude in density. We wish to examine the differences of the column density distributions derived from dust extinction, scattering, and emission, and to determine to what extent the observations are consistent with the standard dust models. From Herschel data, we calculate the column density distribution that is compared to the corresponding data derived in the near-infrared regime from the reddening of the background stars, and from the surface brightness attributed to light scattering. We construct three-dimensional radiative transfer models to describe the emission and the scattering. The scattered light traces low column densities of A_V~1mag better than the dust emission, remaining useful to A_V ~ 10-15 mag. Based on the models, the extinction and the level of dust emission are surprisingly consistent with a sub-millimetre dust emissivity typical of diffuse medium. However, the intensity of the scattered light is very low at the centre of the densest clump and this cannot be explained without a very low grain albedo. Both the scattered light and dust emission indicate an anisotropic radiation field. The modelling of the dust emission suggests that the radiation field intensity is at least three times the value of the normal interstellar radiation field. The inter-comparison between the extinction, light scattering, and dust emission provides very stringent constraints on the cloud structure, the illuminating radiation field, and the grain properties.
    Astronomy and Astrophysics 06/2012; 544. DOI:10.1051/0004-6361/201219084 · 4.38 Impact Factor
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