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

Astronomy and Astrophysics (Impact Factor: 4.48). 02/2012; 542. DOI: 10.1051/0004-6361/201118420
Source: arXiv

ABSTRACT 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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    ABSTRACT: We investigate the far-infrared (IR) dust emission for 20 local star forming galaxies from the Key Insights on Nearby Galaxies: A Far-IR Survey with Herschel (KINGFISH) sample. We model the far-IR/submillimeter spectral energy distribution (SED) using images from Spitzer Space Telescope and Herschel Space Observatory. We calculate the cold dust temperature (T(cold)) and emissivity (beta) on a pixel by pixel basis (where each pixel ranges from 0.1-3 kpc^2) using a two temperature modified blackbody fitting routine. Our fitting method allows us to investigate the resolved nature of temperature and emissivity variations by modeling from the galaxy centers to the outskirts (physical scales of ~15-50 kpc, depending on the size of the galaxy). We fit each SED in two ways: (1) fit T(cold) and beta simultaneously, (2) hold beta constant and fit T(cold). We compare T(cold) and beta with star formation rates (calculated from L(Halpha) and L(24)), the luminosity of the old stellar population (traced through L(3.6), and the dust mass surface density (traced by 500 micron luminosity, L(500)). We find a significant trend between SFR/L(500) and T(cold), implying that the flux of hard UV photons relative to the amount of dust is significantly contributing to the heating of the cold, or diffuse, dust component. We also see a trend between L(3.6)/L(500) and beta, indicating that the old stellar population contributes to the heating at far-IR/submillimeter wavelengths. Finally, we find that when beta is held constant, T(cold) exhibits a strongly decreasing radial trend, illustrating that the shape of the far-IR SED is changing radially through a galaxy, thus confirming on a sample almost double in size the trends observed in Galametz et al. (2012).
    The Astrophysical Journal 05/2014; 789(2). DOI:10.1088/0004-637X/789/2/130 · 6.28 Impact Factor
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    ABSTRACT: We present a new 3.3 mm continuum map of the OMC-2/3 region. When paired with previously published maps of 1.2mm continuum and NH3-derived temperature, we derive the emissivity spectral index of dust emission in this region, tracking its changes across the filament and cores. We find that the median value of the emissivity spectral index is 0.9, much shallower than previous estimates in other nearby molecular clouds. We find no significant difference between the emissivity spectral index of dust in the OMC-2/3 filament and the starless or protostellar cores. Furthermore, the temperature and emissivity spectral index, beta, are anti-correlated at the 4 sigma level. The low values of the emissivity spectral index found in OMC-2/3 can be explained by the presence of millimeter-sized dust grains in the dense regions of the filaments to which these maps are most sensitive. Alternatively, a shallow dust emissivity spectral index may indicate non-powerlaw spectral energy distributions, significant free-free emission, or anomalous microwave emission. We discuss the possible implications of millimeter-sized dust grains compared to the alternatives.
    Monthly Notices of the Royal Astronomical Society 08/2014; 444(3). DOI:10.1093/mnras/stu1596 · 5.23 Impact Factor
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    ABSTRACT: Context. Using observations to deduce dust properties, grain size distribution, and physical conditions in molecular clouds is a highly degenerate problem. Aims. The coreshine phenomenon, a scattering process at 3.6 and 4.5 μm that dominates absorption, has revealed its ability to explore the densest parts of clouds. We want to use this effect to constrain the dust parameters. The goal is to investigate to what extent grain growth (at constant dust mass) inside molecular clouds is able to explain the coreshine observations. We aim to find dust models that can explain a sample of Spitzer coreshine data. We also look at the consistency with near-infrared data we obtained for a few clouds. Methods. We selected four regions with a very high occurrence of coreshine cases: Taurus–Perseus, Cepheus, Chameleon and L183/L134. We built a grid of dust models and investigated the key parameters to reproduce the general trend of surface bright- nesses and intensity ratios of both coreshine and near-infrared observations with the help of a 3D Monte-Carlo radiative transfer code. The grid parameters allow to investigate the effect of coagulation upon spherical grains up to 5 μm in size derived from the DustEm diffuse interstellar medium grains. Fluffiness (porosity or fractal degree), ices, and a handful of classical grain size distributions were also tested. We used the near– and mostly mid–infrared intensity ratios as strong discriminants between dust models. Results. The determination of the background field intensity at each wavelength is a key issue. In particular, an especially strong background field explains why we do not see coreshine in the Galactic plane at 3.6 and 4.5 μm. For starless cores, where detected, the observed 4.5 μm / 3.6 μm coreshine intensity ratio is always lower than ∼0.5 which is also what we find in the models for the Taurus–Perseus and L183 directions. Embedded sources can lead to higher fluxes (up to four times greater than the strongest starless core fluxes) and higher coreshine ratios (from 0.5 to 1.1 in our selected sample). Normal interstellar radiation field conditions are sufficient to find suitable grain models at all wavelengths for starless cores. The standard interstellar grains are not able to reproduce observations and, due to the multi-wavelength approach, only a few grain types meet the criteria set by the data. Porosity does not affect the flux ratios while the fractal dimension helps to explain coreshine ratios but does not seem able to reproduce near–infrared observations without a mix of other grain types. Conclusions. Combined near– and mid–infrared wavelengths confirm the potential to reveal the nature and size distribution of dust grains. Careful assessment of the environmental parameters (interstellar and background fields, embedded or nearby reddened sources) is required to validate this new diagnostic.
    Astronomy and Astrophysics 07/2014; 572. DOI:10.1051/0004-6361/201424081 · 4.48 Impact Factor

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