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 $mu$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.

Dust properties inside molecular clouds from coreshine modeling and observations

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 $mu$m 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 $mu$m 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 $mu$m. For starless cores, where detected, the observed 4.5 $mu$m / 3.6 $mu$m coreshine intensity ratio is always lower than $sim$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.

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.