Direct evidence of dust growth in L183 from MIR light scattering

Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission). We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction AV map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree agree with the AV map and compare them to the Spitzer data. The Spitzer data of L183 show emission in the 3.6 and 4.5 micron bands, while the 5.8 micron band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 micron, which is not seen. Hence, we interpret this emission to be MIR cloudshine. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 micron.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and Astrophysic

On the thermal behaviour of small iron grains

The optical properties of small spherical iron grains are derived using a Kramers-Kronig-consistent model of the dielectric function including its dependence on temperature and size. Especially discussed is the effect of the size dependence, which results from the limitation of the free path of the free electrons in the metal by the size of the grain, on the absorption behaviour of small iron spheres and spheroids. The estimated absorption properties are applied to study the temperature behaviour of spherical and spheroidal grains which are heated by the interstellar radiation field.Comment: 12 pages, 16 figure

The Slope of the Near Infrared Extinction Law

We determine the slope of the near infrared extinction power law (A$_{\lambda} \propto \lambda^{-\alpha}$) for 8 regions of the Galaxy between l$\sim27^{\circ}$ and $\sim100^{\circ}$. UKIDSS Galactic Plane Survey data are compared, in colour-colour space, with Galactic population synthesis model data reddened using a series of power laws and convolved through the UKIDSS filter profiles. Monte Carlo simulations allow us to determine the best fit value of $\alpha$ and evaluate the uncertainty. All values are consistent with each other giving an average extinction power law of $\alpha$=2.14$^{+0.04}_{-0.05}$. This is much steeper than most laws previously derived in the literature from colour excess ratios, which are typically between 1.6 and 1.8. We show that this discrepancy is due to an inappropriate choice of filter wavelength in conversion from colour excess ratios to $\alpha$ and that effective rather than isophotal wavelengths are more appropriate. In addition, curved reddening tracks, which depend on spectral type and filter system, should be used instead of straight vectors.Comment: Accepted by MNRAS: 11/08/09. 13 pages, 10 figures, 2 table

The discrete dipole approximation for periodic targets I. theory and tests

The discrete-dipole approximation (DDA) is a powerful method for calculating absorption and scattering by targets that have sizes smaller than or comparable to the wavelength of the incident radiation. The DDA can be extended to targets that are singly- or doubly-periodic. We generalize the scattering amplitude matrix and the 4 x 4 Mueller matrix to describe scattering by singly- and doubly-periodic targets, and show how these matrices can be calculated using the DDA. The accuracy of DDA calculations using the open-source code DDSCAT is demonstrated by comparison to exact results for infinite cylinders and infinite slabs. A method for using the DDA solution to obtain fields within and near the target is presented, with results shown for infinite slabs.Comment: 19 pages, 7 figures, submitted to J. Opt. Soc. Am.

Radiative properties of visible and subvisible Cirrus: Scattering on hexagonal ice crystals

One of the main objectives of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) is to provide a better understanding of the physics of upper level clouds. The focus is on just one specific aspect of cirrus physics, namely on characterizing the radiative properties of single, nonspherical ice particles. The basis for further more extensive studies of the radiative transfer through upper level clouds is provided. Radiation provides a potential mechanism for strong feedback between the divergence of in-cloud radiative flux and the cloud microphysics and ultimately on the dynamics of the cloud. Some aspects of ice cloud microphysics that are relevant to the radiation calculations are described. Next, the Discrete Dipole Approximation (DDA) is introduced and some new results of scattering by irregular crystals are presented. The Anomalous Diffraction Theory (ADT) was adopted to investigate the scattering properties of even larger crystals. In this way the scattering properties of nonspherical particles were determined over a range of particle sizes

The Discovery of Vibrationally-Excited H_2 in the Molecular Cloud near GRB 080607

GRB 080607 has provided the first strong observational signatures of molecular absorption bands toward any galaxy hosting a gamma-ray burst. Despite the identification of dozens of features as belonging to various atomic and molecular (H_2 and CO) carriers, many more absorption features remained unidentified. Here we report on a search among these features for absorption from vibrationally-excited H_2, a species that was predicted to be produced by the UV flash of a GRB impinging on a molecular cloud. Following a detailed comparison between our spectroscopy and static, as well as dynamic, models of H_2* absorption, we conclude that a column density of 10^{17.5+-0.2} cm^{-2} of H_2* was produced along the line of sight toward GRB 080607. Depending on the assumed amount of dust extinction between the molecular cloud and the GRB, the model distance between the two is found to be in the range 230--940 pc. Such a range is consistent with a conservative lower limit of 100 pc estimated from the presence of Mg I in the same data. These distances show that substantial molecular material is found within hundreds of pc from GRB 080607, part of the distribution of clouds within the GRB host galaxy.Comment: Submitted to ApJL, 6 pages emulate

Dust Emission from Evolved and Unevolved HII Regions in the Large Magellanic Cloud

We present a study of the dust properties of 12 classical and superbubble HII regions in the Large Magellanic Cloud. We use infrared photometry from Spitzer (8, 24, 70, and 160 \mum bands), obtained as part of the Surveying the Agents of a Galaxy's Evolution (SAGE) program, along with archival spectroscopic classifications of the ionizing stars to examine the role of stellar sources on dust heating and processing. Our infrared observations show surprisingly little correlation between the emission properties of the dust and the effective temperatures or bolometric magnitudes of stars in the HII regions, suggesting that the HII region evolutionary timescale is not on the order of the dust processing timescale. We find that the infrared emission of superbubbles and classical HII regions shows little differentiation between the two classes, despite the significant differences in age and morphology. We do detect a correlation of the 24 \mum emission from hot dust with the ratio of 70 to 160 \mum flux. This correlation can be modeled as a trend in the temperature of a minority hot dust component, while a majority of the dust remains significantly cooler.Comment: 15 pages, 5 figures. Accepted to Ap