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Register a new userDust/ice mixing in cold regions and solid-state water in the diffuse interstellar medium
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Whether ice in cold cosmic environments is physically separated from the silicate dust or mixed with individual silicate moieties is not known. However, different grain models give very different compositions and temperatures of grains. The aim of the present study is a comparison of the mid-IR spectra of laboratory silicate-grains/water-ice mixtures with astronomical observations to evaluate the presence of dust/ice mixtures in interstellar and circumstellar environments. The laboratory data can explain the observations assuming reasonable mass-averaged temperatures for the protostellar envelopes and protoplanetary disks demonstrating that a substantial fraction of water ice may be mixed with silicate grains. Based on the combination of laboratory data and infrared observations, we provide evidence of the presence of solid-state water in the diffuse interstellar medium. Our results have implications for future laboratory studies trying to investigate cosmic dust grain analogues and for future observations trying to identify the structure, composition, and temperature of grains in different astrophysical environments.
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Supersonic turbulence is a large reservoir of suprathermal energy in the interstellar medium. Its dissipation, because it is intermittent in space and time, can deeply modify the chemistry of the gas. We further explore a hybrid method to compute the chemical and thermal evolution of a magnetized dissipative structure, under the energetic constraints provided by the observed properties of turbulence in the cold neutral medium. For the first time, we model a random line of sight by taking into account the relative duration of the bursts with respect to the thermal and chemical relaxation timescales of the gas. The key parameter is the turbulent rate of strain a due to the ambient turbulence. With the gas density, it controls the size of the dissipative structures, therefore the strength of the burst. For a large range of rates of strain and densities, the models of turbulent dissipation regions (TDR) reproduce the CH+ column densities observed in the diffuse medium and their correlation with highly excited H2. They do so without producing an excess of CH. As a natural consequence, they reproduce the abundance ratios of HCO+/OH and HCO+/H2O, and their dynamic range of about one order of magnitude observed in diffuse gas. Large C2H and CO abundances, also related to those of HCO+, are another outcome of the TDR models that compare well with observed values. The abundances and column densities computed for CN, HCN and HNC are one order of magnitude above PDR model predictions, although still significantly smaller than observed values.
We study infrared emission of 17 isolated, diffuse clouds with masses of order solar masses, to test the hypothesis that grain property variations cause the apparently low gas-to-dust ratios that have been measured in those clouds. Maps of the clouds were constructed from WISE data and directly compared to the maps of dust optical depth from Planck. The mid-infrared emission per unit dust optical depth has a significant trend toward lower values at higher optical depths. The trend can be quantitatively explained by extinction of starlight within the clouds. The relative amounts of PAH and very small grains traced by WISE, compared to large grains tracked by Planck, are consistent with being constant. The temperature of the large grains significantly decreases for clouds with larger dust optical depth; this trend is partially due to dust property variations but is primarily due to extinction of starlight. We updated the prediction for molecular hydrogen column density, taking into account variations in dust properties, and find it can explain the observed dust optical depth per unit gas column density. Thus the low gas-to-dust ratios in the clouds are most likely due to `dark gas that is molecular hydrogen.
We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, linear and circular polarisation of light. The model is set-up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500nm. Dust grains with radii larger than 6nm are spheroids. Spheroidal dust particles have a factor 1.5 - 3 larger absorption cross section in the far IR than spherical grains of the same volume. Mass estimates derived from submillimeter observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified, so that absolute element abundances are not direct input parameters. The model is confronted with the average properties of the Milky Way, which seems to represent dust in the solar neighbourhood. It is then applied to four specific sight lines including the reflection nebula NGC2023. For these sight lines, we present linear and circular spectro-polarimetric observations obtained with FORS/VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributor to the polarisation.
This paper presents the first results of comparison of Planck along with IRAS data with Green Bank Telescope 21-cm observations in 14 fields covering more than 800 square degrees at high Galactic latitude. Galactic dust emission for fields with average HI column density lower than 2 x 10^20 cm^-2 is well correlated with 21-cm emission. The residual emission in these fields, once the HI-correlated emission is removed, is consistent with the expected statistical properties of the cosmic infrared background fluctuations. Fields with larger column densities show significant excess dust emission compared to the HI column density. Regions of excess lie in organized structures that suggest the presence of hydrogen in molecular form, though they are not always correlated with CO emission. Dust emission from intermediate-velocity clouds is detected with high significance. Its spectral properties are consistent with, compared to the local ISM values, significantly hotter dust (T~20 K), lower sub-millimeter dust opacity, and a relative abundance of very small grains to large grains about four times higher. These results are compatible with expectations for clouds that are part of the Galactic fountain in which there is dust shattering and fragmentation. Correlated dust emission in HVCs is not detected; the average of the 99.9% confidence upper limits to the emissivity is 0.15 times the local ISM value at 857 and 3000 GHz, in accordance with gas phase evidence for lower metallicity and depletion in these clouds. Unexpected anti-correlated variations of the dust temperature and emission cross-section per H atom are identified in the local ISM and IVCs, a trend that continues into molecular environments. This suggests that dust growth through aggregation, seen in molecular clouds, is active much earlier in the cloud condensation and star formation processes.
The Planck-HFI all-sky survey from 353 to 857GHz combined with the 100 microns IRAS show that the dust properties vary in the diffuse ISM at high Galactic latitude (1e19<NH<2.5e20 H/cm2). Our aim is to explain these variations with changes in the ISM properties and grain evolution. Our starting point is the latest core-mantle dust model. It consists of small aromatic-rich carbon grains, larger amorphous carbon grains with aliphatic-rich cores and aromatic-rich mantles, and amorphous silicates with Fe/FeS nano-inclusions covered by aromatic-rich carbon mantles. We explore whether variations in the radiation field or in the gas density distribution in the diffuse ISM could explain the observations. The dust properties are also varied in terms of mantle thickness, Fe/FeS inclusions, carbon abundance, and size distribution. Variations in the radiation field intensity and gas density distribution cannot explain the observed variations but radiation fields harder than the standard ISRF may participate in creating part of them. We further show that variations in the grain mantle thickness coupled with changes in the grain size distribution can reproduce most of the observations. We put a limit on the mantle thickness of the silicates (~10-15nm), and find that aromatic-rich mantles are needed for the carbon grains (at least 5-7.5nm thick). We also find that changes in the carbon abundance in the grains could explain part of the observed variations. Finally, we show that varying the composition of Fe/FeS inclusions in the silicates cannot account for the variations. With small variations in the dust properties, we are able to explain most of the variations in the dust emission observed by Planck-HFI in the diffuse ISM. We also find that the small realistic changes in the dust properties that we consider almost perfectly match the anti-correlation and scatter in the observed beta-T relation.
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