V. Guillet

Dust coagulation processes as constrained by far-infrared observations

Aims. We develop a simple model of coagulated dust particles of two sizes (3.5 and 60 nm radius) to understand the nature and the effects of coagulation, which could explain the evolution of the far-infrared (FIR) dust opacity observed in the transition between the diffuse and the dense interstellar medium (ISM) (nH > 10 3 cm −3 ). Methods. Using the discrete-dipole approximation (DDA) method, we have calculated the absorption coefficient, directly proportional to the opacity, of coagulated grains with varying numbers of sub-grains and of different grain composition. Results. We show that, in the transition from diffuse to dense clouds, an increase in the FIR opacity by a factor of about 2.7 is possible and a decrease in the grain temperature by up to 3−4 K can be explained by the presence of coagulated aggregates composed of four big grains and 4000 very small grains (40% of the volume of the BGs). The coagulation of very small grains into the aggregates leads to a decrease in the 60 μm emission. Conclusions. This model can explain the observed increase in opacity at long wavelengths, the decrease in temperature from the diffuse ISM to denser regions with the coagulation of grains into aggregates and the absence of the 60 μm emission with the coagulation of very small grains onto the surface of the big grains.

Shocks in dense clouds - III. Dust processing and feedback effects in C-type shocks

Aims. We study the impact of grain-grain collisions in C-type shocks propagating in dense clouds (n H ≥ 10 4 cm -3 ) on the evolution of the dust size distribution, the shock dynamics and the release of chemical species into the gas phase such as SiO. Methods. Our shock code for transverse C-type shocks is extended to self-consistently couple the charge, dynamics and evolution of the size distributions of silicate and carbon grains with the shock dynamics. Dust processes included are sputtering in gas-grain collisions, vaporisation and shattering in grain-grain collisions. Results. Grain shattering and its feedback onto the dynamics of C-type shocks is found to be only significant at densities higher than ∼10 5 cm -3 . Numerous small grains are produced in the shock through the fragmentation of large grains. C shocks are therefore shorter and warmer when shattering is included. Vaporisation is more efficient than sputtering at destroying grain cores. In particular, vaporisation destroys dust at low shock velocities where sputtering is inefficient (20-25 km s -1 ). Unlike sputtering, vaporisation produces SiO early in the shock, which may affect the SiO emission line profiles. All these effects are found to be negligible below a density of ∼10 4 cm -3 .

Shocks in dense clouds - I. Dust dynamics

Aims. A new multi-fluid approach to the dust dynamics in transverse shocks in dense clouds is presented with the aim of modelling the dust processing in C- and J-type shocks. Methods. We have augmented an existing steady-state shock code to include the effects of an MRN size distribution of grain cores with icy mantles. The dust charge distribution and its evolution is considered in detail and included in the ionization balance. The 2-D grain dynamics are determined, including the effects of grain inertia and charge fluctuations, paying particular attention to the gyration of the charged grains around the magnetic field lines and the feedback of the ionization state on grain dynamics. Results. We find that the critical velocity for C shocks increases with the gas density but that it is only weakly dependent on a high abundance of PAHs and on the photodetachment of electrons by secondary photons induced by cosmic-rays. The detailed dust dynamics in C shocks is shown to comprise two distinct phases: 1) a short gyration phase followed by 2) a long term drift phase. In J shocks only the first gyration phase is present. In C shocks propagating through molecular clouds (n H = 10 4 cm -3 ), large grains (»100 A) remain coupled to the magnetic field during the second phase. However, a high abundance of PAHs can lead to a shortage of electrons in the gas and the decoupling of large grains in the shock tail. Large grains are decoupled from the magnetic field all through the C shock in high density clouds (n H = 10 6 cm -3 ). In all C shocks small grains (≃100 A) remain strongly coupled to the magnetic field, whereas very small grains («100 A) are subject to stochastic dynamics. As long as they are charged very small grains remain strongly coupled to the magnetic field but tend to couple to the neutral gas everytime they become neutral. We have investigated the effects of an electric field along the shock direction in C shocks and find that it does not significantly modify the relative velocities between grains. The derived grain dynamics can be used to study dust processing in C and J shocks in dense clouds through the effects of gas-grain and grain-grain collisions.

Aggregate dust connections and emissivity enhancements

Context. Observations of cold condensed clouds in the interstellar medium show an enhancement in the dust emissivity at long wavelengths. Model calculations with the discrete-dipole approximation (DDA) can explain this enhancement with the coagulation of dust particles into aggregates. Aims. We study the nature of grain-grain contacts and their effects on the aggregate optical properties. Methods. We use DDA and the T-matrix method (TMM) to calculate the absorption properties of aggregate dust grains and analyse where and why the enhancement in the emissivity occurs. Results. We find that the absorption coefficient changes with material composition and with the contact area between monomers. A larger contact area, with DDA, compared to a zero-point contact with TMM, results in an enhancement of the absorption coefficient for wavelengths where the considered material has a large value n (the real part of the refractive index). Conclusions. DDA seems to be the most realistic way of taking into account “real” inter-particle contact effects in aggregate particles.

Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium

We compare the performance of several dust models in reproducing the dust spectral energy distribution (SED) per unit extinction in the diffuse interstellar medium (ISM). We use our results to constrain the variability of the optical properties of big grains in the diffuse ISM, as published by the Planck collaboration. We use two different techniques to compare the predictions of dust models to data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared emission spectrum to recover the dust extinction and the intensity of the interstellar radiation field (ISRF). Second, we infer the ISRF intensity from the total power emitted by dust per unit extinction, and then predict the emission spectrum. In both cases, we test the ability of the models to reproduce dust emission and extinction at the same time. We identify two issues. Not all models can reproduce the average dust emission per unit extinction: there are differences of up to a factor

Polarization measurements analysis II. Best estimators of polarization fraction and angle

\sim2

Shocks in dense clouds - IV. Effects of grain-grain processing on molecular line emission

between models, and the best accord between model and observation is obtained with the more emissive grains derived from recent laboratory data on silicates and amorphous carbons. All models fail to reproduce the variations in the emission per unit extinction if the only variable parameter is the ISRF intensity: this confirms that the optical properties of dust are indeed variable in the diffuse ISM. Diffuse ISM observations are consistent with a scenario where both ISRF intensity and dust optical properties vary. The ratio of the far-infrared opacity to the

The cycling of carbon into and out of dust

V

Dust models compatible with Planck intensity and polarization data in translucent lines of sight

band extinction cross-section presents variations of the order of

Interplay of dust alignment, grain growth, and magnetic fields in polarization: lessons from the emission-to-extinction ratio

\sim20\%