Context. The evolution of amorphous hydrocarbon materials, a-C(:H), principally resulting from ultraviolet (UV) photon absorptioninduced processing, are likely at the heart of the variations in the observed properties of dust in the interstellar medium. Aims. The consequences of the size-dependent and compositional variations in a-C(:H), from aliphatic-rich a-C:H to aromatic-rich a-C, are studied within the context of the interstellar dust extinction and emission. Methods. Newly-derived optical property data for a-C(:H) materials, combined with that for an amorphous forsterite-type silicate with iron nano-particle inclusions, a-Sil Fe , are used to explore dust evolution in the interstellar medium.Results. We present a new dust model that consists of a power-law distribution of small a-C grains and log-normal distributions of large a-Sil Fe and a-C(:H) grains. The model, which is firmly anchored by laboratory-data, is shown to quite naturally explain the variations in the infrared (IR) to far-ultraviolet (FUV) extinction, the 217 nm UV bump, the IR absorption and emission bands and the IR-mm dust emission. Conclusions. The major strengths of the new model are its inherent simplicity and built-in capacity to follow dust evolution in interstellar media. We show that mantle accretion in molecular clouds and UV photo-processing in photo-dominated regions are likely the major drivers of dust evolution.

Context. The dust properties inferred from the analysis of Planck observations in total and polarized emission challenge current dust models. Aims. We propose new dust models compatible with polarized and unpolarized data in extinction and emission for translucent lines of sight (0.5 < A V < 2.5). Methods. We amended the DustEM tool to model polarized extinction and emission. We fit the spectral dependence of the mean extinction, polarized extinction, SED and polarized SED with PAHs, astrosilicates and amorphous carbon (a-C). The astrosilicate population is aligned along the magnetic field lines, while the a-C population may be aligned or not.Results. With their current optical properties, oblate astrosilicate grains are not emissive enough to reproduce the emission to extinction polarization ratio P 353 /p V derived with Planck data. Models using prolate astrosilicate grains with an elongation a/b = 3 and an inclusion of 20% of porosity succeed. The spectral dependence of the polarized SED is steeper in our models than in the data. Models perform slightly better when a-C grains are aligned. A small (6%) volume inclusion of a-C in the astrosilicate matrix removes the need for porosity and perfect grain alignment, and improves the fit to the polarized SED.Conclusions. Dust models based on astrosilicates can be reconciled with Planck data by adapting the shape of grains and adding inclusions of porosity or a-C in the astrosilicate matrix. 1 The version of DustEM with polarization can be downloaded from http://www.ias.u-psud.fr/DUSTEM. 2 http://dustemwrap.irap.omp.eu 1. R P/p = P 353 /p V = [5.4 ± 0.5] MJy sr −1 ,

Context. Observations of SiO line emission in shocks in star-forming regions indicate that silicate dust destruction must be occurring in these dense regions. Current models rely on predictions for dust destruction by sputtering in C-type shock waves. However, J-type shocks may also be relevant for interpreting the widely-observed optical line emission from species such as O I and Fe II. Aims. In this work we explore, for the first time, dust destruction in J-type shocks slower than 50 km s −1 . Methods. We follow the dust trajectories throughout the shock using a model for the dust dynamics that allows us to solve the shock structure and at the same time calculate the degree of dust processing. We include the effects of sputtering in gas-grain collisions, and vaporisation and shattering in grain-grain collisions. Results. We find that the amount of silicon released into the gas phase is a few percent. The dominant destructive process is vaporisation, not sputtering. The degree of dust destruction increases with the shock velocity but decreases as the preshock density increases. Conclusions. Our results compare well with that of C-type shock models. J-type shocks are therefore reasonable candidates for an interpretation of SiO line emission in molecular outflows and jets.

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 Å) 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 Å) remain strongly coupled to the magnetic field, whereas very small grains ( 100 Å) 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.

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) (n H > 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.

Observational evidence seems to indicate that the depletion of interstellar carbon into dust shows rather wide variations and that carbon undergoes rather rapid recycling in the interstellar medium (ISM). Small hydrocarbon grains are processed in photo-dissociation regions by UV photons and by ion and electron collisions in interstellar shock waves and by cosmic rays. A significant fraction of hydrocarbon dust must therefore be re-formed by accretion in the dense, molecular ISM. A new dust model [Jones et al., A&A, 2013, 558, A62] shows that variations in the dust observables, in the diffuse interstellar medium (n H ≤ 10 3 cm −3 ), can be explained by systematic and environmentally-driven changes in the small hydrocarbon grain population. Here we explore the consequences of gas-phase carbon accretion onto the surfaces of grains in the transition regions between the diffuse ISM and molecular clouds [e.g., Jones, A&A, 2013, 555, A39]. We find that significant carbonaceous dust re-processing and/or mantle accretion can occur in the outer regions of molecular clouds and that this dust will have significantly different optical properties from the dust in the adjacent diffuse ISM. We conclude that the (re-)processing and cycling of carbon into and out of dust is perhaps the key to advancing our understanding of dust evolution in the ISM.

Context. Dust emission is increasingly used as a tracer of the mass in the interstellar medium. With the combination of Planck and Herschel observatories, we now have both the spectral coverage and the angular resolution required to observe dense and cold molecular clouds. However, as these clouds are optically thick at short wavelengths but optically thin at long wavelengths, it is tricky to conclude anything about dust properties without a proper treatment of the radiative transfer. Aims. Our aim is to disentangle the effects of radiative transfer and dust properties on the variations in the dust emission at long wavelengths. This enables us to provide observers with tools to analyse the dust emission arising from dense clouds. Methods. We model cylindrical clouds with visual extinctions between 1 and 20 mag, illuminated by the standard interstellar radiation field, and carry out full radiative transfer calculations using a Monte Carlo code. Dust temperatures are solved using the DustEM code for amorphous carbons and silicates representative of dust at high Galactic latitude (DHGL), carbon and silicate grains coated with carbon mantles, and mixed aggregates of carbon and silicate. We also allow for variations in the optical properties of the grains with wavelength and temperature. We determine observed colour temperatures, T colour , and emissivity spectral indices, β colour , by fitting the dust emission with modified blackbodies using a standard χ 2 fitting method, in order to compare our models with observational results. Results. Radiative transfer effects can explain neither the low T colour , the increased submillimetre emissivity measured at the centre of dense clouds, nor the observed β colour − T colour anti-correlation for the models considered. Adding realistic noise to the modelled data, we show that it is unlikely to be the only explanation of the β colour − T colour anti-correlation observed in starless clouds, which may instead be explained by intrinsic variations in the grain optical properties with temperature. Similarly the higher submillimetre emissivity and the low T colour have to originate in variations in the grain optical properties, probably caused by their growth to form porous aggregates. We find it difficult to infer the nature of the grains from the spectral variations in their emission, owing to radiative transfer effects for λ 300 μm, and to the mixture of different grain populations for longer wavelengths. Finally, the column density is underestimated when determined with modified blackbody fitting because of the discrepancy between T colour and the "true" dust temperature in the innermost layers of the clouds.

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 .