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The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov–Gerasimenko, has been detecting individual dust particles of mass larger than 10−10 kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time. The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at sizes >1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from models of 67P's trail. At sizes

IOP Publishing [Society Publisher] Silicon is present in interstellar dust grains, meteorites and asteroids, and to date 13 silicon-bearing molecules have been detected in the gas phase toward late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those in the atmosphere of evolved stars. We have found that the chemistry of Si, H, and H2 efficiently produces silane (SiH4), disilane (Si2H6) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected toward the carbon-rich star IRC +10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H2 are a plausible source of silane in C-rich asymptotic giant branch stars, although its contribution to the total SiH4 abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH4 and Si2H6 at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si–O–Si and Si–OH groups at the expense of SiH moieties, which implies that if this kind of grain is present in the interstellar medium, it will probably be processed into silicates through the interaction with water ices covering the surface of dust grains. We thank the European Research Council for funding support under Synergy grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). Also, we acknowledge partial support from the Spanish MINECO through grants MAT2017-85089-c2-1R, FIS2016-77726-C3-1-P, FIS2016-77578-R, AYA2016-75066-C2-1-P and RyC-2014-16277. Support from the FotoArt-CM Project (P2018/NMT 4367) through the Program of R&D activities between research groups in Technologies 2013, cofinanced by European Structural Funds, is also acknowledged. G.T.C. acknowledges funding from the Comunidad Autonoma de Madrid (PEJD-2018-PRE/IND-9029). Peerreview

13 pags., 12 figs., 2 tabs., Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch stars (AGBs). In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene, are the most abundant species after H and CO. In a previous study, we addressed the chemistry of carbon (C and C) with H showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene, and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C) with CH. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene, and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a nonnegligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in the gas phase are incorporated into nanometric-sized dust analogs, which consist of a complex mixture of sp, sp, and sp hydrocarbons with amorphous morphology., We thank the European Research Council for funding support under Synergy grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). Also, partial support from the Spanish Research Agency (AEI) through grants MAT2017-85089-c2- 1R, FIS2016-77578-R, FIS2016-77726-C3-1-P, AYA2016- 75066-C2-1-P, and RyC-2014-16277 is acknowledged. Support from the FotoArt-CM Project (P2018/NMT 4367) through the Program of R&D activities between research groups in Technologies 2013, co-financed by European Structural Funds, is also acknowledged. G.TC. acknowledges funding from the Comunidad Autónoma de Madrid (PEJD-2018-PRE/ IND-9029)