Focus Meeting 10 - Abstracts


Heterogeneous chemistry on nano dust in the terrestrial and planetary atmospheres (including Titan)

Plane, John

Cosmic dust particles are produced in the solar system from the sublimation of comets as they orbit close to the sun, and also from asteroidal collisions between Mars and Jupiter. Recent advances in interplanetary dust modelling provide much improved estimates of the fluxes of cosmic dust particles into planetary (and lunar) atmospheres throughout the solar system. Combining the dust particle size and velocity distributions with a new chemical ablation model enables the injection rates of individual elements to be predicted as a function of location and time. This information is essential for understanding a variety of atmospheric impacts, including the formation of layers of metal atoms and ions, the subsequent production of meteoric smoke particles, and the role of these particles in ice cloud nucleation and heterogeneous chemistry. Specific examples that will be discussed are: in the terrestrial atmosphere, the formation of mesospheric and stratospheric ice clouds, and polar vortex chemistry; for Venus, the oxidation of CO and removal of O2 on meteoric smoke particles in the hot troposphere; for Mars, production of an Mg+ layer which has recently been observed by the MAVEN spacecraft, and the formation of metal carbonate-rich ice particles which nucleate CO2 clouds in the Martian mesosphere; and for Titan, the production of benzene in the troposphere by the cyclo-trimerization of acetylene on dust particles.

Nano dust and the far-ultraviolet interstellar extinction

Jiang, Biwei

The continuing rise of extinction in the far-ultraviolet (shortward of about 2000 angstrom) is observed in the Milky Way, and the nearby galaxies including the Magellanic Clouds and M31. This phenomenon indicates the presence of very small (~200 angstrom) particles in the interstellar medium. Yet, the carrier(s) responsible for this far-UV rising has not been certainly identified. Some candidates are suggested, from large molecules to very small dust grains. This talk will review the far-UV extinction laws in various environments, both Galatic and extra-galactic, and the dust models to explain the laws.

Dusty plasma interactions near the Moon and in the system of Mars

Popel, Sergey

We present results of recent self-consistent studies which consider dust and dusty plasmas at the Moon and in the system of Mars. These studies are associated with the future space missions Luna-25 and Luna-27 as well as Phobos-Grunt 2 and ExoMars 2020. The dusty plasma system over the Moon includes charged dust, photoelectrons, and electrons and ions of the solar wind and Earth's magnetosphere. The electrostatically ejected dust population can exist in the near-surface layer over the Moon while the dust appearing in the lunar exosphere owing to impacts of meteoroids present everywhere. Dusty plasmas are shown to be formed in the surface layer over the illuminated part of Mars' satellites Phobos and Deimos owing to photoelectric and electrostatic processes. In view of a weak gravitational field, dust particles rising over the surfaces of Phobos and Deimos are larger than those over the surface of the Moon. In this case, the role of adhesion, which is a significant process preventing the separation of dust particles from the lunar surface, is much smaller on Phobos and Deimos. We discuss also dusty plasmas in Martian atmosphere. This work was supported by the Russian Foundation for Basic Research (project no. 18-02-00341).

Low-temperature formation of silicate and oxide nano-dust

Kimura, Yuki

  Fundamental physical and chemical properties of dust are essential to understand whole processes of material evolution in a history of the universe.  Especially, surface free energy and sticking probability have large uncertainty despite critical parameters to establish dust formation model based on nucleation theories.  Nucleation from vapor to solid has a large hindrance because of disadvantage for creation of a new surface.  To overcome the large barrier for phase transition, larger supersaturation is required.  Especially, in a gas outflow of late-type stars, dust is only able to form at very low-temperature compared with thermal equilibrium because of rareness of solid materials for heterogeneous nucleation.  Recent years, we have tackled to know how dust forms in such extreme condition; how large supersaturation is required, how different physical properties they have in an environment far from thermal equilibrium, and whether dust formation follows classical nucleation theory or multistep nucleation.  Our microgravity experiments using sounding rockets gave us following results.  The sticking probability of Fe to be solid from supersaturated gas is as low as 0.002% against 1 as conservatively thought [1].  Formation of alumina dust around oxygen-rich late-type stars and its 13 µm feature was successfully duplicated by a specially designed experimental system.  Our laboratory experiments using an in-situ IR measurement system of dust analogues during nucleation and growth succeeded reproduction of the spectrum of astronomical silicate with Mg-bearing silicate particles and found two step crystallization process that a liquid droplet form from a supersaturated gas at first and, then, forsterite nucleates and grows from the supercooled droplet [2].  These findings provide a solid basis for elaborating models of condensation of dust in the universe.[1] Kimura, et al., Science Advances, 3 (2017) e1601992.[2] shizuka, Kimura, Sakon, ApJ, 803 (2015) 88.

Nano dust in stellar atmospheres and winds

Höfner, Susanne

The extended dynamical atmospheres of cool luminous giant stars are places where solid particles condense out of the gas. This stardust leaves its marks on the observable stellar spectra, and also on the structure and dynamics of the stellar atmospheres, since radiation pressure on the newly-formed grains is a key factor for driving the massive winds of evolved stars. In recent years, considerable progress has been made in understanding these processes, and in characterizing the properties of the dust particles. In particular, improvements in high-angular-resolution techniques have led to spatially resolved observations of the dust-forming atmospheric layers of close-by cool giant stars, making detailed comparisons with predictive models possible. I will give an overview of these developments, including results from both observations and numerical simulations.

C60 in interstellar and circumstellar environments

Cami, Jan

In recent years, fullerenes (and in particular C60) have been detected in a variety of astrophysical environments – from the circumstellar carbon-rich surroundings of evolved stars to interstellar reflection nebulae and young stellar objects. Understanding how these species form, evolve and respond to their environment yields important insights into astrochemistry and the characteristics of large aromatics in space, thought to be the main reservoir of organic material in space.In this talk, I will present an overview of what we have learned about cosmic fullerenes from multi-wavelength astronomical observations, theoretical calculations and recent laboratory experiments, and show how fullerenes have significantly changed our understanding of interstellar chemistry. I will discuss the conditions that appear to be conducive to the formation and/or detection of fullerenes, and highlight some of the difficulties we still face in understanding the formation of fullerenes, especially in planetary nebulae.

Nano dust as a possible cause of hot emission in planetary debris disks

Su, Kate

Planetary debris disks are tenuous disks consisting of dust replenished by collisions of leftover planetesimals and cometary activity, events that are driven through gravitational shepherding and stirring by planets. Their large surface area makes these disks detectable through infrared thermal emission and/or optical scattered light, providing insights into the nature of unseen minor-body populations and the underlying planetary architecture. The majority of the disks show warm and cold dust emission in a structure analogous to that of minor body belts in the solar system with asteroid- and/or Kuiper-belt components. A significant fraction of main sequence stars observed interferometrically in the near-infrared have extended excess emission that has been attributed to very small (<200 nm) hot dust in the vicinity of the stars. Various mechanisms have been proposed to explain the origins of these nanograins, particularly with regard to how they are retained in the presence of significant radiative pressure forces around early-type stars. Similar to the nanograins in the inner solar system, these nano particles are likely the products of (1) sublimation from stellar grazing planetesmials, (2) photoelectric charged by the stellar wind, and (3) magnetically trapped by the magnetic field of the star. I will review recent observational and theoretical studies of nanograins in planetary debris disks, and discuss future observational tests in these systems.

Experiments on the formation of carbonaceous dust and molecules

Jäger, Cornelia

Carbonaceous dust is present in nearly all astrophysical environments proving its existence by the interstellar extinction, IR spectra, and elemental depletion patterns. Dust grains absorb and scatter stellar light and reemit the absorbed energy at infrared and millimeter wavelengths. They affect the formation rate of H2 and organic molecules being formed on the surface or in ice layers covering grain surfaces in the interstellar medium (ISM). Asymptotic giant branch stars and supernovae (SNe) are major dust factories. However, dust grains can be efficiently processed and completely destroyed in SN shocks and have to be re-formed in the ISM. In order to understand all dust-related astrochemical processes, laboratory experiments are desperately required. Experimental studies on the nucleation and condensation of carbonaceous dust have demonstrated that carbon nanoparticle and molecules, including polycyclic aromatic hydrocarbons (PAHs) and fullerenes, condense in gas-phase condensation processes at high temperature. In addition, recent experimental studies have demonstrated that organic molecules and carbonaceous solids can also be formed at conditions prevailing in molecular clouds. Refractory fullerene-like carbon particles were formed in cryogenic ice layers at about 10 K.  In the talk, I will compare experimental results on the formation pathways of carbonaceous matter at high and low temperatures and discuss the role of precursors and chemically matured molecules such as PAHs, carbon chains, and fullerenes. The formation of refractory carbonaceous grains is either governed by PAHs or by chain-like carbon molecules. However, carbon grains can also be a source for the formation of carbonaceous molecules. Reactions on the surface or at the interface between dust and ice may lead to the erosion of dust and formation of new molecular species.

Fullerenes and PAHs in the Interestellar Medium

iglesias groth, susana

We will review theoretical work, astronomical observations and measurements inlaboratory of the new form of carbon known as fullerenes and their  hydrogenated forms (fulleranes). These molecules can be responsible for diffuse interstellar bands, the UV "bump", main feature in the extinction curves observed in many lines of vision of our Galaxy and other galaxies and the anomalous microwave emission discovered in several regions of star formation, in molecular clouds and HII regions. Recent detections of C60 and C70 fullerenes in planetary nebulae in our Galaxy and in the Magellanic cloud reinforce the hypothesis that fullerenes and fulleranes are common in the interstellar medium and could contribute significantly to these processes. Other potential agents of anomalous microwave emission processes and interstellar extinction bands are polycyclic aromatic hydrocarbons (PAHs). We will summarize the efforts made to achieve the identification of the simplest PAHs, naphthalene and anthracene, in regions of anomalous microwave emission and the results I have obtained on the search for fullerenes in protoplanetary discs.

Spatially resolved studies of Diffuse Interstellar Bands in Galaxies outside of the Local Group

Monreal Ibero, Ana

Diffuse Interstellar Bands (DIBs) are non-stellar weak absorption features of unknown origin found in the spectra of different astronomical objects when they are viewed through one or several clouds of Interstellar Medium. Galaxies other than ours offer the opportunity of study the behavior of DIBs under physical (e.g. radiation field) and chemical (e.g. metallicity and relative abundances) different to those typically found in the Milky Way. This can in turn, put further constrains on the nature of the agents creating these features. Because of their weakness, studies targeting extragalactic DIBs are relatively scarce. This is a research that will certainly blossom at the E-ELT era. However, we can already start paving the way.In this talk, we will illustrate how MUSE can help us in this quest. We will use as examples some results on two highly reddened systems. In the first one, AM 1353-272, we established a gradient of DIB strength in a galaxy at more than 150 Mpc (Monreal-Ibero et al. 2015, A&A, 576, 3). In the second one, The Antennae Galaxy, we measured the strength of the l5780 and l5797 DIBs in more than 100 independent line of sights, thus mapping these DIBs for the first time in a system outside the Local Group (Monreal-Ibero et al. 2017, A&A, in press). The distribution of DIB strength was compared with that of atomic hydrogen, molecular gas, and PAHs as traced by the emission in the mid-infrared. In both cases, DIB stregth correlates well with extinction, similar to results for the Milky Way.

Mixed Aromatic Aliphatic organic nanoparticles (MAON) as carriers of unidentified infrared emission bands

Kwok, Sun

The unidentified infrared emission (UIE) phenomenon consists of a family of emission bands, broad emission plateaus, all superimposed on an underlying continuum.  While the emission bands are almost certainly due to the stretching and bending modes of aromatic and aliphatic groups, the exact vibrational modes of these bands and the chemical structure of the carrier are not known.  We report results of quantum chemistry calculations of large (>100 carbon atoms) molecules with mixed aromatic/aliphatic structures with the goal of identifying the origin of the UIE bands and explore various possibilities of the chemical nature of the UIE carrier.

Small-scale clustering of nano-dust grains in turbulence

Mattsson, Lars

We investigate the clustering and dynamics of nano-sized particles (nano-dust) in high-resolution (1024^3) simulations of homogeneous isothermal hydrodynamic turbulence. It is well established that large grains will decouple from a turbulent gas flow, while small grains will tend to trace the motion of the gas. However, small grains may still cluster in a turbulent flow (small-scale clustering), which increases the rate of grain-grain interaction. In combination with the fact that nano-dust grains may be abundant, and the increased interaction rate due to turbulent motions, aggregation of nano-dust in, e.g., interstellar clouds, may be quite efficient. We conclude by a brief discussion of charged nano-dust grains how they are different to the passive-scalar type dust in the present simulations.

Orion Bar as a window to evolution of small carbonaceous dust grains

Murga, Maria

The Orion Bar is one of the most well-known photodissociation regions (PDR). An enormous volume of observational data in various spectral ranges makes the Orion Bar a versatile tool for checking theoretical ideas. Specifically, it allows studying small carbonaceous grains, which reveal themselves through mid-infrared (IR) emission bands. Their lifecycle strongly depends on the external conditions that vary dramatically within this object. Thus it is possible to follow the evolutionary changes of dust at different conditions within a single object. Observational variations of the mid-IR bands (at 3.3-3.4, 6.2, 7.7, 8.6, 11.3 mkm) and abundance of some small hydrocarbons (C2H, C4H,c-C3H2, l-C3H, etc.) indicate changes in dust size, ionization stage and fraction of grains with aliphatic bonds. We utilize theoretical modeling of photo-processing of carbonaceous grains to interpret these evolutionary clues. Specifically, we consider photo-destruction of hydrogenated amorphous carbon (HAC) grains that causes restructuring of aliphatic-rich material to aromatic-rich material under ultraviolet radiation, formation of species like polycyclic aromatic hydrocarbons and the smallest hydrocarbons like C2H and finally the decrease of the average grain size. The dehydrogenation of aromatic-rich grains and ionization stage of grains along with corresponding changes in IR-spectra are taken into consideration. We construct synthetic maps of emission in the observed mid-IR bands across the Orion Bar and compare them with observed maps available in archives. We make conclusions about efficiency of photo-destruction processes in PDRs and about the abundance of HAC grains.

Dust formation from vapor through multistep nucleation in the astrophysical environments

Tanaka, Kyoko K.

Cosmic dust grains are believed to form in outflows in the late stages of evolution of stars such as AGB stars and supernovae.  The condensation and crystallization processes are important for understanding the origin of cosmic dusts and have seen by various observations.  For instance, the silicate dusts condense in outflows with amorphous structure, as evidenced by the broad and smooth appearance of around 9.7 micron spectrum of silicate.  Some observations suggest an increase in the fraction of crystalline as it cools from an intrinsic change in optical properties of the dust (Waters et al. A&A 315, L361,1996).  Despite the transition from vapor to solid is a familiar process, the process is not fully understood yet.  One reason is that size of nuclei is usually very small (< nm) and the properties of nuclei are poorly understood.  In the study, we present molecular dynamics (MD) simulations of vapor-to-solid phase transition with a simple potential model (Lennard-Jones type) and discuss the transition process.  In the simulations, the nuclei of supercooled liquid appear and growth. After the growth of nuclei, the crystallizations of supercooled nano-clusters are observed and the crystallized nano clusters have various structures of metastable phase (Tanaka et al.  Phys. Rev. E 96, 022804, 2017).  Our simulations indicate that the vapor-to-solid transition occurs through multistep nucleation which is vapor-to-liquid nucleation (first step nucleation) and crystallization in the supercooled liquid droplets (second step nucleation), even though the temperature is much lower than the triple temperature. Recent experimental studies support the multiple processes of nucleation for various substances including silicate materials (Ishizuka et al. ApJ, 803:88, 2015).  Our results with the experiments indicate that the multistep nucleation is a common phenomenon in the first stage of condensation from vapor to solid in the astrophysical environments.

Spatial distribution of the fullerene C60 in the planetary nebula IC418

Manchado, Arturo

We have studied the spatial distribution of the fullereneC60 in the planetary nebula IC 418, and compare with the dust and PAH emission structure.A ring-like extended structure (at a distance of 6300 AU) is seen at all IR wavelengths.However, after continuum subtraction the dust continuumemission at 9.8 micron , peaks close to the central star while the broad 9-13 micron emissiontogether with the PAH emission show a clear ring-like extended emission.On the contrary the C60 17.4 micron, emission is mainly located at the northeast,extending from the central star to the outer regions of the nebula.This 17.4 micron fullerene emission may be a photo-product of the 9-13 micron carrier or containperhaps contribution from other fullerene-based species like hydrogenated fullerenes.

Iron dust growth in the Galactic interstellar medium: clues from element depletions

Zhukovska, Svitlana

Iron is severely depleted from the interstellar gas compared to interstellar silicon. We address a long-standing question ``Where is the missing interstellar iron?'' using a model of dust evolution inhomogeneous, multiphase interstellar medium based on hydrodynamic simulations. The model includes dependence of dust destruction in SN shocks and growth by accretion of gas-phase metals on local physical conditions. In order to reproduce the observed trend of interstellar Fe depletion with gas density, our model requires that solid iron resides in two dust components: (i) metallic iron nanoparticles with sizes in the range of 1—10 nm and (ii) small inclusions in silicate grains.

Carbon Nano Dust in the Interstellar Medium: Nanodiamonds, Graphene, and Carbon Nanotubes

Li, Aigen

Carbon is exclusively formed in the hot interiors of stars through the fusion reactions of three alpha particles (i.e., helium nuclei) and expelled into the interstellar medium (ISM) through stellar outflows and/or supernova explosions in the late stages of stellar evolution. As the fourth most abundant element in the universe and due to its unique property to form three different types of chemical bonds through sp1, sp2, and sp3 hybridizations, carbon can be stabilized in various multi-atomic structures with different molecular configurations (i.e., allotropes), including amorphous carbon, graphite, diamond, polycyclic aromatic hydrocarbon (PAH), fullerenes, graphene, and carbon nanotubes (CNTs).In this presentation I will focus on nanodiamonds, graphene, and CNTs. I will present (1) our DFT calculations of the electronic and vibrational transitions of graphene and CNTs as a function of carbon atoms (NC), (2) the infrared (IR) emission spectra of nanodiamonds, graphene and CNTs which are stochastically excited by single photons in the ISM, and (3) the possible contribution of nanodiamonds, graphene and CNTs to the UV interstellar extinction. The model-calculated UV extinction and IR emission spectra of nanodiamonds, graphene and CNTs will then be compared with the astronomical observations, allowing us to constrain the abundances of these nano species in the ISM. The possible connection of graphene and CNTs with the mysterious diffuse interstellar bands (DIBs) will also be examined.

Polycyclic Aromatic Hydrocarbons in Protoplanetary Disks

Seok, Ji Yeon

Protoplanetary disks (PPDs) where planet formation takes place frequently emanate unidentified infrared emission (UIE) features in their infrared (IR) spectra. Major UIE features appear at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 µm, which are commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules. PAHs play crucial roles in the evolution of PPDs physically and chemically. Exposed to ultraviolet and visible photons from the central star, PAHs re-emit the absorbed photons through their vibrational relaxation, including the C-H stretching mode at 3.3 µm, C-C stretching at 6.2 and 7.7 µm, C-H in-plane bending at 8.6 µm, and C-H out-of-plane bending at 11.3 and 12.7 µm. The relative strength of the PAH emission bands prominently vary depending on the physical properties of PAHs such as their size (NC) and charge state (fion), which sensitively reflect the physical conditions of PPDs. We present model calculations to quantitatively interpret the PAH features observed in various PPDs, taking a wide range of both stellar properties (effective temperature and luminosity) and PAH properties (size, charge state, and radial distance from a central star) into account. The range of effective temperature considered is between 3,500 K and 30,000K, and PAH sizes are 16< NC < 4000. In particular, we generate grid diagrams of the 6.2 µm to 7.7 µm band ratio versus the 11.3 µm to 7.7 µm band ratio as a diagnostic tool, which allow us to directly compare observed PAH band ratios with the model calculations and to infer the PAH size and charge state as well as the local physical conditions where PAHs reside. We discuss applications to observational data and characteristics of PAHs in PPDs.

Processing of nano dust particles in galaxies

Onaka, Takahsi

Dust grains control thermal balance and chemistry in the interstellar medium (ISM) and their infrared emission is often used as a useful tracer of the star-formation.  Therefore, the understanding of the lifecycle of dust grains in the ISM is vital for the study of star-formation and galaxy evolution.  While several theoretical studies have investigated the lifecycle of dust grains, observational studies of dust processing are scarce.  Smallest grains, or nano dust particles, are thought to be most vulnerable to the environmental conditions and thus a good indicator for the dust processing.  A family of emission bands in the near- to mid-infrared are attributed to nano-sized dust containing polycyclic aromatic hydrocarbons (PAHs) or PAH-like atomic groups.  Thus, the lifecycle of PAHs in the ISM is particularly interesting.  However, even the formation site of the band carriers (hereafter PAHs) remains unclear.  Recent observations of the PAH emission in nearby galaxies, NGC1569, NGC2782, and NGC 7727, with the AKARI satellite indicate that PAHs may be formed by fragmentation of larger carbonaceous grains (Onaka et al. 2010, A&A, 514, A15; Onaka et al. 2018, ApJ, 853, 31).  In addition, observations of merger galaxies show the presence of PAHs and the paucity of nano-sized dust particle larger than PAHs (very small grains, hereafter VSGs) in extended structures formed by the merger events, suggesting that PAHs may be produced by fragmentation of VSGs (Onaka et al. 2018).  Comparison with the dust model further suggests that present models cannot explain the mid-infrared PAH emission and the very faint emission at 24µm properly.  The emissivity of PAHs in these harsh conditions may need to be revised.  In this presentation, we report the latest results of AKARI observations and discuss the properties of nano-dust particles and dust processing in harsh environments in galaxies.

Formation and interaction of nano dust in planetary debris discs

Mann, Ingrid

The circum-stellar planetary debris discs are produced from planetesimals in a similar way as asteroids and comets produce the interplanetary dust. The debris disc dust is typically observed by thermal emission in mid-infrared and located at large distance from the star, comparable to the solar system’s Kuiper belt. Similar to the solar system, it is produced by dust-dust collisions, but with higher rates and the dust clouds are denser. Most of the small dust particles are pushed away from the vicinity of the star by radiation pressure. Some stars show, however thermal emission spectra that suggest the existence of nm-sized dust relatively close to the star (i.e. hot debris discs). This presentation addresses the formation of this dust in the collisional fragmentation process, the dynamics including electromagnetic forces and the thermal emission brightness. The findings are compared to the dust in the solar system, especially in the vicinity of the Sun, the region that will be explored with the space missions Solar Orbiter and Parker Solar Probe. 

Dust dynamics in adaptative-mesh-refinement codes : application to protostellar collapses.

Lebreuilly, Ugo

The interstellar medium is essentially composed of gas and a small amount of dust,  with  an average dust-to-gas ratio of 1% and a grain size distribution well approximated by power laws (Mathis et al, 1977). In dense regions, such as the molecular clouds or the prestellar cores, these properties are not well constrained because dust dynamics is particularly affected by pressure and density gradients.  Most of current studies do not consider a possible variation of dust-to-gas in these objects, supposing that dust is frozen in the gas. I will present an implementation of dust and gas mixture  dynamics in the adaptive-mesh-refinement code RAMSES (Lebreuilly & Commerçon, In. prep.). The method use the monofluid formalism in the diffusion approximation. I will show the validation  tests of our implementation, i.e., the dustyshock, dustywave or dusty-diffusion tests (Laibe & Price, 2011). Finally I will present a first application to protostellar collapse and disk formation.

The role of alumina in triggering stellar outflows

Gobrecht, David

An important dust component in AGB stars is aluminum oxide or alumina (stoichiometric formula Al2O3) showing a spectral emission feature around ~13 µm attributed to Al-O streching and bending modes (Posch et al. 1999, Sloan et al. 2003). Alumina presolar grains are also found in pristine meteorites with typical sizes of a few tens of nm to µms (Stroud et al. 2004). Owing to their refractory nature (thermal stability) and the large abundance of Al- and O-bearing compounds, alumina grains are thought to represent the first condensates to emerge in the atmospheres of AGB stars. In the bulk phase, alumina exists predominantly in two crystalline forms (corundum and clay). The properties of nanoparticles with sizes below ~50 nm, however, differ significantly from bulk properties. Quantum and surface effects of these small particles lead to non-crystalline structures, whose characteristics (geometry, coordination, density, energy) may differ by orders of magnitude, compared to the bulk material. A top-down approach, like classical nucleation theory, is thus not applicable.Therefore, we follow a bottom-up approach, starting with the smallest stoichiometric clusters (Al2O3, Al4O6). Then, we successively build up larger-sized clusters. We present the results of the quantum-mechanical structure calculations of (Al2O3)n clusters with n=1-10, including potential energies, rotational constants, charge distributions and structure-specific infrared spectra (vibrational frequencies and intensities). The vibrational IR spectra can be compared directly with observations and laboratory experiments on meteorites. Moreover, a vibration analysis is required to construct an appropriate partition function (including the vibrational contribution). The latter is needed to accurately compute the thermodynamic potentials (enthalpy, entropy and Gibbs free energy) in circumstellar conditions (p=10-5 - 10 Pa, T=500 - 6000 K) that differ significantly from standard conditions (p=105 Pa, T=298 K).

Constraining dust properties in circumstellar envelopes of carbon stars in the Magellanic Clouds: optical constants and grain size of carbon dust

Nanni, Ambra

In galaxies with sub-solar metallicity, such as the Magellanic Clouds (MCs), a large fraction of the thermally pulsing asymptotic giant branch (TP-AGB) stars evolve through the carbon-rich phase (C-stars), shaping the near and mid-infrared (NIR and MIR) colours of the resolved stellar populations. Specifically, the spectra of C-stars are largely affected by the presence of carbon dust that condenses in their circumstellar envelopes (CSEs).The study of dust growth and radiative transfer in the CSEs of these stars allows us to investigate the properties of carbon dust.The main uncertainties in the input physics of such radiative transfer models are related to the choice of the grain size distribution and of the optical constants for carbon dust. The former cannot be directly derived from observations, while for the latter several sets of lab measurements, very different from each other, are available.The results obtained by using different combinations of those inputs can be tested against the observations of thousands of C-stars in the MCs, providing constraints on the carbon dust properties.To achieve this goal, we follow dust growth and the outflow acceleration, coupled with a radiative transfer code, through the CSEs of C-stars evolving along some selected TP-AGB tracks, for which we compute spectra and colours.By requiring our models to simultaneously reproduce several observed infrared colour-colour diagrams, we found the best agreement with the observations of the MCs for nano dust particles of sizes between 0.035-0.1 microns, rather than by larger grains, of 0.2-0.7 microns. The inability of large grains to reproduce NIR and MIR colours is independent of the adopted optical data set and the deviations between models and observations tend to increase for increasing grain sizes.We also identify a possible trend between the grain size or grain structure (more graphite-like or diamond like) and the mass-loss rate.

The 30 micron sources in galaxies with different metallicities

Szczerba, Ryszard

I will present an analysis and comparison of the 30 micron dust features seen in the Spitzer Space Telescope spectra of 207 carbon-rich Asymptotic Giant Branch (AGB) stars, post-AGB objects, and planetary nebulae located in the Milky Way, the Magellanic Clouds or the Sagittarius Dwarf Spheroidal galaxy, which are characterised by different average metallicities. Our analysis uses the ``Manchester method'' as a basis for estimating the temperature of dust for the carbon-rich AGB stars and the planetary nebulae in our sample. For post-AGB objects we changed the wavelength ranges used for temperature estimation, because of the presence of the 21 micron feature on the short wavelength edge of the 30 micron feature. We have found that the average feature is identical for different metallicities, for AGB and post-AGB stars, while is shifted to significantly longer wavelengths in case of planetary nebulae.  We have produced online catalogues of photometric data and Spitzer IRS spectra for all objects that show the 30 micron feature. These resources are available online.

Dust Cloud at Mars

Andersson, Laila

Mars has a rich dust environment, including incoming interstellar dust and dust lofted up from the atmosphere. There is no indication that the moons of Mars contribute significantly to the dust environment. The conclusion that some of the observed dust originates from the atmosphere is based on observations of dust plumes with densities up to ~2 #/m3 extending to altitudes of 1000 km. All observations presented here are from electric field instrument measurements, which provide only limited information about dust origins.  Further complicating the measurements, the spacecraft frequency moves from high-density plasma to low-density plasma changing the signatures of dust detection. This presentation will discuss how to separate interplanetary dust from planetary dust and present the time variation of the observed dust over the ~4 years of MAVEN spacecraft operation around Mars. 

Formation, Processing and Properties of Nano-Silicate Dust: A Bottom-up Computional Modelling Approach

Bromley, Stefan

Nanosilicates, although thought to be highly abundant [1], are difficult to characterise by experiment and observation due to their small size and corresponding non-bulk-like properties. We present a complementary approach to understanding the formation, structure and properties of nanosized silicate dust grains based on an atomistic bottom-up computational modelling [2]. Our approach is independent of assumptions based on bulk materials properties, is not limited to any particular chemistry or specific thermodynamic conditions, and provides a solid basis for kinetic modelling.Our approach can provide detailed and quantitative insights in three important areas. First, we provide a realistic account of circumstellar heteromolecular silicate dust formation [3] while highlighting deficiencies in classical nucleation theory accounts based on SiO aggregation [4]. Second, we give insights into the reactivity of nanosilicates with respect to their role as ice condensation nuclei [5,6] and their influence on H2 formation/dissociation [6,7]. Finally, we show how we can explicitly model detailed process of nucleation and growth from molecular precursors (e.g. SiO, Mg, H2O) of nanosilicates with diameters of up to 50 nm. A direct link between our models (with 10s to 1000s of atoms) and observation can be made via computing infrared spectra directly from the atomic vibrational modes in the nanoparticles.[1] Draine, B. T.; Li, A. Astrophys. J. 551 (2001) 807.[2] Bromley, S. T. ; Goumans, T. P. M. ; Herbst, E. ; Jones A. P.; Slater, B. Phys. Chem. Chem. Phys.16 (2014) 18623.[3] Goumans, T. P. M.; Bromley, S. T., Mon. Not. R. Astron. Soc. 420 (2012) 3344.[4] Gomez, J. C.; Plane, J. C.; Bromley, S. T.; Phys. Chem. Chem. Phys. 18 (2016) 26913.[5] Goumans, T. P. M.; Bromley, S. T., Mon. Not. R. Astron. Soc. 414, 1285 (2011).[6] Kerkeni, B; Bacchus-Montabonel, M-C.; Bromley, S. T., Mol. Astrophys. 7, (2017) 1.[7] Kerkeni, B.; Bromley, S T. Mon. Not. R. Astron. Soc. 435 (2013) 1486. 

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