Harald Mutschke

Aluminum Oxide and the Opacity of Oxygen-rich Circumstellar Dust in the 12-17 Micron Range

Amorphous alumina (Al2O3) was produced by a sol-gel technique in order to make available its optical constants for possible astrophysical applications. Gradual annealing showed that the X-ray amorphousness of alumina ended somewhere between 723 and 873 K. Above this transition point, the structure changes into disordered γ-Al2O3. At T > 1273 K, crystalline α-Al2O3 (corundum) is formed. Mie calculations show that amorphous alumina exhibits a wide Al-O vibrational band, peaking at 11.5-11.8 μm and having a steep blue and an extended red wing. It may be an important contributor to the continuous opacity between the silicate bands in oxygen-rich circumstellar envelopes, whereas it is ruled out for the explanation of the 13 μm band. An average 13 μm band profile was derived from 51 IRAS low-resolution spectra of bright Mira stars and semiregular variables. Its shape, which is satisfactorily represented by a Lorentz profile, can be reproduced by Mie calculations with the data of α-Al2O3, but not with those of γ-Al2O3. The calculations show that the 13 μm band profile of α-Al2O3 is sensitive to grain shape. If α-Al2O3 is the absorber, a second band should be present at 21 μm. A close correlation was found between the strengths of the 13 μm band and the 10 μm silicate band. It suggests that the 13 μm band carrier could also be somehow connected with silicate dust. Experimental arguments supporting this attribution are presented.

Steps toward interstellar silicate mineralogy - VII. Spectral properties and crystallization behaviour of magnesium silicates produced by the sol-gel method

Amorphous silicate particles are generally assumed to be the main dust component in the envelopes of oxygen-rich evolved stars and may be considered the precursors of the pure crystalline enstatite and forsterite particles detected by ISO. We present optical constants in the broad wavelength range 0.2-500 µm for a unique series of pure amorphous Mg-silicates (Mg/Si in the range 0.7-2.4). They have been prepared by the sol-gel process, a chemical technique based on the condensation of Mg- and Si-hydroxides in a liquid phase. The salient feature of these Mg-silicates is the very small content of Si-OH bonds in the silicate network, which considerably reduces the activation energy of crystallization and, thus, decreases the temperature threshold for crystallization as well as crystallization time. The astrophysical relevance of our sol-gel silicates is shown by a comparison of optically thin model spectra based on dust emissivities with ISO-SWS spectra of AGB stars and with 10 µm emission profiles of such stars obtained by ground-based spectroscopy. As paradigmatic cases of AGB spectra with respect to the appearance of the silicate bands, TY Dra (slender bands and deep trough between them) and R Cas (broad bands and widely filled-up trough) were used, for which ISO-SWS spectra are available. The dust emissivity derived from TY Dra can be excel- lently reproduced by the models, suggesting that the dust grains consist indeed of pure amorphous Mg-silicates. Satisfactory agreement was also found with the mean 10 µm profiles of some groups of AGB stars and supergiants. Spectra with strong dust emission in the silicate trough like R Cas require additional contributions by other dust components, probably oxides. A rough orientation on the spectral properties of such potential trough opacity contributors has been obtained by subtracting a pure silicate spectrum (TY Dra) from a spectrum with a nearly filled trough and a less pronounced 20 µm band (R Cas). In agreement with other amorphous silicates, the spectral index of the new silicate analogues amounts to −2.

DUst around NEarby Stars. The survey observational results

Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system counterparts are the asteroid and Edgeworth-Kuiper belts. Aims. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. Methods. We used Herschel/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 mu m were obtained, complemented in some cases with observations at 70 mu m, and at 250, 350 and 500 mu m using SPIRE. The observing strategy was to integrate as deep as possible at 100 mu m to detect the stellar photosphere. Results. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of similar to 12.1% +/- 5% before Herschel to similar to 20.2% +/- 2%. A significant fraction (similar to 52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70-160 mu m range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.

Matrix-isolated Nano-sized Carbon Grains as an Analog for the 217.5 Nanometer Feature Carrier

The effect of particle shape or particle clustering on the extinction behavior of nano-sized hydrogenated carbon grains is investigated experimentally. The particles were extracted by a molecular beam technique at different condensation and clustering states and isolated in an argon matrix for UV spectroscopy. The state of clustering in the samples was controlled by transmission electron microscopy analysis. The simple spherical morphology of the matrix-isolated nonagglomerated particles permitted the derivation of reliable optical constants. A clear correlation was found between the measured UV feature width and the degree of particle clustering, in agreement with theoretical investigations based on the derived optical data. Therefore, the results prove unambiguously the expectation that the optical properties of carbonaceous grain material are strongly influenced by the particle shape and the clustering degree. For particles produced in hydrogen-containing atmospheres, the UV extinction peak was shifted blueward to a position close to the 217.5 nm hump. This shift was found to be nearly independent of the amount of hydrogen in the condensation zone. We also discuss the infrared spectra of the hydrogenated carbon materials. The astrophysical implications of the results are discussed with regard to the observational as well as the elemental abundance constraints.

Formation of Polycyclic Aromatic Hydrocarbons and Carbonaceous Solids in Gas-Phase Condensation Experiments

Carbonaceous grains represent a major component of cosmic dust. In order to understand their formation pathways, they have been prepared in the laboratory by gas-phase condensation reactions such as laser pyrolysis and laser ablation. Our studies demonstrate that the temperature in the condensation zone determines the formation pathway of carbonaceous particles. At temperatures lower than 1700 K, the condensation by-products are mainly polycyclic aromatic hydrocarbons (PAHs) that are also the precursors or building blocks for the condensing soot grains. The low-temperature condensates contain PAH mixtures that are mainly composed of volatile three to five ring systems. At condensation temperatures higher than 3500 K, fullerene-like carbon grains and fullerene compounds are formed. Fullerene fragments or complete fullerenes equip the nucleating particles. Fullerenes can be identified as soluble components. Consequently, condensation products in cool and hot astrophysical environments such as cool and hot asymptotic giant branch stars or Wolf-Rayet stars should be different and should have distinct spectral properties.

TEMPERATURE DEPENDENCE OF THE SUBMILLIMETER ABSORPTION COEFFICIENT OF AMORPHOUS SILICATE GRAINS

We have measured mass absorption coefficients of amorphous silicate materials for wavelengths between 100 μm and 2 mm (5-100 cm-1) and at temperatures between 300 and 10 K. For both interstellar analog MgSiO3 and simple silica SiO2, we find evidence for a strong temperature and frequency dependence. We define two distinct wavelength regimes, 500 μm-1 mm and 100-250 μm, for which the absorption coefficient presents different trends with frequency. To evaluate this frequency dependence, we fit our absorption coefficient using two power laws with spectral index β that varies with temperature. We do not find a significant variation of β with temperature between 100 and 250 μm, whereas between 500 μm and 1 mm a pronounced anticorrelation between T and β exists. Globally, β-values decrease from 2.5 to 1.5 between 10 and 300 K. This anticorrelation for interstellar analog grains has the same trend as the one observed using the balloon-borne experiment PRONAOS. We show that physisorbed water is not responsible for the observed temperature and frequency dependence and that OH groups could be at the origin of the submillimeter properties of the materials. As discussed in the literature, OH groups are often related to tunneling processes in two-level systems (TLS). In the case of the more complex MgSiO3 silicates, TLS could also be produced by the Mg+2 ions, which act as network modifiers, similar to how they act with OH groups.

Identification of iron sulphide grains in protoplanetary disks

Sulphur is depleted in cold dense molecular clouds with embedded young stellar objects, indicating that most of it probably resides in solid grains. Iron sulphide grains are the main sulphur species in cometary dust particles, but there has been no direct evidence for FeS in astronomical sources, which poses a considerable problem, because sulphur is a cosmically abundant element. Here we report laboratory infrared spectra of FeS grains from primitive meteorites, as well as from pyrrhotite ([Fe, Ni]1-xS) grains in interplanetary dust, which show a broad FeS feature centred at ∼23.5 micrometres. A similar broad feature is seen in the infrared spectra of young stellar objects, implying that FeS grains are an important but previously unrecognized component of circumstellar dust. The feature had previously been attributed to FeO. The observed astronomical line strengths are generally consistent with the depletion of sulphur from the gas phase, and with the average Galactic sulphur/silicon abundance ratio. We conclude that the missing sulphur has been found.

Infrared Spectroscopy of Nano-sized Carbon Grains Produced by Laser Pyrolysis of Acetylene: Analog Materials for Interstellar Grains

The infrared spectroscopic behavior of nano-sized carbon grains produced by laser-driven pyrolysis of acetylene (C2H2) is presented with respect to the internal structure of the particles investigated by electron energy loss spectroscopy and high-resolution transmission electron microscopy. Carbon grain samples were synthesized at different condensation conditions, and the effect of the pyrolysis temperature and pyrolysis mode (pulsed versus continuous wave) on the carbon structure was investigated. The size distribution of the carbon clusters synthesized in the flow reactor was determined by means of time-of-flight mass spectroscopy. Despite the CH absorption features attributed to saturated aliphatic hydrocarbons adsorbed by the grain surfaces, the infrared spectra of the neat carbon grains show only weak CH features. The investigations show that the formation and growth of polycyclic aromatic structural units are involved in the carbon grain condensation process. We conclude that the low feature-to-continuum ratio in the IR spectra of the grains is a typical property of carbonaceous dust formed by the pyrolysis of acetylene. The lack of observational evidence for hydrocarbon dust in the outflows of carbon-rich asymptotic giant branch (AGB) stars can be rationalized by our spectroscopic results. An evolutionary sequence of the circumstellar carbonaceous material during the AGB to planetary nebula (PN) phase transition can be deduced from our results by comparison with the IR spectral behavior of carbonaceous grain materials synthesized in other condensation systems.

Infrared optical properties of spinels A study of the carrier of the 13, 17 and 32m emission features observed in ISO-SWS spectra of oxygen-rich AGB stars ?

In a previous paper, we have proposed magnesium aluminium spinel to be the carrier of the 13 and 17 m band features observed in the ISO spectra of some red giants. The IR optical properties of spinel strongly depend on its chemistry and its internal structure. To study the dependence of spinels IR-spectra on its aluminium content, we have synthesized a number of crystals with dierent Al/Mg-ratios. Additionally, we performed an annealing experiment to investigate the phase transition between ordered and partially disordered spinel taking place at about 1200 K. We derived sets of optical constants of our natural, annealed and synthetic spinels in order to calculate the absorption eciencies of small (sub-m-sized) spherical particles. Thereby, it turned out that natural as well as near-stoichiometric synthetic spinel can indeed be considered as a suitable candidate for the carrier of the 13m feature observed in the spectra of some oxygen rich circumstellar shells. To illustrate this, we reinvestigated the mean prole of the residual dust emission in the 12{18m wavelength range (i.e., in the so-called trough region between the two silicate bands). The reality of the emission feature at 16.8m is conrmed by our new investigation. We demonstrated that it is not an instrumental artifact since it is not present in ISO spectra of K-stars. In the course of our laboratory work, we found a third prominent emissivity maximum of spinel at 32m. This feature could also be detected in the spectra of the brightest 13m band emission sources; we derived its mean band prole, too. The new sets of optical constants here presented have been made available for public access in the electronic database http://www.astro.uni-jena.de.

Transient dust in warm debris disks - Detection of Fe-rich olivine grains

Context. Debris disks trace remnant reservoirs of leftover planetesimals in planetary systems. In the past years, a handful of “warm” debris disks have been discovered in which emission in excess starts in the mid-infrared. An interesting subset of these warm debris disks shows emission features in mid-infrared spectra, which points towards the presence of μm-sized dust grains, with temperatures above hundreds K. Given the ages of the host stars, the presence of these small grains is puzzling, and raises questions about their origin and survival in time. Aims. This study focuses on determining the mineralogy of the dust around seven debris disks with evidence for warm dust, based on Spitzer/IRS spectroscopic data, to provide new insights into the origin of the dust grains. Methods. We developed and present a new radiative transfer code (Debra) dedicated to spectral energy distribution (SED) modeling of optically thin disks. The Debra code is designed such that it can simultaneously determine dust composition and disk properties. We used this code on the SEDs of seven warm debris disks, in combination with recent laboratory experiments on dust optical properties. Results. We find that most, if not all, debris disks in our sample are experiencing a transient phase, suggesting a production of small dust grains on relatively short timescales. Dust replenishment should be efficient on timescales of months for at least three sources. From a mineralogical point of view, we find that crystalline pyroxene grains (enstatite) have low abundances compared to crystalline olivine grains. The main result of our study is that we find evidence for Fe-rich crystalline olivine grains (Fe/[Mg + Fe] ∼ 0.2) for several debris disks. This finding contrasts with studies of gas-rich protoplanetary disks, where Fe-bearing crystalline grains are usually not observed. Conclusions. These Fe-rich olivine grains, and the overall differences between the mineralogy of dust in Class II disks compared to debris disks suggests that the transient crystalline dust in warm debris disk is of a new generation. We discuss possible crystallization routes to explain our results, and also comment on the mechanisms that may be responsible for the production of small dust grains.