arXiv subjects

Harald Mutschke

Publications and source records attributed to Harald Mutschke.

16 recordsLinked to original sources

arXiv2024

Trapped Water on Silicates in the Laboratory and in Astrophysical Environments

Existence of strongly bound water molecules on silicate surfaces, above the desorption temperature of water ice, has been first predicted by computational studies and recently demonstrated by laboratory experiments. Such trapped water may be present in various astrophysical environments and there is now evidence for its presence in the diffuse interstellar medium and in extraterrestrial particles. We present here new results of a laboratory study of the phenomenon of trapping (strong bonding) of water molecules by silicates. We show that the efficiency of trapping is strongly dependent on the properties and composition of the surface. Our results point out that the presence of trapped water should be due to the hydrophilic properties of the silicate surface and that the nature of trapping is physical (physisorption rather than chemisorption). We demonstrate that water can be trapped on silicates up to the temperatures of about 470 K, which speaks for the presence of wet silicate grains in the terrestrial planet formation zone in planet-forming disks. Studying the thermal and UV stability of trapped water, we conclude that the detection of trapped water in the diffuse ISM speaks for its efficient continuous formation. We discuss our results as relevant to fundamental scientific questions, such as the oxygen depletion problem, the origin of water on Earth, and the formation of rocky planets.

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arXiv2022

Sub-mm/mm optical properties of real protoplanetary matter derived from Rosetta/MIRO observations of comet 67P

Optical properties are required for the correct understanding and modelling of protoplanetary and debris discs. By assuming that comets are the most pristine bodies in the solar system, our goal is to derive optical constants of real protoplanetary material. We determine the complex index of refraction of the near-surface material of comet 67P/Churyumov-Gerasimenko by fitting the sub-millimetre/millimetre observations of the thermal emission of the comet's sub-surface made by the Microwave Instrument for the Rosetta Orbiter (MIRO) with synthetic temperatures derived from a thermophysical model and radiative-transfer models. According to the two major formation scenarios of comets, we model the sub-surface layers to consist of pebbles as well as of homogeneously packed dust grains. In the case of a homogeneous dusty surface material, we find a solution for the length-absorption coefficient of $α\approx 0.22~\mathrm{cm^{-1}}$ for a wavelength of 1.594 mm and $α\geq 3.84~\mathrm{cm^{-1}}$ for a wavelength of 0.533 mm and a constant thermal conductivity of $0.006~\mathrm{Wm^{-1}K^{-1}}$. For the pebble scenario, we find for the pebbles and a wavelength of 1.594 mm a complex refractive index of $n = (1.074 - 1.256) + \mathrm{i} \, (2.580 - 7.431)\cdot 10^{-3}$ for pebble radii between 1 mm and 6 mm. Taking into account other constraints, our results point towards a pebble makeup of the cometary sub-surface with pebble radii between 3 mm and 6 mm. The derived real part of the refractive index is used to constrain the composition of the pebbles and their volume filling factor. The optical and physical properties are discussed in the context of protoplanetary and debris disc observations.

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arXiv2019

Constraining the detectability of water ice in debris disks

Water ice is important for the evolution and preservation of life. Identifying the distribution of water ice in debris disks is therefore of great interest in the field of astrobiology. Furthermore, icy dust grains are expected to play important roles throughout the entire planet formation process. However, currently available observations only allow deriving weak conclusions about the existence of water ice in debris disks. We investigate whether it is feasible to detect water ice in typical debris disk systems. We take the following ice destruction mechanisms into account: sublimation of ice, dust production through planetesimal collisions, and photosputtering by UV-bright central stars. We consider icy dust mixture particles with various shapes consisting of amorphous ice, crystalline ice, astrosilicate, and vacuum inclusions. We calculated optical properties of inhomogeneous icy dust mixtures using effective medium theories, that is, Maxwell-Garnett rules. Subsequently, we generated synthetic debris disk observables, such as spectral energy distributions and spatially resolved thermal reemission and scattered light intensity and polarization maps with our code DMS. We find that the prominent $\sim$ 3 $μ\rm{m}$ and 44 $μ\rm{m}$ water ice features can be potentially detected in future observations of debris disks with the James Webb Space Telescope and the Space Infrared telescope for Cosmology and Astrophysics. We show that the sublimation of ice, collisions between planetesimals, and photosputtering caused by UV sources clearly affect the observational appearance of debris disk systems. In addition, highly porous ice tends to produce highly polarized radiation at around 3 $μ\rm{m}$. Finally, the location of the ice survival line is determined by various dust properties such as a fractional ratio of ice versus dust, physical states of ice, and the porosity of icy grains.

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arXiv2018

Low temperature optical properties of interstellar and circumstellar icy silicate grain analogues in the mid-infrared spectral region

Two different silicate/water ice mixtures representing laboratory analogues of interstellar and circumstellar icy grains were produced in the laboratory. For the first time, optical constants, the real and imaginary parts of the complex refractive index, of such silicate/water ice mixtures were experimentally determined in the mid-infrared spectral region at low temperatures. In addition, optical constants of pure water ice and pure silicates were derived in the laboratory. Two sets of constants were compared, namely, measured constants calculated from the transmission spectra of silicate/water ice samples and effective constants calculated from the optical constants of pure silicates and pure water ice samples using different mixing rules (effective medium approaches). Differences between measured and effective constants show that a mixing (averaging) of optical constants of water ice and silicates for the determination of the optical properties of silicate/ice mixtures can lead to incorrect results. Also, it is shown that a part of water ice molecules is trapped in/on silicate grains and does not desorb up to 200 K. Our unique data are just in time with respect to the new challenging space mission, James Webb Space Telescope, which will be able to bring novel detailed information on interstellar and circumstellar grains, and suitable laboratory data are extremely important for the decoding of astronomical spectra.

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arXiv2017

Constraints on the structure of hot exozodiacal dust belts

Recent interferometric surveys of nearby main-sequence stars show a faint but significant near-infrared excess in roughly two dozen systems, i.$\,$e. around $10\,\%$ to $30\,\%$ of stars surveyed. This excess is attributed to dust located in the immediate vicinity of the star, the origin of which is highly debated. We used previously published interferometric observations to constrain the properties and distribution of this hot dust. Considering both scattered radiation and thermal reemission, we modelled the observed excess in nine of these systems. We find that grains have to be sufficiently absorbing to be consistent with the observed excess, while dielectric grains with pure silicate compositions fail to reproduce the observations. The dust should be located within $\sim\,0.01-1\,au$ from the star depending on its luminosity. Furthermore, we find a significant trend for the disc radius to increase with the stellar luminosity. The dust grains are determined to be below $0.2-0.5\,μ\textrm{m}$, but above $0.02-0.15\,μ\textrm{m}$ in radius. The dust masses amount to $(0.2-3.5)\times10^{-9}\,M_\oplus$. The near-infrared excess is probably dominated by thermal reemission, though a contribution of scattered light up to $35\,\%$ cannot be completely excluded. The polarisation degree predicted by our models is always below $5\,\%$, and for grains smaller than $\sim0.2\,μ\textrm{m}$ even below $1\,\%$. We also modelled the observed near-infrared excess of another ten systems with poorer data in the mid-infrared. The basic results for these systems appear qualitatively similar, yet the constraints on the dust location and the grain sizes are weaker.

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arXiv2014

Absorption of crystalline water ice in the far infrared at different temperatures

The optical properties of ice in the far infrared are important for models of protoplanetary and debris disks. In this report we derive a new set of data for the absorption (represented by the imaginary part of the refractive index $κ$) of crystalline water ice in this spectral range, including a detailed inspection of the temperature dependence, which had not been done in such detail before. We measured the transmission of three ice layers with different thicknesses at temperatures $\vartheta = 10...250$K and present data at wavelengths $λ=80...625$ microns. We found a change in the spectral dependence of $κ$ at a wavelength of $175 \pm 6$ microns. At shorter wavelengths, $κ$ exhibits a constant flat slope and no significant temperature dependence. Long-ward of that wavelength, the slope gets steeper and has a clear, approximately linear temperature dependence. This change in the behaviour is probably caused by a characteristic absorption band of water ice. The measured data were fitted by a power-law model that analytically describes the absorption behaviour at an arbitrary temperature. This model can readily be applied to any object of interest, for instance a protoplanetary or a debris disk. To illustrate how the model works, we simulated the spectral energy distribution (SED) of the resolved, large debris disk around the nearby solar-type star HD 207129. Replacing our ice model by another, commonly used data set for water ice results in a different SED slope at longer wavelengths. This leads to changes in the characteristic model parameters of the disk, such as the inferred particle size distribution, and affects the interpretation of the underlying collisional physics of the disk.

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arXiv2013

Optical constants of refractory oxides at high temperatures

Many cosmic dust species, among them refractory oxides, form at temperatures higher than 300 K. Nevertheless, most astrophysical studies are based on the room-temperature optical constants of solids, such as corundum and spinel. A more realistic approach is needed for these materials, especially in the context of modeling late-type stars. We aimed at deriving sets of optical constants of selected, astrophysically relevant oxide dust species with high melting points. A high-temperature-high-pressure-cell and a Fourier-transform spectrometer were used to measure reflectance spectra of polished samples. For corundum (alpha-Al$_2$O$_3$), spinel (MgAl$_2$O$_4$), and alpha-quartz (SiO$_2$), temperature-dependent optical constants were measured from 300 K up to more than 900 K. Small particle spectra were also calculated from these data. All three examined oxides show a significant temperature dependence of their mid-IR bands. For the case of corundum, we find that the 13$μ$m emission feature - seen in the IR spectra of many AGB stars - can very well be assigned to this mineral species. The best fit of the feature is achieved with oblate corundum grains at mean temperatures around 550 K. Spinel remains a viable carrier of the 13$μ$m feature as well, but only for T < 300 K and nearly spherical grain shapes. Under such circumstances, spinel grains may also account for the 31.8$μ$m band that is frequently seen in sources of the 13$μ$m feature and which has not yet been identified with certainty.

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arXiv2012

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

(Abridged) Debris disks trace remnant reservoirs of leftover planetesimals in planetary systems. A handful of "warm" debris disks have been discovered in the last years, where emission in excess starts in the mid-infrared. An interesting subset within these warm debris disks are those where emission features are detected in mid-IR spectra, which points towards the presence of warm micron-sized dust grains. Given the ages of the host stars, the presence of these grains is puzzling, and questions their origin and survival in time. This study focuses on determining the mineralogy of the dust around 7 debris disks with evidence for warm dust, based on Spitzer/IRS spectroscopic data, in order to provide new insights into the origin of the dust grains. We present a new radiative transfer code dedicated to SED modeling of optically thin disks. We make use of this code on the SEDs of seven warm debris disks, in combination with recent laboratory experiments on dust optical properties. 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. From a mineralogical point of view, we find that enstatite grains have small abundances compared to crystalline olivine grains. The main result of our study is that we find evidences 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. The presence of Fe-rich olivine grains, and the overall differences between the mineralogy of dust in Class II disks compared to debris disks suggest that the transient crystalline dust is of a new generation. We discuss possible crystallization routes to explain our results, and comment on the mechanisms that may be responsible for the production of small dust grains.

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arXiv2012

Recent Results of Solid-State Spectroscopy

Solid state spectroscopy continues to be an important source of information on the mineralogical composition and physical properties of dust grains both in space and on planetary surfaces. With only a few exceptions, artificially produced or natural terrestrial analog materials, rather than 'real' cosmic dust grains, are the subject of solid state astrophysics. The Jena laboratory has provided a large number of data sets characterizing the UV, optical and infrared properties of such cosmic dust analogs. The present paper highlights recent developments and results achieved in this context, focussing on 'non-standard conditions' such as very low temperatures, very high temperatures and very long wavelengths.

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arXiv2012

High-Temperature Optical Constants of Dust Analogues for the Solar Nebula

The dust in protoplanetary disks is influenced by a lot of different processes. Besides others, heating processes are the most important ones: they change not only the physical and chemical properties of dust particles, but also their emission spectra. In order to compare observed infrared spectra of young stellar systems with laboratory data of hot (up to 700°C) circumstellar dust analogues, we investigate materials, which are important constituents of dust in protoplanetary disks. We calculated the optical constants by means of a simple Lorentzian oscillator fit and apply them to simulations of small-particle emission spectra in order to compare our results with real astronomical spectra of AGB-stars and protoplanetary disks.

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arXiv2011

Near-infrared absorption properties of oxygen-rich stardust analogues: The influence of coloring metal ions

Several astrophysically relevant solid oxides and silicates have extremely small opacities in the visual and near-infrared in their pure forms. Datasets for the opacities and for the imaginary part k of their complex indices of refraction are hardly available in these wavelength ranges. We aimed at determining k for spinel, rutile, anatase, and olivine, especially in the near-infrared region. Our measurements were made with impurity-containing, natural, and synthetic stardust analogs. Two experimental methods were used: preparing small sections of natural minerals and synthesizing melt droplets under the electric arc furnace. In both cases, the aborption properties of the samples were measured by transmission spectroscopy. For spinel (MgAl2O4), anatase, rutile (both TiO2), and olivine ((Mg,Fe)2SiO4), the optical constants have been extended to the visual and near-infrared. We highlight that the individual values of k and the absorption cross section depend strongly on the content in transition metals like iron. Based on our measurements, we infer that k values below 10^(-5) are very rare in natural minerals including stardust grains, if they occur at all. Data for k and the absorption cross section are important for various physical properties of stardust grains such as temperature and radiation pressure. With increasing absorption cross section due to impurities, the equilibrium temperature of small grains in circumstellar shells increases as well. We discuss why and to what extent this is the case.

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arXiv2010

Optical properties of cosmic dust analogs: A review

Nanometer- and micrometer-sized solid particles play an important role in the evolutionary cycle of stars and interstellar matter. The optical properties of cosmic grains determine the interaction of the radiation field with the solids, thereby regulating the temperature structure and spectral appearance of dusty regions. Radiation pressure on dust grains and their collisions with the gas atoms and molecules can drive powerful winds. The analysis of observed spectral features, especially in the infrared wavelength range, provides important information on grain size, composition and structure as well as temperature and spatial distribution of the material. The relevant optical data for interstellar, circumstellar, and protoplanetary grains can be obtained by measurements on cosmic dust analogs in the laboratory or can be calculated from grain models based on optical constants. Both approaches have made progress in the last years, triggered by the need to interpret increasingly detailed high-quality astronomical observations. The statistical theoretical approach, spectroscopic experiments at variable temperature and absorption spectroscopy of aerosol particulates play an important role for the successful application of the data in dust astrophysics.

astro-ph.IM↗

arXiv2008

Collisional and Thermal Emission Models of Debris Disks: Towards Planetesimal Population Properties

Debris disks around main-sequence stars are believed to derive from planetesimal populations that have accreted at early epochs and survived possible planet formation processes. While debris disks must contain solids in a broad range of sizes - from big planetesimals down to tiny dust grains - debris disk observations are only sensitive to the dust end of the size distribution. Collisional models of debris disks are needed to "climb up" the ladder of the collisional cascade, from dust towards parent bodies, representing the main mass reservoir of the disks. We have used our collisional code to generate five disks around a sun-like star, assuming planetesimal belts at 3, 10, 30, 100, and 200 AU with 10 times the Edgeworth-Kuiper-belt mass density, and to evolve them for 10 Gyr. Along with an appropriate scaling rule, this effectively yields a three-parametric set of reference disks (initial mass, location of planetesimal belt, age). For all the disks, we have generated spectral energy distributions (SEDs), assuming homogeneous spherical astrosilicate dust grains. A comparison between generated and actually observed SEDs yields estimates of planetesimal properties (location, total mass etc.). As a test and a first application of this approach, we have selected five disks around sun-like stars with well-known SEDs. In four cases, we have reproduced the data with a linear combination of two disks from the grid (an "asteroid belt" at 3 AU and an outer "Kuiper belt"); in one case a single, outer component was sufficient. The outer components are compatible with "large Kuiper belts" of 0.2-50 earth masses (in the bodies up to 100 km in size) with radii of 100-200 AU.

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arXiv2005

Infrared Extinction by Aggregates of SiC Particles

Particle shape and aggregation have a strong influence on the spectral profiles of infrared phonon bands of solid dust grains. In this paper, we use a discrete dipole approximation, a cluster-of-spheres code following the Gerardy-Ausloos approach and a T-matrix method for calculating IR extinction spectra of aggregates of spherical silicon carbide (SiC) particles. We compare the results obtained with the three different methods and discuss differences in the band profiles.

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arXiv2005

Infrared extinction by homogeneous particle aggregates of SiC, FeO and SiO2: comparison of different theoretical approaches

Particle shape and aggregation have a strong influence on the spectral profiles of infrared phonon bands of solid dust grains. Calculating these effects is difficult due to the often extreme refractive index values in these bands. In this paper, we use the Discrete Dipole Approximation (DDA) and the T-matrix method to compute the absorption band profiles for simple clusters of touching spherical grains. We invest reasonable amounts of computation time in order to reach high dipole grid resolutions and take high multi-polar orders into account, respectively. The infrared phonon bands of three different refractory materials of astrophysical relevance are considered - Silicon Carbide (SiC), Wustite (FeO) and Silicon Dioxide (SiO2). We demonstrate that even though these materials display a range of material properties and therefore different strengths of the surface resonances, a complete convergence is obtained with none of the approaches. For the DDA, we find a strong dependence of the calculated band profiles on the exact dipole distribution within the aggregates, especially in the vicinity of the contact points between their spherical constituents. By applying a recently developed method to separate the material optical constants from the geometrical parameters in the DDA approach, we are able to demonstrate that the most critical material properties are those where the real part of the refractive index is much smaller than unity.

astro-ph↗

arXiv2004

Reconsidering the origin of the 21 micron feature: Oxides in carbon-rich PPNe?

The origin of the so-called "21 micron" feature which is especially prominent in the spectra of some carbon-rich protoplanetary nebulae (PPNe}) is the matter of a lively debate. A large number of potential band carriers have been presented and discarded within the past decade. The present paper gives an overview of the problems related to the hitherto proposed feature identifications, including the recently suggested candidate carrier silicon carbide. We also discuss the case for spectroscopically promising oxides. SiC is shown to produce a strong resonance band at 20-21 micron if coated by a layer of silicon dioxide. At low temperatures, core-mantle particles composed of SiC and amorphous SiO$_2$ indeed have their strongest spectral signature at a position of 20.1 micron, which coincides with the position of the "21 micron" emission band. The optical constants of another candidate carrier that has been relatively neglected so far -- iron monoxide -- are proven to permit a fairly accurate reproduction of the "21 micron" feature profile as well, especially when low-temperature measurements of the infrared properties of FeO are taken into account. As candidate carrier of the "21 micron" emission band, FeO has the advantage of being stable against further oxidation and reduction only in a narrow range of chemical and physical conditions, coinciding with the fact that the feature, too, is detected in a small group of objects only. However, it is unclear how FeO should form or survive particularly in carbon-rich PPNe.

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