D. Fabian

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.

Structural processing of enstatite by ion bombardment

During their lifetime, cosmic dust silicates suffer from a continuous processing by annealing, cosmic ray and UV irradiation, destruction and possibly also interstellar recondensation. Since the discovery that a significant proportion of star- dust silicates leaves their star in crystalline form, the question arose as to why the interstellar silicate dust component does not show any indication of crystallinity. Amorphization due to ion irradiation is one possible explanation for the effect. In this paper, the results of irradiation experiments of submicrometre-sized clinoenstatite (MgSiO3) particles with 400 keV Ar + and 50 keV He + ions are presented. The irradiation doses have been varied between 1×10 16 and 1×10 18 ions/cm 2 for He + ions and 1×10 14 up to 5×10 14 ions/cm 2 for Ar + ions. These doses are comparable to those values that an interstellar silicate grain should be exposed to during its average life-time of 4×10 8 years. Threshold values for amorphization have been amounted to 1×10 17 and 3×10 14 ions/cm 2 for 50 keV He + and 400 keV Ar + ions. Besides the structural changes in the microcrystallites morphological modifications in the grains, but no change of the chemical composition are found. Conclusions of potential astrophysical relevance have been drawn.

A database of optical constants of cosmic dust analogs

We describe the current state and future of the WWW Jena-Petersburg database of optical constants (JPDOC) that also contains references to papers and links to internet resources related to measurements or calculations of the optical constants of materials of astronomical interest. The most important part of the JPDOC are data measured in broad wavelength ranges and partly at low temperatures in the Jena Laboratory. To demonstrate the use of these data, we show as examples infrared refractive indices of crystalline and amorphous magnesium silicates, spinel, and hydrogenated amorphous carbon and calculate the absorption cross-sections of small particles composed of these materials.

Infrared Properties of Solid Titanium Oxides: Exploring Potential Primary Dust Condensates

We present optical constants and opacities of solid TiO2 ,T i 2O3, magnesium and calcium titanates, largely based on laboratory measurements. These dust species deserve interest as potential primary condensates in oxygen-rich dusty environments. Of the three known solid TiO2 phases, only one (rutile) has been extensively studied so far with respect to its mid-IR optical properties. We compare these with our measurements of the optical constants of anatase, brookite, and CaTiO3. Furthermore, for several Mg-Ti-oxides, powder transmission spectra are shown. While all known TiO2 modifications have their strongest bands between 13 and 13.5 lm (for spherical particles), CaTiO3, MgTiO3, and other Mg titanates have principal maxima of their absorption coefficients between 14 and 19 lm. This makes a spectroscopic identification of circumstellar Ti oxide particles rather difficult, because both the 13 and the 14–19 lm region are crowded with other features in the spectra of oxygen-rich circumstellar shells. Subject headings: circumstellar matter — methods: laboratory — molecular data — solar system: formation — stars: AGB and post-AGB — stars: mass loss

Towards the identification of circumstellar hibonite

We have performed IR reflectance measurements on crystalline CaAl12O19 (hibonite) and derived optical constants for both principal directions of the electric field vector relative to the crystallographic c-axis. Based on the resulting dielectric functions, we calculated small particle spectra and found six maxima of the absorption cross section for spherical dust grains at the wavelengths 12.3, 15.9, 21.1, 25.1, 34.4, and 78.7m. For nonspherical particles, strong shifting and broadening of the two strongest resonances at 12.3 and 15.9m occurs. The signal-to-noise-ratio of ISO spectra of oxygen-rich AGB stars is not sucient for a definite identification of these bands, even though there is some weak indication of their presence. We propose additional observations to clarify this point.

Study of the amorphization process of MgSiO3 by ion irradiation as a form of dust processing in astrophysical environments

Abstract The study of the crystalline-to-amorphous transition of silicates triggered by ion irradiation is useful for the understanding of the structural modification of interstellar dust grains. We focused our experiments on the mineral enstatite (MgSiO3), an essential part of the polycrystalline silicate dust. Within our studies He+-, Li+-, Ne+- and Ar+-ions with energies between 50 and 400 keV were used. Irradiations were carried out at 70 K. The samples were analyzed by transmission electron microscopy in combination with electron diffraction and infrared reflection spectroscopy. It was found, that the number of displacements necessary to amorphize enstatite is higher for lower ion masses. The amorphization threshold increases from 0.3 to about 5 dpa in the case of 400 keV Ar+- and 50 keV He+-ions, respectively. This can be explained by the dilution of the collision cascades with decreasing ion mass and with an in situ annealing process with enhanced ratio between electronic and nuclear energy loss, respectively.