Infrared optical properties of spinels

Abstract

Abstract. In a previous paper, we have proposed magnesium aluminium spinel to be the carrier of the 13 and 17 µm 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 spinel's IR-spectra on its aluminium content, we have synthesized a number of crystals with different 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 efficiencies of small (sub-µm-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 13 µm feature observed in the spectra of some oxygen rich circumstellar shells. To illustrate this, we reinvestigated the mean profile of the residual dust emission in the 12-18 µm wavelength range (i.e., in the so-called trough region between the two silicate bands). The reality of the emission feature at 16.8 µm is confirmed 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 32 µm. This feature could also be detected in the spectra of the brightest 13 µm band emission sources; we derived its mean band profile, 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.

Optical constants of refractory oxides at high temperatures

“…In a perfect spinel crystal, the trivalent Al cations are octahedrally coordinated by oxygen ions, while the bivalent Mg cations are tetrahedrally coordinated. However, it has been noted by Tropf & Thomas (1991) and shown in detail by Fabian et al (2001) that annealing induces the Mg and Al ions to change their sites, such that a part of the Al ions is occupying the tetrahedral sites and a part of the Mg ions is obversely located in the octahedral sites. This structural transition takes place at 1023−1073 K and is apparently irreversible at least at short time scales (Tropf & Thomas 1991;Fabian et al 2001).…”

“…However, it has been noted by Tropf & Thomas (1991) and shown in detail by Fabian et al (2001) that annealing induces the Mg and Al ions to change their sites, such that a part of the Al ions is occupying the tetrahedral sites and a part of the Mg ions is obversely located in the octahedral sites. This structural transition takes place at 1023−1073 K and is apparently irreversible at least at short time scales (Tropf & Thomas 1991;Fabian et al 2001). Therefore, synthetic spinel crystals are in particular characterized by this deviation from the perfect crystal, since high temperature conditions are needed for their production.…”

“…Results of annealing A&A 553, A81 (2013) experiments with spinel can be found in Fabian et al (2001). The optical constants derived in the present paper will be made publicly available 2 .…”

Fitting of dust spectra with genetic algorithms

“…The MIR spectrum of CE And is dominated by a classical amorphous silicate emission (Fabian et al 2001;Heras & Hony 2005). As a starting point for the dust composition amorphous melilite, amorphous olivine and crystalline magnesiowustite, respectively f 4 , f 5 and f 6 , were chosen.…”

Infrared spectroscopy of asymptotic giant branch stars in the Galactic bulge

“…Several observations, mostly Infrared Astronomical Satellite (IRAS)/Low Resolution Spectrometer (LRS) and ISO/Short Wavelength Spectrometer (SWS), have revealed the presence of many different dust compounds in O-rich AGB stars (Vardya, de Jong & Willems 1986;Onaka, de Jong & Willems 1989;Sloan & Price 1995;Waters et al 1996;Speck et al 2000;Fabian et al 2001;Cami 2002;Posch et al 2002;Blommaert et al 2006) and many -but not all -of these compounds are part of the theoretical condensation sequence. Indeed, AGB stars with high massloss rates show copious amounts of silicates, including crystalline olivines and pyroxenes.…”