Optical constants of refractory oxides at high temperatures

Abstract

Context. 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. Aims. We aimed at deriving sets of optical constants of selected, astrophysically relevant oxide dust species with high melting points. Methods. A high-temperature, high-pressure cell and a Fourier-transform spectrometer were used to measure reflectance spectra of polished samples. For corundum (α-Al 2 O 3 ), spinel (MgAl 2 O 4 ), and α-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. Results. All three examined oxides show a significant temperature dependence of their mid-IR bands. For the case of corundum, we find that the 13 μm 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 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 μm band that is frequently seen in sources of the 13 μm feature and which has not yet been identified with certainty.

Dynamic atmospheres and winds of cool luminous giants

“…In the case of Al 2 O 3 the characteristic mid-IR features have been explored extensively in the astrophysical literature, both observationally and with laboratory measurements on stardust analogue materials (e.g., Koike et al 1995;Begemann et al 1997;Zeidler et al 2013). The visual to near-IR absorption coefficients, on the other hand, are a major source of uncertainty.…”

Dusty wind of W Hydrae

“…10) adopted the dust components identified by Justtanont et al (2004). In Paper I, the region of the ISO spectrum between 8 and 30 μm of the ISO spectrum was fitted, but no attempt was made to fit the 13 μm feature as its carrier is still uncertain (see Posch et al 1999;Zeidler et al 2013, and references therein). A total dust mass-loss rate of 2.8 × 10 −10 M year −1 was obtained.…”

“…Although the opacities of crystalline Al 2 O 3 show significant variations with temperature (Zeidler et al 2013), to our knowledge, no studies have been performed that probe the effect of temperature variations on the amorphous Al 2 O 3 features. In the context of our model, only the strength of the emission is sensitive to temperature, and therefore the total grain mass needed to reproduce the infrared excess is a function of grain temperature.…”

“…In fact, grains consisting of pure crystalline Al 2 O 3 (Zeidler et al 2013) and those consisting of a crystalline Al 2 O 3 core and a silicate mantle (Kozasa & Sogawa 1997a) have been proposed as the carriers of the unidentified 13 μm feature. We tried to fit the 13 μm feature of W Hya with pure crystalline Al 2 O 3 grains in the GBDS using the optical constant measured by Zeidler et al (2013).…”

“…In fact, grains consisting of pure crystalline Al 2 O 3 (Zeidler et al 2013) and those consisting of a crystalline Al 2 O 3 core and a silicate mantle (Kozasa & Sogawa 1997a) have been proposed as the carriers of the unidentified 13 μm feature. We tried to fit the 13 μm feature of W Hya with pure crystalline Al 2 O 3 grains in the GBDS using the optical constant measured by Zeidler et al (2013). We can only obtain a good fit by considering spherical (rather than DHS) particles and the optical constants for crystalline Al 2 O 3 grains at 973 K. Our models show that the mass abundance of crystalline Al 2 O 3 grains compared to the amorphous ones would be very small, around 5% by mass.…”