Better Alternatives to “Astronomical Silicate”: Laboratory-Based Optical Functions of Chondritic/Solar Abundance Glass With Application to Hd 161796

Abstract

Astronomical" or "circumstellar" silicate optical functions (real and imaginary indices of refraction n(λ) and k(λ)) have been previously derived from compositionally and structurally disparate samples; past values were compiled from different sources in the literature, and are essentially kluges of observational, laboratory, and extrapolated or interpolated values. These synthetic optical functions were created because astronomers lack the quantitative data on amorphous silicates at all wavelengths needed for radiative transfer modeling. This paper provides optical functions that (1) are created with a consistent methodology, (2) use the same sample across all wavelengths, and (3) minimize interpolation and extrapolation wherever possible. We present electronic data tables of optical functions derived from mid-ultraviolet to far-infrared laboratory transmission spectra for two materials: iron-free glass with chondritic/solar atmospheric abundances, and metallic iron. We compare these optical functions to other popular n, k data used to model amorphous silicates (e.g., "astronomical" or "circumstellar" silicate), both directly and in application to a simple system: the dust shell of the post-AGB star HD 161796. Using the new optical functions, we find that the far-IR profile of model SEDs are significantly affected by the ratio of glass to iron. Our case study on HD161796 shows that modeling with our new optical functions, the mineralogy is markedly different from that derived using synthetic optical functions and suggests a new scenario of crystalline silicate formation.

Modeling Cosmic Dust: How to Use Optical "Constants"

“…As will be seem in other papers in this volume, the value of β is not so simple and depends and many factors including composition, crystal structure, grain size and temperature (see also [39,40]. Figure 7 shows the real (n, top panel) and imaginary (k, bottom panel) parts of complex refractive indices from the most commonly used synthetic optical functions [28,29,30] together with new lab-based data [41]. The new optical functions are measure directly from a single sample of iron-free silicate with otherwise solar abundances (see [35]) over a large wavelength range.…”

“…The new optical functions are measure directly from a single sample of iron-free silicate with otherwise solar abundances (see [35]) over a large wavelength range. It is clear that the synthetic optical functions have significantly different profiles to this new "real" dataset which will soon be publicly available [41].…”

“…Cosmic Silicate (SPH14) Figure 7: Comparison of synthetic complex refractive indices for "astronomical silicate" from [28,29] with laboratory-derived optical function of a cosmic abundance silicate from [41] (details of the infrared portion and the sample synthesis available in [35].…”

Comparative Studies of the Dust around Red Supergiant and Oxygen-Rich Asymptotic Giant Branch Stars in the Local Universe

“…An O-rich GRAMS model from Sargent et al (2011) has been fit to its optical/infrared spectral energy distribution, the stellar photosphere has been subtracted from the Spitzer Infrared Spectrograph spectrum, and the residuals are the black points in this plot being fit by the model (solid orange line). The model is of similar style to those of Sargent et al (2009), except the components with size given in the legend are for spherical dust grains whose optical constants are those of the glassy silicates of cosmic composition from Speck et al (2015).…”

Identifying circumstellar dust around oxygen-rich mira variables with maser emission via continuum elimination

“…Speck et al (2011) produces infrared spectra of a silicate glass for which the ratios of the major cations (Mg, Si, Al, Na, Ti) were the same as those of chondrites/the solar system at large but excluded iron. This glass sample was then used to produce the complex refractive index for Cosmic Silicate, where the same sample was measure spectroscopically from 0.2𝜇m to 200𝜇m(Speck et al 2015).…”