Revisiting astronomical crystalline forsterite in the UV to near-IR

Abstract

Optical functions (n and k) of cosmic dust species like forsterite (Mg 2 SiO 4 ) are required at all wavelengths to quantify the temperature and amount of dust. Astronomers combine optical functions of forsterite and olivine in different ways, which will affect radiative transfer models. We investigated what recent updates to the ultraviolet-visible-near-infrared (UV-VIS-NIR) laboratory spectra of forsterite and the choice of forsterite n, k dataset will have on radiative transfer models. We measured the UV-VIS-NIR transmission spectra of synthetic forsterite, MgO, SiO 2 , olivine (Fo90), and meteoritic olivine (pallasite). We derived optical functions for these and compared the UV-IR behavior of our k, absorption cross-section C abs , and total flux to that of "astronomical silicate" and olivine. Laboratory-derived k is substantially lower than "astronomical silicate" k at λ ∼ 0.2-5 μm. In the IR, different laboratory n and k produce equivocal C abs , whereas total flux is different for "astronomical silicate" versus laboratory n, k. From 0.35-5 μm, the choice of "forsterite" k values has the most effect on modeled quantities. For environments with significant UV flux, astronomers should use recent UV-VIS-NIR laboratory n, k.

Thermodynamic and optical thickness corrections to diffusive radiative transfer formulations with application to planetary interiors

“…In the IR (<3000 cm −1 ), absorption coefficients are large, near 10 3 mm −1 [e.g., Hofmeister and Bowey , ]. For the near‐IR with high transparency, A is difficult to measure being <0.005 mm −1 for olivine from 3000 to 6000 cm −1 [ Pitman et al ., ]. This requires extinction distances of >2 m, which is very small compared to the distances across which temperature changes in the mantle.…”

“…In addition, transparency of anhydrous silicates and oxides in the near‐IR spectral region only requires that k rad be strongly frequency dependent and that an average absorption coefficient can only be used over limited spectral ranges (e.g., A in the near‐IR region differs by orders of magnitude from A in the visible: Figure ) [also see Shankland et al ., ; Palik , ; Hofmeister , ; Pitman et al ., ]. The present paper provides an alternative derivation of k rad and calculates a generic, but rough, estimate for k rad for the Earth's mantle.…”

Far-infrared continuum absorption of olivine at low temperatures

“…Apart from the diagnostic lattice-vibration bands, the value of the continuum absorption coefficient in adjacent spectral ranges is also important. The absorption of stellar radiation, which determines the temperature of dust in stellar environments, takes place in spectral regions, where minerals are often only weakly absorbing (Zeidler et al, 2011;Pitman et al, 2013), except for conducting, or semiconducting, compounds. Similarly, if dust is located at a larger distance from a star, its thermal radiation may occur at very long infrared wavelengths extending significantly beyond the range of lattice-vibration bands where the emissivity is determined by a low continuum opacity as well.…”

A new interpretation of Serkowski’s polarization law

“…They show that pure SiO2, MgO and Mg2SiO4 (forsterite ) are nearly transparent in the visible, much more so than the "astronomical silicates" defined by to represent IS dust (e.g. Zeidler et al 2011, Pitman et al 2013). However, Scott and Duley (1996) deduced the complex index of refraction of amorphous forsterite and enstatite (MgSiO3 or Mg2Si6O6) from reflection measurements in the range 0.1 to 20 µm.…”