Iron(II) sulphides in cosmic dust: Optical constants depending on iron deficiency and temperature

Modelling of spectral energy distributions of protoplanetary and debris disks relies on optical constants of the dust components. Iron(II) sulphides are a very likely component of the dust in these objects, and they could influence the dust opacity substantially due to their large optical constants at long wavelengths. The composition of these compounds (Fe_ 1-x S, x=0,..,0.125) may considerably influence the optical constants. Complex refractive index data of iron(II) sulphides are scarce in the literature, especially for iron-deficient varieties, and not yet available over wide wavelength ranges. Our aim with this work is to enlarge the database by deriving optical constants from 210 nm to 2 mm wavelength for several varieties. We studied temperature dependencies from 300 K to 10 K to understand their influence for different astronomical environments. Polished samples of meteoritic troilite (stoichiometric FeS) and well-characterised terrestrial pyrrhotites were subjected to reflection ellipsometry from the ultraviolet-visible to the mid-infrared and to reflection spectroscopy from ultraviolet to sub-millimetre wavelengths. We derived the complex refractive index by minimisation of Drude and multiple Lorentz oscillator models. Phonon bands were investigated by polarised radiation in the infrared and at lower temperatures. We discovered a binary spectroscopic behaviour between iron depleted and stoichiometric(-like) iron(II) sulphides. For the latter, we found that free charge carrier excitations only appear at sub-millimetre wavelengths, while for all pyrrhotites they are dominant from the mid-infrared to far-infrared. Troilite shows a strong temperature dependency for the lattice vibrations, whereas variations of the pyrrhotite spectra are within the instrumentation uncertainties. The optical constants of iron(II) sulphides depend strongly on the iron to sulphur ratio. Due to their metal-like spectroscopic properties, non-stoichiometric iron(II) sulphides can contribute to dust emission without being directly detectable by infrared bands.