Infrared emission spectroscopy of nanograins in the laboratory for understanding hot exozodiacal dust

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The main objective of the project is to obtain information that will lead to or contribute to the identification of the materials that compose hot exozodiacal dust disks (exozodis) and the Solar F-corona. Exozodis are frequently observed and are thought to consist of submicronic grains possibly as hot as 2000 K because of their location very close to the star. The composition and therefore the origin of these grains are currently undetermined. The same issues concern the dust grains of the F-corona. Given that the presence of hot exozodiacal grains is revealed by their thermal emission at near- and mid-infrared wavelengths, the information we aim to obtain consists of the optical constants of relevant materials at high temperatures in the infrared wavelength range. These data will allow us to compute thermal emission spectra for different grain geometries and sizes, which we will compare to astronomical observation data, available or being acquired, in order to identify or constrain the composition of hot exozodis and that of the F-corona. The project materials are selected considering properties exhibited by or expected from hot exozodiacal dust grains such as chemical composition and sublimation temperature. Hence, we have chosen carbon allotropes, Fe, Fe2O3, Fe3O4, Fe3C, and SiC, a list that we keep open. Their optical constants will be derived from the analysis of Fourier-transform infrared (FTIR) thermal emission spectra complemented with FTIR reflection spectra. Heating the materials in steps from room temperature to 1800 K for emission spectroscopy, to 1000 K for reflection spectroscopy, will allow us to characterize the temperature dependence of the optical constants. In order to perform FTIR thermal emission measurements, an objective of the project is to bring this technique into our laboratory. The existing FTIR spectrometer will be equipped with an extension for thermal emission measurements at temperatures up to 1800 K. It will be designed, built, installed, and tested during the first stage of the project. The thermal emission spectra will be measured using samples in the relevant form of nanopowder and also in that of bulk. The comparison of simulated emission efficiencies based on the bulk data with directly measured nanopowder emissivities will allow us to evaluate the effect of grain geometry on thermal emission and to assess the usefulness of models for predicting grain properties. These results, applied to the interpretation of astronomical observations, will improve our understanding of the physics and chemistry in debris disks and will assist astronomers in their search for exo-Earths. Moreover, the thermal emission spectra, made available through a publicly accessible database, will be useful to the study of hot dust in the inner region of protoplanetary disks.