Abstract

A database of computed infrared spectra of proto-silicate clusters and silicate aggregates

Ciska Kemper (ASIAA)

Stefan Bromley (ICREA & Universitat de Barcelona) Albert Rimola (Universitat Autònoma de Barcelona)

Interstellar grain models (Desert et al. 1990; Li & Draine, 2001) typically include a “Very Small Grain” (VSG) component, of which the optical properties cannot be described using bulk material measurements. VSGs (a≲20 nm) may be responsible for the UV bump in the interstellar extinction curve and a significant fraction of the mid-infrared emission. While traditionally Polycyclic Aromatic Hydrocarbons (PAHs) and other carbonaceous species are thought to be the main constituents of the VSG population, recent suggestions that silicate grains may form in the interstellar medium (Rouillé et al. 2013) imply that a corresponding proto-silicate population must be present as well. Bromley & Goumans (2012) have explored possible chemical pathways for the first steps toward silicate condensation, resulting in a number of plausible proto-silicate clusters, for which the molecular structures and energies have been determined. For the purpose of observational astronomy, especially in the context of JWST-MIRI spectroscopy, we are currently calculating infrared spectra for proto-silicate clusters ranging from just a handful of atoms to some 50 atoms. We are exploring stoichiometry (olivine, pyroxene, and non-stoichiometric compositions) and Fe-content, and present calculated spectra of the five to ten lowest energy clusters that we know of, for each composition. Furthermore, we are planning to expand this database with larger clusters for which the structures have been calculated by aggregating different proto-silicate clusters (Rimola et al. in prep.). Calculations of large clusters will also be used to quantify the amount of lattice displacements needed (by e.g. cosmic ray hits) for the infrared spectrum to appear “amorphous”. The database of calculated infrared spectroscopy of proto-silicate clusters and silicate aggregates will be made available to the astronomical community for comparison with observational data. We envision a data delivery similar to the PAH database (http://www.astrochem.org/pahdb; Boersma et al. 2014), which consists of the raw database, but also tools that allow the user to fit observed spectra with species in the database.

Mode of presentation: poster