Mid-infrared Extinction and Fresh Silicate Dust Toward the Galactic Center

Abstract

Abstract We interpret the interstellar extinction observed toward the Galactic center (GC) in the wavelength range λ = 1–20 μm. Its main feature is the flat extinction at 3–8 μm whose explanation is still a problem for cosmic dust models. We search for the structure and chemical composition of dust grains that could explain the observed extinction. In contrast to earlier works, we use laboratory measured optical constants and consider particles of different structures. We show that a mixture of compact grains of aromatic carbon and of some silicate is better suited for reproducing the flat extinction in comparison with essentially porous grains or aliphatic carbon particles. Metallic iron should be located inside the particle, i.e., it cannot form layers on silicate grains as the extinction curves then become very peculiar. We find a model including aromatic carbonaceous particles and three-layered particles with an olivine-type silicate core, a thin, very porous layer and a thin envelope of magnetite that provides a good (but still not perfect) fit to the observational data. We suggest that such silicate dust should be fresh, i.e., recently formed in the atmospheres of late-type stars in the central region of the Galaxy. We assume that this region has a radius of about 1 kpc and produces about half of the observed extinction. The remaining part of extinction is caused by a “foreground” material being practically transparent at .