What Can Infrared Spectra Tell Us about the Crystallinity of Nanosized Interstellar Silicate Dust Grains?

Infrared (IR) spectroscopy is the main technique used to identify and characterize silicate dust grains in astronomical environments. From IR spectra, the fraction of crystalline dust particles can be estimated and used to help understand the processing of dust occurring in different astronomical environments such as the interstellar medium (ISM) and circumstellar shells. Narrow spectral lines are assigned to crystalline grains, while broad signals are usually assumed to originate from amorphous material. Herein, we accurately calculate the IR spectra and energetic stabilities of several amorphous and crystalline silicate nanograins with an astronomically common Mg-rich olivinic (Mg2SiO4) stoichiometry and with sizes ranging from hundreds to thousands of atoms. First, unlike at larger length scales, crystalline forsterite-like grains at the nanoscale are found to be energetically metastable with respect to amorphous grains. However, from our careful analysis, we further show that the IR spectra of such nanosilicate grains cannot be unambiguously used to identify their structural nature. In particular, our work indicates that amorphous and crystalline silicate nanograins both exhibit broad IR spectra typical of noncrystalline grains, raising potential issues for estimates of the fraction of crystalline silicate dust in the ISM.