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.