How Does Temperature Affect the Infrared Vibrational Spectra of Nanosized Silicate Dust?

Magnesium-rich silicates are pervasive throughout the universe and are found in many astronomical environments as small dust grains. Most of the information we have about such silicate dust comes from infrared (IR) observations, whereby the prominent similar to 9.7 mu m Si-O stretching and 18-20 mu m O-Si-O bending vibrational bands are used as identifying signatures. During the initial stages of silicate dust nucleation around aging stars, the nascent grains are nanosized and heated to 800-1000 K. Later, the larger fully formed grains are ejected into the cold interstellar medium and are subsequently heavily processed (e.g., by shattering and sputtering), likely leading to an abundant population of nanosilicate grains. Such ultrasmall grains can eventually form a dust component of protoplanetary disks, where they can be again heated to a few hundred degrees. Although nanosilicate grains are astrochemically important and are likely carriers of the ubiquitous anomalous microwave emission, they have yet to be unambiguously identified through observation. The advent of the James Webb Space Telescope, with IR instruments of unrivalled sensitivity covering the 5-28 mu m wavelength range, promises to open a window on these elusive, but presumably abundant, species. Here, we examine Mg-rich ultrasmall silicate clusters with both olivinic (Mg2SiO4) and pyroxenic (MgSiO3) stoichiometries and accurately calculate their IR spectra at temperatures between 0 and 800 K. Although this temperature range is below that required to anneal bulk magnesium silicates (similar to 1000 K), we show that nanosized grains can exhibit significant anharmonic motions and premelted structural conformational fluxionality even at 400 K. As a result, we demonstrate that the IR spectra of nanosilicates are highly sensitive to temperature, which should thus be considered when attempting to identify these species.