On the Crystallinity of Silicate Dust in the Interstellar Medium

TL;DR

Understanding the mineralogy of silicate dust in the interstellar medium is vital for insights into cosmic dust evolution, with recent spectroscopic studies refining the estimated crystalline fraction to about 3-5%.

Contribution

This paper refines the upper limit of silicate crystallinity in the ISM by considering ice mantles, updating previous estimates based on spectroscopic data.

Findings

Crystalline silicate fraction in the ISM is approximately 3-5%.

Infrared spectroscopy constrains the silicate crystallinity in the ISM.

Ice mantles on silicate cores influence the observed crystallinity estimates.

Abstract

An accurate knowledge of the mineralogy (chemical composition and crystal structure) of the silicate dust in the interstellar medium (ISM) is crucial for understanding its origin in evolved stars, the physical and chemical processing in the ISM, and its subsequent incorporation into protostellar nebulae, protoplanetary disks and cometary nuclei where it is subjected to further processing. While an appreciable fraction of silicate dust in evolved stars, in protoplanetary disks around pre-main sequence stars, in debris disks around main sequence stars, and in cometary nuclei is found to be in crystalline form, very recent infrared spectroscopic studies of the dust along the sightline toward the Galactic center source Sgr A* placed an upper limit of ~1.1% on the silicate crystalline fraction, well below the previous estimates of ~5% or ~60% derived from the observed 10 micron absorption profile for the local ISM toward Cyg OB2 No.12. Since the sightline toward SgrA contains molecular cloud materials as revealed by the detection of the 3.1 and 6.0 micron water ice absorption features, we argue that by taking into account the presence of ice mantles on silicate cores, the upper limit on the degree of silicate crystallinity in the ISM is increased to ~3--5%.