Absorption at 11 microns in the interstellar medium and embedded sources: evidence for crystalline silicates

TL;DR

This study presents evidence that the 11 micron absorption feature in astronomical spectra is caused by crystalline silicates, particularly crystalline olivine, suggesting early crystallization or preservation of silicates in space.

Contribution

It provides new ground-based and space-based spectral data supporting crystalline silicates as the carrier of the 11 micron feature, indicating their presence in various cosmic environments.

Findings

The 11 micron feature matches crystalline olivine emissivity.

The feature is associated with a polarisation signature.

Crystalline silicates are present in the ISM and embedded sources.

Abstract

An absorption feature is occasionally reported around 11 microns in astronomical spectra, including those of forming stars. Candidate carriers include water ice, polycyclic aromatic hydrocarbons (PAHs), silicon carbide, crystalline silicates or even carbonates. All are known constituents of cosmic dust in one or more types of environments, though not necessarily together. In this paper we present new ground-based 8-13 micron spectra of one evolved star, several embedded young stellar objects (YSOs) and a background source lying behind a large column of the interstellar medium (ISM) toward the Galactic Centre. Our observations, obtained at a spectral resolution of approximately 100, are compared with previous lower resolution data, as well as data obtained with the Infrared Space Observatory (ISO) on these and other targets. By presenting a subset of a larger sample our aim is to establish the reality of the feature and subsequently speculate on its carrier. All evidence points toward crystalline silicate. For instance, the 11 micron band profile is well matched with the emissivity of crystalline olivine. Furthermore, the apparent association of the absorption feature with a sharp polarisation signature in the spectrum of two previously reported cases suggests a carrier with a relatively high band strength compared to amorphous silicates. If true, this would either set back the evolutionary stage in which silicates are crystallised, either to the embedded phase or even before within the ISM, or else the silicates ejected from the outflows of evolved stars retain some of their crystalline identity during their long residence in the ISM.