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