Possible identification of the 45-µm ice signature in Orion

Abstract

THE composition and size distribution of interstellar dust are controversial matters. Even the ice contribution to the well known and conspicuous 3-µm feature of the extinction curve has recently been questioned. We now report a study of the middle-infrared spectrum of dust emission. This spectral region is of interest because it includes the 45-µm band of water ice which, unlike the 3-µm feature, is characteristic of the ice lattice—being essentially due to stretching vibrations of the H–O bond. The 45-µm band also carries information on the physical state and temperature of the ice¹. If the ice is amorphous, the band is wide (∼19 µm) and, at 100 K, peaks at 45 µm; crystalline ice produces a narrow peak ∼ 2 µm wide at 43.5 µm, for a temperature of 100 K; these characteristics are all temperature dependent. Note that the only possible crystalline phase at pressures below 1 bar is Ic (cubic) or Ih (hexagonal), for both of which the above statements are true. The relevant lattice vibrations are essentially translational and probably associated with the transverse optical branch of the dispersion diagram. Our results suggest an emission due to predominantly amorphous ice.