... waves

Appendices are only available in electronic form via http://www.aanda.org

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... mantles

The possibility that silicon may be present in grain mantles has been considered previously by Walmsley et al. (1999) and Schilke et al. (2001), in the context of photon-dominated regions.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...%

The possibility that solid SiO at the higher concentrations might be detectable in the infrared needs careful evaluation. The fundamental transition should fall around 8 $\mu$m, in a region of the infrared which is congested, owing to emission from ices and silicates. The first harmonic at 4 $\mu$m would probably also be difficult to detect (Dartois, personal communication).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.