Polyoxymethylene co-polymers on grains

Abstract

Conditions prevalent in dense molecular clouds are shown to favour the polymerization of H₂CO molecules and the deposition of formaldehyde co-polymer mantles, with typical radii ∼10⁻⁵ cm, on smaller refractory grains. If a significant fraction of such co-polymer coated grains are expelled with systematic gas flows into the general interstellar medium, these moderately refractory grains may be responsible for the bulk of interstellar extinction and polarization at optical wavelengths. Mie calculations for a mixture consisting of iron, graphite and POM particles are presented as an example where POM grains of radii 0.15 μ dominate the extinction at optical wavelengths, providing a satisfactory overall fit to a range of extinction data. A size distribution of POM needles with a ‘mean’ radius 0.15 μ also provides good agreement with data on interstellar linear as well as circular polarization. Suitably end-capped and stabilized co-polymer-coated grains, with either silicate or graphite cores, may survive at temperatures ∼450 K under interstellar ambient conditions and be responsible for the 10 μ emission feature in many sources. Theoretically computed band profiles of the 10 μ-feature in POM coated grains, in general, provide better agreement with observations than most types of silicate grains considered so far. We also note that an unexplained dip at λ≃10 μ in the 8–12 μ feature of the infrared source OH 231.8+4.2 may be a signature of POM grains; likewise, a persistent 3.3 μ emission feature in many different types of infrared source could be attributed to the CH stretching mode in formaldehyde co-polymer grains.