Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Aliphatic hydrocarbons exhibit an absorption feature at 3.4 µm observed toward sources that sample diffuse regions of the interstellar medium. The absorbers responsible for this feature are assumed to reside in some component of interstellar dust, but the physical nature of the particles (size, shape, structure, etc.) is uncertain. Observations of interstellar polarization provide discrimination. Since the grains that carry the silicate absorption feature are known to be aligned, polarization across the 3.4 µm hydrocarbon feature can be used to test the silicate core-organic refractory mantle grain theory. Although the 3.4 µm feature has been observed to be devoid of polarization for one line of sight toward the Galactic center, a corresponding silicate polarization measurement for the same line of sight was not available. Here, we present spectropolarimetric observations toward GCS 3-II and GCS 3-IV toward the Galactic center, where the 9.7 µm silicate polarization has been previously observed. We show that polarization is not detected across the 3.4 µm feature to a limit of 0.06 ± 0.13%
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