Spectropolarimetry of the 3.4 μm Feature in the Diffuse ISM toward the Galactic Center Quintuplet Cluster

Aliphatic hydrocarbons exhibit an absorption feature at 3.4 μm, observed toward sources that sample diffuse regions of the interstellar medium (ISM). 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 of 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% (GCS 3-II) and 0.15% ± 0.31% (GCS 3-IV), well below the lowest available prediction of polarization on the basis of the core-mantle model. We conclude that the hydrocarbons in the diffuse ISM do not reside on the same grains as the silicates, and likely form a separate population of small grains.