Spectropolarimetric constraints on the nature of the 3.4 micron absorber in the interstellar medium

Spectropolarimetry of the 3.4 mu m aliphatic C-H stretch feature, generally attributed to carbonaceous dust in the diffuse interstellar medium, has been carried out in the line of sight from the Galactic center source Sagittarius A IRS 7. The feature is unpolarized (Delta p/Delta tau < 0.2): the upper limit for polarization is well below that expected on the basis of a model in which the carrier molecules are associated with the aligned silicate component of interstellar dust, for example, as an organic or carbonaceous mantle on a silicate core. The simplest explanation is that the 3.4 mu m carrier resides in a population of small, nonpolarizing carbonaceous grains, physically separate from the silicates and sharing many characteristics with the carriers of the 217.5 nm extinction bump.