Aj. Adamson et al., Spectropolarimetric constraints on the nature of the 3.4 micron absorber in the interstellar medium, ASTROPHYS J, 512(1), 1999, pp. 224-229
Spectropolarimetry of the 3.4 mu m aliphatic C-H stretch feature, generally
attributed to carbonaceous dust in the diffuse interstellar medium, has be
en carried out in the line of sight from the Galactic center source Sagitta
rius A IRS 7. The feature is unpolarized (Delta p/Delta tau < 0.2): the upp
er limit for polarization is well below that expected on the basis of a mod
el 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 ca
rrier resides in a population of small, nonpolarizing carbonaceous grains,
physically separate from the silicates and sharing many characteristics wit
h the carriers of the 217.5 nm extinction bump.