A TEST CASE FOR THE ORGANIC REFRACTORY MODEL OF INTERSTELLAR DUST

摘要

The organic refractory model of interstellar dust postulates that the grains primarily responsible for visual extinction consist of silicate cores coated with organic refractory matter (ORM) supposed to form by photochemical processing of ice mantles. We identify the line of sight to the early-type star Elias 25, in the ρ Oph dark cloud, as a test for local production of ORM in an environment where mantled grains are exposed to a substantial ultraviolet radiation field. Infrared spectra of Elias 25 in the 2.4-4.0 μm region were used to examine the 3 μm ice profile and to search for the C+H bond absorption feature near 3.4 μm that occurs in laboratory-synthesized ORM. We find that the ice feature in Elias 25 has an unusually deep 3.1-3.7 μm wing, but no detectable 3.4 μm feature is present in the wing profile. An upper limit on the τ_(3.4)/A_V ratio is set that is a factor of 4 lower than that determined from the feature detected in the diffuse interstellar medium toward Cygnus OB2 No. 12. Our results are consistent with other evidence indicating that the carrier of the 3.4 μm feature is destroyed, rather than produced, inside dense clouds.