3.4 Micron Feature on the Shoulder of Ice-band Absorptions in Three Luminous Young Stellar Objects: Iras 18511+0146, Iras 21413+5442, and Iras 04579+4703

An absorption feature at 3.4 lm has been detected in the long-wavelength wing of the 3.1 lm H2O ice feature in three young stellar objects: IRAS 18511+0146, IRAS 21413+5442, and IRAS 04579+4703. The profile of the 3.4 lm absorption in IRAS 18511+0146 has distinct subfeatures at 3.38, 3.41, and 3.48 lm and is not similar to those found in molecular clouds but is similar to those in the diffuse interstellar medium, such as the lines of sight toward the Galactic center and Cyg OB2 12. Spectropolarimetry of the 3.4 lm feature in IRAS 18511+0146 shows no excess polarization accompanying the feature, which is consistent with the observation toward the Galactic center source IRS 7 by Adamson et al.The 3.4 lm absorption in IRAS 18511+0146 probably occurs in the diffuse interstellar medium intervening in the line of sight to the molecular cloud where the H2O ice absorption occurs. Furthermore, the 3.4 lm absorption carrier seems to reside in a population of nonpolarizing grains in the diffuse interstellar medium, physically separate from other polarizing grains.