A Study of Absorption Features in the 3 Micron Spectra of Molecular Cloud Sources with H 2O Ice Bands

Abstract
New 3.3--3.6~$mu$m spectra were obtained of nine young stellar objects embedded in molecular clouds. An absorption feature at $sim$3.47~$mu$m (2880~cm$^{-1}$) with FWHM$sim$0.09 $mu$m (80 cm$^{-1}$), first identified by Allamandola et al. (1992), was definitively detected toward seven objects, and marginally in the other two. The feature is better correlated with H$_2$O ice than with the silicate dust optical depth in the data obtained to date. Assuming the feature is due to a C--H stretch absorption, the abundance of the C--H bonds averaged along the lines of sight is closely related to that of H$_2$O ice. We interpret the correlation with H$_2$O ice as indicating that the C--H bonds form together with H$_2$O ice on grain surfaces in the molecular clouds, though other formation mechanisms are not ruled out. A second absorption feature at 3.25 $mu$m (3080 cm$^{-1}$) was detected toward NGC7538/IRS 1 and S140/IRS 1; this feature was first detected in spectra of MonR2/IRS 3 (Sellgren, Smith, & Brooke 1994; Sellgren et al. 1995). There is as yet insufficient data to tell whether this feature is better correlated with H$_2$O ice or silicates.
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