New Three Micron Spectra of Young Stellar Objects with H_2O Ice Bands

Abstract

We present new groundbased 3 $μ$m spectra of 14 young stellar objects with H_2O ice absorption bands. The broad absorption feature at 3.47 $μ$m was detected toward all objects and its optical depth is correlated with the optical depth of H_2O ice, strengthening an earlier finding. The broad absorption feature at 3.25 $μ$m was detected toward two more sources and an upper limit is given for a third source. The optical depths of the 3.25 $μ$m feature obtained to date are better correlated with the optical depth of the refractory silicate dust than with that of H_2O ice. If this trend is confirmed, this would support our proposed identification of the feature as the C--H stretch of aromatic hydrocarbons at low temperature. An absorption feature at 3.53 $μ$m due to solid methanol was detected for the first time toward MonR2/IRS2, as well as toward W33A and GL 2136. The wavelengths of the CH_3OH features toward W33A, GL 2136, and NGC7538/IRS9 can be fit by CH_3OH-rich ices, while the wavelength of the feature toward MonR2/IRS2 suggests an H_2O-rich ice environment. Solid methanol abundances toward GL 2136, NGC7538/IRS9, and MonR2/IRS2 are 3-5 % relative to H_2O ice. There is an additional narrow absorption feature near 3.47 $μ$m toward W33A. For the object W51/IRS2, spatially resolved spectra from 2 to 4 $μ$m indicate that the H_2O ice is located predominantly in front of the eastern component and that the H_2O ice extinction is much deeper than previously estimated. For the object RNO 91, spectra from 2 to 4 $μ$m reveal stellar (or circumstellar) CO gas absorption and deeper H_2O ice extinction than previously estimated.