2001ApJ...558..185N

We present high-resolution (R∼1500-2000) spectra of the 4.27 µm asymmetric stretching feature of solid CO₂ in eight lines of sight observed with the Short Wavelength Spectrometer of the Infrared Space Observatory. Two of the sources are field stars located behind the Taurus molecular cloud; the others are young stellar objects (YSOs) of predominantly low-to-intermediate mass. We find a significant source-to-source variation in the solid CO₂/H₂O abundance ratio in our sample: two lines of sight, Elias 18 and RAFGL 989, have CO₂ abundances of ∼34%-37%, considerably higher than in other lines of sight studied to date. In agreement with a previous study of Elias 16, we confirm a substantial (∼20%) abundance of solid CO₂ relative to H₂O in the quiescent intracloud medium. We compare the CO₂ profiles with laboratory spectra of interstellar ice analogs from the Leiden Observatory Laboratory database.
Result. show that the 4.27 µm profiles toward field stars and embedded low-mass objects are remarkably similar to each other and seem to originate mostly in cold H₂O-rich ice. In two higher mass YSOs (RAFGL 989 and S255 IRS1), the profiles are clearly different, and at least the latter source shows signs of thermal processing.