2001A&A...365..144D

We present a study of the 3.1 micron absorption band, attributed to the OH stretching mode of the water molecule in the form of solid ice, observed in many protostellar lines of sight by the SWS instrument on board ISO. Using ice optical constants and Mie theory, we obtain reasonable fits to the peculiar band shapes observed in twelve sources. The fits clearly show that some scattering effects arise in this absorption band, as the grain sizes used in the calculations are of the order of a few tenths of a micron. In the fit residuals, we search for the ν1 and ν3 vibrations of the ammonia molecule which fall in the same spectral region, leading only to upper limits for the NH₃ content of a few percents of the total water ice content in these different lines of sight. We also discuss the occurrence of a 3.47µm absorption band which could be related to the formation of an ammonia hydrate in the ice mantles. On the assumption that this band is due merely to this hydrate and with the help of relevant laboratory experiments, we show that ammonia represents therefore at most 5% in abundance relative to water ice in these interstellar grain mantles. Finally, this study sheds light on the "3.1µm/6µm column density paradox" obtained when comparing the water ice absorption band at 3.1 (stretching mode) and 6µm (bending mode). We show that this paradox is resolved by considering different extinction regimes, in which scattering affects only the 3.1µm band whereas pure absorption dominates at 6µm.