Combined VLT ISAAC/ISO SWS spectroscopy of two protostellar sources - The importance of minor solid state features

We discuss the L band infrared spectra of two massive protostars in the region of the so-called 3.47 mum absorption feature. The high sensitivity VLT-ISAAC spectra of the two differently evolved massive protostars GL 989 and GL 2136, together with dedicated infrared laboratory experiments on ice molecular interactions, allow us to propose the identification of this feature with the formation of an ammonia hydrate. Combined with an analysis of additional ISO observations, the derived ratio of solid NH3/H2O present in ice mantles is less than or equal to 7%. This amount fits well with the nitrogen cosmic abundance and the chemical evolution of ices where other nitrogen containing OCN- and NH4+ ions are observed.