SIMBAD references

We discuss the L band infrared spectra of two massive protostars in the region of the so-called 3.47µm 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.