Combined VLT ISAAC/ISO SWS spectroscopy of two protostellar sources

The importance of minor solid state features

¹ “Astrochimie Expérimentale”, IAS-CNRS, Bât. 121, Université Paris Sud, 91405 Orsay Cedex, France
² Department of Physics and Astronomy, Univeristy College London, WC1E 6BT, UK
³ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, Netherlands (Based on observations collected at the European Southern Observatory, Chile (ESO 164.I-0605(A)). ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.)

Corresponding author: E. Dartois, emmanuel.dartois@ias.u-psud.fr

Received: 27 May 2002
Accepted: 26 August 2002

Abstract

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