Infrared observations have clearly shown the presence of absorption features (e.g., at 3.1, 4.67, 4.9, 6.0, 6.8, µm) along the line of sight of embedded young stellar objects and field stars obscured by dense molecular clouds. These features are characteristic of simple molecules such as H2 O, CO, OCS, CH3 OH in the solid state. These molecules are believed to be frozen in icy grain mantles formed by accretion (e.g., CO) or reaction of gas phase species (e.g., H and O to form H2 O) onto preexisting refractory grain cores.
On the basis of laboratory experiments and theoretical calculations it has been possible to predict the presence of solid CO2 in icy grain mantles (Tielens & Hagen 1982; d'Hendecourt et al. 1985). However because of telluric absorption its detection is impossible with ground-based and airborne telescopes. D'Hendecourt & Jourdain de Muizon (1989) have reported the detection of the CO2 bending mode at about 15.2 µm with the low resolution spectrometer (LRS) on board IRAS along the line of sight of 3 infrared sources. Whittet & Walker (1991) carried out a systematic search of the LRS database for sources known to have the 3 µm absorption feature and found no discernible 15.2 µm absorption band except the ones already studied by D'Hendecourt & Jourdain de Muizon (1989).
With the Infrared Space Observatory it is now possible to observe both the stretching and bending modes of solid CO2. In fact solid CO2 has already been detected with ISO-SWS and ISOPHOT-S along the line of sight of several embedded sources (de Graauw et al. 1996a; Whittet et al. 1996; Gürtler et al. 1996) showing that it is a ubiquitous constituent of icy grain mantles in molecular clouds.
NGC7538 is a complex of HII regions and molecular clouds of about one degree of diameter situated in the Perseus spiral arm at 2700 pc. South-east of the centre of the visible nebula three very compact HII regions called NGC7538 IRS1, IRS2, and IRS3 are found. IRS1 has infrared properties characteristic of pre-main sequence objects and appears to be the youngest and the most luminous (more than 104 L ) of the three sources. Being a very luminous source still embedded in the dense dusty envelope in which the star is forming, IRS1 is a perfect candidate for studying interstellar absorption features. Indeed, features of H2 O (at 3 µm), CO (at 4.67 µm) and CH4 (at 7.6 µm) ices have been observed towards this source from the ground (Willner et al. 1982; Lacy et al. 1984, 1991).
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998