Weak ice absorption features at 7.24 and 7.41 MU M in the spectrum of the obscured young stellar object W 33A
de Graauw, Th.; van Dishoeck, E. F.; Whittet, D. C. B.; Schutte, W. A.; Tielens, A. G. G. M.; Boogert, A. C. A.; Ehrenfreund, P.; Gerakines, P. A.; Greenberg, J. M.; Chiar, J. E.
Netherlands, United States
Abstract
ISO-SWS observations of the highly obscured young stellar object W 33A reveal two broad absorption features centered at 7.24 and 7.41 mu m. Comparison to interstellar ice analogs shows that the 7.24 mu m band can be well matched by the CH deformation mode of formic acid (HCOOH), while the 7.41 mu m band can be fitted both by the formate ion (HCOO(-) ) and acetaldehyde (CH_3HCO). The laboratory spectra reveal additional strong features of these molecules which should make a more definite identification straightforward. While an assignment of the 7.24 mu m band to the -CH_3 deformation mode of aliphatic species may be considered, the absence of a corresponding strong CH stretching mode argues against this possibility. These results and the earlier tentative detection of HCOOH towards NGC7538:IRS9 suggest that formic acid is a general component of the ices in the vicinity of embedded high-mass young stellar objects. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.