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Astron. Astrophys. 343, 966-976 (1999)

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Weak ice absorption features at 7.24 and 7.41 µm in the spectrum of the obscured young stellar object W 33A *

W.A. Schutte 1, A.C.A. Boogert 2, A.G.G.M. Tielens 2, D.C.B. Whittet 3, P.A. Gerakines 3, J.E. Chiar 4, P. Ehrenfreund 1, J.M. Greenberg 1, E.F. vanDishoeck 1 and Th. deGraauw 5

1 Raymond and Beverly Sackler Observatory Laboratory at Leiden University, P.O. Box 9513, 2300 RA, Leiden, The Netherlands (schutte@strw.leidenuniv.nl)
2 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV, Groningen, The Netherlands
3 Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
4 NASA-Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
5 SRON, P.O. Box 800, 9700 AV, Groningen, The Netherlands

Received 27 August 1998 / Accepted 10 December 1998


ISO-SWS observations of the highly obscured young stellar object W 33A reveal two broad absorption features centered at 7.24 and 7.41 µm. Comparison to interstellar ice analogs shows that the 7.24 µm band can be well matched by the CH deformation mode of formic acid (HCOOH), while the 7.41 µm band can be fitted both by the formate ion ([FORMULA]) and acetaldehyde ([FORMULA]). 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 µm band to the [FORMULA] 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.

Key words: infrared: ISM: lines and bands – ISM: molecules – ISM: abundances – stars: individual: NGC 7538:IRS 9 – stars: individual: W 33A – methods: laboratory

* 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.

Send offprint requests to: W.A. Schutte (schutte@strw.leidenuniv.nl)

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999