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Astron. Astrophys. 315, L333-L336 (1996)

The 6.0 and 6.8 img1.gifm absorption features in the spectrum of NGC 7538: IRS9*

W.A. Schutte1, A.G.G.M. Tielens2, D.C.B. Whittet3, A. Boogert4, P. Ehrenfreund1, Th. de Graauw4, T. Prusti5, E.F. van Dishoeck1, and P. Wesselius4,

1 Leiden Observatory, P. O. Box 9513, 2300 RA Leiden, The Netherlands
2 NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA
3 Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
4 SRON Laboratory for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands
5 ISO Science Operations Centre, Astrophysics Division, ESA, Villafranca del Castillo,, P.O. Box 50727, E-28080 Madrid, Spain

Received 15 July 1996 / Accepted 22 August 1996

Abstract. Short Wavelength Spectrometer observations of the 6.0 and 6.8 img1.gifm absorption features towards the obscured young stellar object NGC 7538: IRS9 are presented and compared to the features of a number of species measured in the solid phase. It is found that while solid img7.gif provides the bulk of the 6 img1.gifm band, additional absorption is required near 5.83 img1.gifm and 6.25 img1.gifm. It is proposed that formic acid (HCOOH) is a plausible candidate for the blue excess, while the red excess could be related to the C-C stretching mode in aromatic structures. Subsequently, a number of candidates for the 6.8 img1.gifm band are reviewed. Unfortunately, none of these appears to satisfy the observational constraints and no plausible identification can at this point be presented.

Key words: ISM: dust, extinction - ISM: molecules - infrared: ISM: lines and bands - stars: pre-main-sequence - stars: individual: NGC 7538: IRS9

* ISO is 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.

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