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Astron. Astrophys. 353, 349-362 (2000)

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ISO-SWS observations of interstellar solid 13CO2: heated ice and the Galactic 12C[FORMULA]C abundance ratio *

A.C.A. Boogert 1,2, P. Ehrenfreund 3, P.A. Gerakines 4, A.G.G.M. Tielens 1,2, D.C.B. Whittet 4, W.A. Schutte 3, E.F. van Dishoeck 3, Th. de Graauw 1,2, L. Decin 5 and T. Prusti 6

1 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
2 SRON, P.O. Box 800, 9700 AV Groningen, The Netherlands
3 Leiden Observatory, P. O. Box 9513, 2300 RA Leiden, The Netherlands
4 Rensselaer Polytechnic Institute, Department of Physics, Applied Physics and Astronomy, Troy, NY 12180, USA
5 Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
6 ISO Data Centre, Astrophysics Division, ESA, Villafranca del Castillo, P.O. Box 50727, 28080 Madrid, Spain

Received 29 July 1998 / Accepted 12 October 1999

Abstract

We present observations of the stretching mode of 13CO2 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory (ISO-SWS). Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to 13CO2 mixed with polar molecules such as [FORMULA], and [FORMULA]. The high-mass protostars GL 2136, GL 2591, S 140 : IRS1, and W 3 : IRS5 show an additional narrow substructure at 2282 [FORMULA] (4.382 [FORMULA]), which we attribute to a polar, [FORMULA]-containing [FORMULA] ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar [FORMULA] ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing.

Furthermore, we determine for the first time the Galactic [FORMULA] ratio from the solid state as a function of Galacto-centric radius. The 12CO2/13CO2 ratio for the local ISM (69[FORMULA]), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase ([FORMULA], H2CO) studies. For the few individual objects for which gas phase values are available, the 12C/13C ratios derived from [FORMULA] tend to be higher compared to CO studies (albeit with [FORMULA] significance only). We discuss the implications of this possible difference for the chemical origin of interstellar [FORMULA].

Key words: ISM: dust, extinction – ISM: molecules – ISM: abundances – infrared: ISM: lines and bands – stars: formation – Galaxy: abundances

* 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.

Present address: California Institute of Technology, Downs Laboratory of Physics 320-47, Pasadena, CA 91125, USA

Present address: Code 691, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

Send offprint requests to: A.C.A. Boogert (boogert@submm.caltech.edu)

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999
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