Astron. Astrophys. 364, 170-178 (2000)

Structure and chemical composition of the silicate dust around OH/IR stars ^*

K. Demyk ¹, E. Dartois ^2,1, H. Wiesemeyer ², A.P. Jones ¹ and L. d'Hendecourt <sup>1

1</sup> IAS-CNRS, Université Paris XI, Batiment 121, 91405 Orsay Cedex, France
² IRAM, 300, rue de la Piscine, 38406 Saint Martin d'Hères, France

Received 31 May 2000 / Accepted 13 August 2000

Abstract

Using a radiative transfer code we have studied the dust shells of the two OH/IR stars IRAS 17004-4119 (OH344.93) and IRAS 17411-3154 (OH357). The ISO-SWS spectra of both sources exhibit deep amorphous silicate absorption bands at 9.8 and 17.5 µm together with crystalline silicate emission bands at 33.6, 40.5 and 43 µm. In both sources the 9.8 µm silicate band shows a shoulder at 11.2 µm. The amorphous silicates are mainly composed of olivine. With the adopted set of optical constants, the amount of amorphous pyroxene-type silicates participating in the absorption cannot exceed 10% of the amorphous silicate mass. The crystalline silicates are identified with enstatite, forsterite and diopside. They represent 35% and 25% of the amorphous silicate mass for IRAS 17004-4119 and IRAS 17411-3154, respectively. In these sources, the 11.2 µm feature is attributed to the absorption of crystalline forsterite which is also observed in emission at 33.6 µm. Water ice is observed in both objects through two bands in the covered wavelength range, in absorption (at 3.09 µm) and in emission (at 43 µm). The presence of water ice and crystalline forsterite bands, observed in emission and absorption, emphasizes the necessity, in such objects, of modelling the radiative transfer in order to interpret the overall spectra. Our results, compatible with classical silicate formation theories, may help to put some constraints on dust formation models.

Key words: stars: late-type stars: individual: IRAS 17411-3154, IRAS 17004-4119 stars: circumstellar matter radiative transfer infrared: stars

* 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: K. Demyk (demyk@ias.fr)

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Modelling
  • 3.1. Radiative transfer modelling
  • 3.2. Dust modelling
• 4. Results
  • 4.1. The NIR opacity
  • 4.2. The 11.2 µm feature
  • 4.3. Water ice
  • 4.4. Composition of the amorphous silicates
  • 4.5. Composition of the crystalline silicates
• 5. Discussion
• 6. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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