Astron. Astrophys. 350, 163-180 (1999)

The composition and nature of the dust shell surrounding the binary AFGL 4106 ^* **

F.J. Molster ¹, L.B.F.M. Waters ^1,2, N.R. Trams ³, H. van Winckel ⁴, L. Decin ⁴, J.Th. van Loon ¹, C. Jäger ⁵, Th. Henning ⁵, H.-U. Käufl ⁶, A. De Koter ¹ and J. Bouwman <sup>1

1</sup> Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² SRON Space Research Laboratory, P.O. Box 800, 9700 AV Groningen, The Netherlands
³ Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands
⁴ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, B-3001 Heverlee, Belgium
⁵ Astrophysical Institute and University Observatory (AIU), Schillergäßchen 2-3, D-07745 Jena, Germany
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany

Received 23 February 1999 / Accepted 16 June 1999

Abstract

We present infrared spectroscopy and imaging of AFGL 4106. The 2.4-5 µm ISO-SWS spectrum reveals the presence of a cool, luminous star () in addition to an almost equally luminous F star (). The 5-195 µm SWS and LWS spectra are dominated by strong emission from circumstellar dust. We find that the dust consists of amorphous silicates, with a minor but significant contribution from crystalline silicates. The amorphous silicates consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be excluded but if present they contain much less Fe than the amorphous olivines. Comparison with laboratory data shows that the pure Mg-end members of the crystalline olivine and pyroxene solid solution series are present. In addition, we find strong evidence for simple oxides (FeO and Al₂O₃) as well as crystalline H₂O ice. Several narrow emission features remain unidentified.

Modelling of the dust emission using a dust radiation transfer code shows that large grains (m) must be present and that the abundance of the crystalline silicates is between 7 and 15% of the total dust mass, depending on the assumed enstatite to forsterite ratio, which is estimated to be between 1 and 3. The amorphous and crystalline dust components in the shell do not have the same temperature, implying that the different dust species are not thermally coupled. We find a dust mass of   expelled over a period of  years for a distance of 3.3 kpc. The F-star in the AFGL 4106 binary is likely a post-red-supergiant in transition to a blue supergiant or WR phase.

Key words: infrared: stars stars: supergiants stars: mass-loss stars: binaries: spectroscopic stars: circumstellar matter stars: individual: AFGL 4106

* 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
** Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: F.J. Molster (frankm@astro.uva.nl)

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Contents

• 1. Introduction
• 2. The observations and data reduction
  • 2.1. ISO observations
    • 2.1.1. SWS
    • 2.1.2. LWS
    • 2.1.3. Comparison with IRAS data
  • 2.2. 10 µm imaging at the ESO 3.6m telescope
• 3. The 2.4-5 µm part of the spectrum; a binary revealed
  • 3.1. Interstellar and circumstellar extinction
• 4. The 5-195 µm part of the spectrum; the dust shell
  • 4.1. The continuum subtraction of the SWS and LWS spectra
    • 4.1.1. The 10-20 µm region
    • 4.1.2. The 20-29.5 µm region
    • 4.1.3. The 29.5-45 µm region
    • 4.1.4. The 45-195 µm region, LWS
  • 4.2. Identifications
    • 4.2.1. Amorphous silicates
    • 4.2.2. Crystalline olivines and pyroxenes
    • 4.2.3. Diopside
    • 4.2.4. Aluminum rich dust
    • 4.2.5. Water ice
    • 4.2.6. Iron oxides
    • 4.2.7. Unidentified features
• 5. Radiative transfer modelling
  • 5.1. Dust species
  • 5.2. The best fit model parameters
  • 5.3. Discussion of the model fit
• 6. Discussion
  • 6.1. The distance and luminosity
  • 6.2. The dust shell
  • 6.3. Formation of crystalline silicates
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999
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