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Astron. Astrophys. 317, 859-870 (1997)


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IRAS 22272 [FORMULA] 5435 - a source with 30 and 21 µm features

R. Szczerba 1, 2, 3, A. Omont 2, K. Volk 1, P. Cox 4, 5 and S. Kwok 1

1 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada  T2N 1N4
2 Institut d'Astrophysique de Paris, C.N.R.S., 98bis bd. Arago, F-75014 Paris, France
3 N. Copernicus Astronomical Center, ul. Rabiaska 8, PL-87-100 Toru, Poland
4 Observatoire de Marseille, 2 pl. Leverrier, F-13248 Marseille Cedex 04, France
5 Max-Planck-Institut für Radioastronomie, Postfach 2024, D-53121 Bonn, Germany

Received 31 October 1995 / Accepted 17 May 1996

Abstract

Recent observations have shown that IRAS 22272 [FORMULA] 5435, a post-AGB object with a cool central star, is one of the few sources known which displays strong 21 and 30 µm features. We present a detailed analysis of the spectral energy distribution and discuss the possible nature of the dust grains in the circumstellar envelope around this source. We find that grains of pure magnesium sulfides could be responsible for the observed 30 µm feature provided they have a high enough temperature and an appropriate broad distribution of shapes inside the shell. The absorption properties of such grains at optical and ultraviolet wavelengths are unknown, so at the present time we are unable to estimate how high a temperature they would have in the circumstellar shell; laboratory measurements of the the optical properties are needed. The 30 µm feature cannot be produced by mantles of magnesium sulfide covering an amorphous carbon core as such grains are shown to have two maxima in their absorption cross sections around a minimum at about 30 µm. Since the material(s) responsible for 21 and 30 µm features is (are) still unknown we constructed an empirical opacity function which provides a reasonable fit to the energy distribution at far-infrared wavelengths. We also show that small particles (with radii smaller than 10 Å) having properties very similar to those of PAH molecules are an indispensable part of dust around this source. For these small grains the quantum treatment of heating and cooling processes is important. Assuming that the central star of IRAS 22272 [FORMULA] 5435 is burning H quiescently in a shell, the parameters of the best fit to the energy distribution of this source indicate a stellar core mass 0.60 [FORMULA] which is presently evolving with no (or with very little) mass loss.

Key words: stars: AGB, post-AGB – circumstellar matter – stars: individual: IRAS 22272+5435 – infrared: stars

Send offprint requests to: R. Szczerba: szczerba@ncac.torun.pl

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998

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