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Astron. Astrophys. 356, 501-516 (2000)

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Infrared imaging and spectroscopy of the Luminous Blue Variables Wra 751 and AG Car * **

R.H.M. Voors 1,2,3, L.B.F.M. Waters 3,4, A. de Koter 3, J. Bouwman 3, P.W. Morris 1,3, M.J. Barlow 5, R.J. Sylvester 5, N.R. Trams 6 and H.J.G.L.M. Lamers 2,1

1 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2 University of Utrecht, Astronomical Institute, Princetonplein 5, 3584 CC Utrecht, The Netherlands
3 University of Amsterdam, Astronomical Institute Anton Pannekoek, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4 Katholieke Universiteit Leuven, Instituut voor Sterrenkunde, Celestijnenlaan 200B, 3001 Heverlee, Belgium
5 University College London, Department of Physics and Astronomy, Gower St., London WC1E 6BT, UK
6 Integral Science Operations, Astrophysics Division of ESA, ESTEC SCI-SAG, PO Box 299, 2200 AG Noordwijk, The Netherlands

Received 10 March 1999 / Accepted 10 January 2000

Abstract

We present ground-based infrared imaging and ISO spectroscopy of the luminous blue variables Wra 751 and AG Car. The images show in both cases a detached shell with a roughly circular distribution of emission. The infrared images of AG Car coincide very well with the optical images. The optical (H[FORMULA]) image of Wra 751 is different from the infrared image; the H[FORMULA] nebula is suggested to be a scattering nebula containing cold dust particles.

Fitting both the images and the spectra consistently with a 1-D radiative transfer model, we derive properties of their dust shells. Wra 751 is surrounded by a dust shell with inner and outer radii of 0.17 and 0.34 pc respectively and a dust mass of 0.017 [FORMULA]. The dust shell of AG Car has inner and outer radii of 0.37 and 0.81 pc respectively and a total dust mass of 0.25 [FORMULA]. Dust mass-loss rates during the formation of the shells are 2.7[FORMULA] and 3.4[FORMULA] [FORMULA] yr-1, respectively. The total dust mass and hence the derived dust mass-loss rates are uncertain by at least a factor of two. For AG Car, the derived dust mass and mass-loss rate are higher than previous estimates. This is mainly caused by the fact that a contribution of very large grains ([FORMULA] 10 µm) is needed to explain the flux levels at longer wavelengths.

Dust models for both objects fail to explain the flux shortward of 15 to 20 µm: a population of small warm grains, not in thermal equilibrium with the central star is necessary to explain this excess. Similarities between dust shells around Wolf-Rayet stars and Wra 751 and AG Car (mass, grain size population, morphology) suggest a similar formation history and imply an evolutionary connection. A similar connection with red supergiants is suggested on the basis of the dust composition and derived time-averaged mass-loss rates.

Key words: stars: circumstellar matter – stars: evolution – stars: individual: Wra 751, AG Car – stars: mass-loss

* based on observations obtained with ISO, an ESA project with instruments funded by ESA Member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA
** based on observations obtained at ESO, La Silla, Chile

Send offprint requests to: L.B.F.M. Waters: rensw@astro.uva.nl

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
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