ISO mid-infrared spectroscopy of Galactic Bulge AGB stars

¹ Inst. voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium e-mail: jorisb@ster.kuleuven.be
² Service d'Astrophysique, CEA/DAPNIA, Saclay, France
³ Physical Research Laboratory, Ahmedabad, India
⁴ Institut d'Astrophysique de Paris, CNRS, Paris, France
⁵ SETI Institute, NASA Ames Research Center, USA
⁶ South African Astronomical Observatory, South Africa
⁷ Leiden Observatory, Leiden, The Netherlands
⁸ CNRS UMR6091, Observatoire de Besançon, Besançon, France
⁹ GEPI, Observatoire de Paris, France
¹⁰ Astrophysics Group, School of Physical & Geographical Sciences, Keele University, UK

Received: 31 July 2006
Accepted: 6 September 2006

Abstract

Aims.To study the nature of Bulge AGB stars and in particular their circumstellar dust, we have analysed mid-infrared spectra obtained with the ISOCAM CVF spectrometer in three Bulge fields.

Methods.The ISOCAM 5–16.5 μm CVF spectra were obtained as part of the ISOGAL infrared survey of the inner Galaxy. A classification of the shape of the 10 μm dust feature was made for each case. The spectra of the individual sources were modelled using a radiative transfer model. Different combinations of amorphous silicates and aluminium-oxide dust were used in the modelling.

Results.Spectra were obtained for 29 sources of which 26 are likely to be Bulge AGB stars. Our modelling shows that the stars suffer mass loss rates in the range of   /yr, which is at the low end of the mass-loss rates experienced on the thermally pulsing AGB. The luminosities range from 1700 to 7700  as expected for a population of AGB stars with M_init of 1.5–2 . In agreement with the condensation sequence scenario, we find that the dust content is dominated by Al₂O₃ grains in this sample of low mass-loss rate stars.