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Astron. Astrophys. 348, 755-767 (1999)

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ISOGAL-DENIS detection of red giants with weak mass loss in the Galactic bulge * ** ***

A. Omont 1, S. Ganesh 1,2, C. Alard 3,1, J.A.D.L. Blommaert 4, B. Caillaud 1, E. Copet 5, P. Fouqué 6, G. Gilmore 7, D. Ojha 8,1, M. Schultheis 1, G. Simon 3, X. Bertou 1, J. Borsenberger 1, N. Epchtein 9, I. Glass 10, F. Guglielmo 1, M.A.T. Groenewegen 11, H.J. Habing 12, S. Kimeswenger 13, M. Morris 14,1, S.D. Price 15, A. Robin 16, M. Unavane 7 and R. Wyse 17

1 Institut d'Astrophysique de Paris, CNRS, 98bis Bd Arago, F-75014 Paris, France
2 Physical Research Laboratory, Navarangpura, Ahmedabad 380009, India
3 DASGAL, Observatoire de Paris, France
4 ISO Data Centre, ESA, Villafranca, Spain
5 DESPA, Observatoire de Paris, France
6 ESO, Santiago, Chile
7 Institute of Astronomy, Cambridge, UK
8 T.I.F.R., Mumbai, India
9 O.C.A., Nice, France
10 SAAO, South Africa
11 MPA, Garching, Germany
12 Leiden Observatory, Leiden, The Netherlands
13 Innsbruck, Austria
14 UCLA, Los Angeles, CA, USA
15 Air Force Research Laboratory, Hanscom AFB, MA, USA
16 Observatoire de Besancon, France
17 The Johns Hopkins University, Baltimore MD, USA

Received 4 February 1999 / Accepted 26 May 1999


The ISOGAL project is a survey of the stellar populations, structure, and recent star formation history of the inner disk and bulge of the Galaxy. ISOGAL combines 15 µm and 7 µm ISOCAM observations with DENIS IJKs data to determine the nature of a source and the interstellar extinction. In this paper we report an ISOGAL study of a small field in the inner Galactic Bulge ([FORMULA] = 0.0o, b = 1.0o, area = 0.035 deg2) as a prototype of the larger area ISOGAL survey of the inner Galaxy. The ISOCAM data are two orders of magnitude more sensitive than IRAS ones, and their spatial resolution is better by one order of magnitude, allowing nearly complete and reliable point-source detection down to [FORMULA] 10 mJy with the LW3 filter (12-18 µm) and [FORMULA] 15 mJy with the LW2 filter (5.5-8 µm). More than 90% of the ISOCAM sources are matched with a near-infrared source of the DENIS survey. The five wavelengths of ISOGAL+DENIS, together with the relatively low and constant extinction in front of this specific field, allow reliable determination of the nature of the sources.

While most sources detected only with the deeper 7 µm observation are probably RGB stars, the primary scientific result of this paper is evidence that the most numerous class of ISOGAL 15 µm sources are Red Giants in the Galactic bulge and central disk, with luminosities just above or close to the RGB tip and weak mass-loss rates. They form loose sequences in the magnitude-colour diagrams [15]/[FORMULA]-[15] and [15]/[7]-[15]. Their large excesses at 15 µm with respect to 2 µm and 7 µm is due to circumstellar dust produced by mass-loss at low rate ([FORMULA]-a few [FORMULA]/yr). These ISOGAL results are the first systematic evidence and study of dust emission at this early stage (Intermediate AGB and possibly RGB-Tip), before the onset of the large mass-loss phase ([FORMULA]/yr). It is thus well established that efficient dust formation is already associated with such low mass-loss rates during this early phase.

About twenty more luminous stars are also detected with larger excess at 7 and 15 µm. Repeated ISOGAL observations suggest that the majority of these are long period variables with large amplitude, probably in the large mass-loss stage with [FORMULA]/yr.

Key words: stars: circumstellar matter – stars: mass-loss – stars: AGB and post-AGB – Galaxy: center – Galaxy: stellar content – infrared: stars

* This is paper no. 4 in a refereed journal based on data from the ISOGAL project
** 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
*** Partly based on observations collected at the European Southern Observatory, La Silla Chile

Send offprint requests to: A. Omont (omont@iap.fr)

© European Southern Observatory (ESO) 1999

Online publication: August 13, 199