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

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 (ℓ = 0.0°, b = 1.0°, 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 ∼ 10 mJy with the LW3 filter ( 12-18 μm) and ∼ 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 ∼tm 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]/Ks-[15] and [15]/[7]-[15]. Their large excesses at 15μm with respect to 2 μm and 7 μm is due to circumstellar dust produced by mass-loss at low rate (Mdust ∼ l0-11-a few 10-10M⊙/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 (M ≥ 10-7M⊙/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 M ≥ 10-7M⊙/yr.