ISOGAL Survey of Baade ' s Windows in the Mid - infraredI

The ISOGAL mid-infrared survey of areas close to the Galactic Plane aims to determine their stellar content and its possible bearing on the history of the Galaxy. The NGC 6522 and Sgr I Baade's Windows of low obscuration towards the inner parts of the Bulge represent ideal places in which to calibrate and understand the ISOGAL colour-magnitude diagrams, which are more diicult to interpret in heavily reddened elds. The survey observations were made with the ISOCAM instrument of the ISO satellite. The lter bands chosen were LW2 (7 m) and LW3 (15 m). The results presented here show that most of the detected objects are late M-type giants on the AGB, with a cut-oo for those earlier than M3{M4. The most luminous members of these two Bulge elds at 7 m are the Mira variables. However, it is evident that they represent the end of a sequence of increasing 15 m dust emission which commences with M giants of earlier sub-type. In observations of late-type giants the ISOCAM 15 m band is mainly sensitive to the cool silicate or aluminate dust shells which overwhelm the photospheric emission. However, in ordinary M-giant stars, the 7 m band is not strongly aaected by dust emission and may be innuenced instead by absorption. The 2 band of water at 6.25 m and the SiO fundamental at 7.9 m are likely contributors to this eeect. Miras are more luminous at 7 m and have redder K 0 {{7] colours than other M giants. Their 7]{{15] colours are consequently bluer than might be expected by extrapolating from warmer M-giants. A group of late M stars has been found which vary little or not at all but have infrared colours typical of well-developed dust shells. Their luminosities are similar to those of 200{300 day Miras but they have slightly redder 7]{{15] colours which form an extension of the ordinary M giant sequence. They may belong to the class of red semi-regular variables. The Mira dust shells show a mid-infrared 7]{{15] colour-period relation. In these two elds there is no component of high-luminosity late-type variables obscured at K and shorter wavelengths such as is seen in the Magellanic Clouds. The upper limit of LPV periods in these elds remains at ca 700 days.