The ISOGAL field FC – 01863 + 00035 : Mid - IR interstellar extinction and stellar populations ⋆ , ⋆ ⋆

A 0.35◦× 0.29◦field centered at l=–18.63◦, b=0.35◦was observed during the ISOGAL survey by ISOCAM imaging at 7μm and 15μm. 648 objects were detected and their brightness are measured. By combining with the DENIS data in the near-infrared J and KS bands, one derives the extinction at 7μm through AKS −A7 = 0.35(AJ − AKS) which yields A7/AV ∼0.03 from the near-IR extinction values of van de Hulst–Glass (Glass 1999). The extinction structure along the line of sight is then determined from the values of J–KS or KS–[7] of the ISOGAL sources identified as RGB or early AGB stars with mild mass-loss. The distribution of AV ranges from 0 to ∼45 and it reflects the concentration of the extinction in the spiral arms. Based on their locations in color-magnitude diagrams and a few cross-identifications with IRAS and MSX sources, the nature of objects is discussed in comparison with the case of a low extinction field in Baade’s Window. Most of the objects are either AGB stars with moderate mass loss rate or luminous RGB stars. Some of them may be AGB stars with high mass loss rate. In addition, a few young stellar objects (YSOs) are present.