2001MNRAS.327.1193S

In this work, a sample of luminous M-type giants in the Baade's Windows towards the inner Galactic bulge is investigated in the near-infrared. The ISOGAL survey at 7 and 15µm has given information concerning the mass-loss rates of these stars and their variability characteristics have been extracted from the MACHO data base. Most are known to be semi-regular variables (SRVs). Here we discuss how their IJHK_S-region colours depend on period and on the presence or absence of mass loss, using results mainly taken from the DENIS and 2MASS surveys.

In order to compare their colours with solar neighbourhood stars, photometric colours on the DENIS, 2MASS and ESO photometric systems have been synthesized for objects in the spectrophotometric atlas of Lanon & Wood. In addition, they have been used to predict the differences in colour indices when stars with strong molecular bands are observed using different photometric systems.

The SRVs are found to inhabit the upper end of the J-K_S, K_S colour-magnitude diagram, lying just below the Miras. High mass-loss rates are associated with high luminosity. The near-infrared colours of the SRVs increase in a general way with period and are the reddest for stars with significant mass loss. The average colours of Mira variables, whose periods start at around 200d in the bulge, are bluer than those of the semi-regulars at this period, particularly in J-H, thanks to the association of deep water vapour bands with large amplitude.