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Description
We present a study of MSX point sources in the Galactic bulge (|l|<3deg, 1deg<|b|<5deg), observed in the A, C, D and E bands (8-21um), with a total area 48 square degrees and more than 7000 detected sources in the MSX D band (15 micron). We discuss the nature of the MSX sources [mostly asymptotic giant branch (AGB) stars], their luminosities, the interstellar extinction, the mass-loss rate distribution and the total mass-loss rate in the bulge. The mid-infrared data of MSX point sources have been combined with the near-infrared (J, H and Ks) data of Two Micron All Sky Survey. The cross-identification was restricted to Ks-band detected sources with Ks<=11mag. However, for those bright MSX D-band sources ([D]<4.0mag), which do not satisfy this criterion, we have set no Ks-band magnitude cut-off. The bolometric magnitudes and the corresponding luminosities of the MSX sources were derived by fitting blackbody curves. The relation between dM/dt and (Ks-[15])0 was used to derive the mass-loss rate of each MSX source in the bulge fields. Except for very few post-AGB stars, planetary nebulae and OH/IR stars, a large fraction of the detected sources at 15 micron (MSX D band) are AGB stars well above the red giant branch tip. A number of them show an excess in ([A]-[D])0 and (Ks-[D])0 colours, characteristic of mass-loss. These colours, especially (Ks-[D])0, enable estimation of the mass-loss rates (dM/dt) of the sources in the bulge fields which range from 10^-7^ to 10^-4^M_{sun}_/yr. Taking into consideration the completeness of the mass-loss rate bins, we find that the contribution to the integrated mass-loss is probably dominated by mass-loss rates larger than 3x10^-7^M_{sun}_/yr and is about 1.96x10^-4^M_{sun}_/yr/deg^2^ in the `intermediate' and `outer' bulge fields of sources with mass-loss rates, dM/dt>3x10^-7^M_{sun}_/yr. The corresponding integrated mass-loss rate per unit stellar mass is 0.48x10^-11^/yr. Apart from this, the various mid- and near-infrared colour-colour and colour-magnitude diagrams are discussed in the paper to study the nature of the stellar population in the MSX bulge fields.
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