2007MNRAS.381.1219O

We present a study of Midcourse Space Experiment (MSX) point sources in the Galactic bulge (|l| < 3°,1° <|b| < 5°), observed in the A, C, D and E bands (8-21µm), with a total area ∼48 deg² and more than 7000 detected sources in the MSX D band (15 µm). 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 K_s) data of Two Micron All Sky Survey. The cross-identification was restricted to K_s-band detected sources with K_s≤ 11 mag. However, for those bright MSX D-band sources ([D] < 4.0 mag), which do not satisfy this criterion, we have set no K_s-band magnitude cut-off. The bolometric magnitudes and the corresponding luminosities of the MSX sources were derived by fitting blackbody curves. The relation between and (K_s- [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 µm (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 (K_s- [D])₀colours, characteristic of mass-loss. These colours, especially (K_s- [D])₀, enable estimation of the mass-loss rates of the sources in the bulge fields which range from 10^–7 to 10^–4M_☉/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^–7M_☉/yr and is about 1.96x10^–4M_☉yr^–1deg^–2 in the `intermediate' and `outer' bulge fields of sources with mass-loss rates,. 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.