High mass - loss AGB stars detected by MSX in the “ intermediate ” and “ outer ” Galactic bulge

We present a study of MSX point sources in the Galactic bulge (|l| < 3, 1 < |b| < 5), observed at A, C, D and E-band (8 to 21 μm), with a total area ∼ 48 deg and more than 7000 detected sources in the MSX D-band (15 μm). We discuss the nature of the MSX sources (mostly AGB stars), their luminosities, the interstellar extinction, the mass-loss rate distribution and the total mass-loss rate in the bulge. The midinfrared data of MSX point sources have been combined with the near-infrared (J , H and Ks) data of 2MASS survey. The cross-identification was restricted to Ks-band detected sources with Ks ≤ 11 mag. However, for those bright MSX D-band sources ([D] < 4.0 mag), which do not satisfy this criteria, 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 Ṁ 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 μm (MSX D-band) are AGB stars well above the RGB 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 (Ṁ) of the sources in the bulge fields which range from 10 to 10 M⊙ 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 3× 10 M⊙ yr −1 and is about 1.96× 10M⊙ yr −1 deg in the “intermediate” and “outer” bulge fields of sources with mass-loss rates, Ṁ > 3 × 10 M⊙ yr . The corresponding integrated mass-loss rate per unit stellar mass is 0.48×10 yr. Apart from this, the various midand 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.