High mass‐loss asymptotic giant branch stars detected by the midcourse space experiment in the ‘intermediate’ and ‘outer’ galactic bulge

Abstract

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 M 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])₀ colours, characteristic of mass‐loss. These colours, especially (K_s−[D])₀, enable estimation of the mass‐loss rates forumla of the sources in the bulge fields which range from 10⁻⁷ to 10⁻⁴ M_⊙ yr⁻¹. 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 × 10⁻⁷ M_⊙ yr⁻¹ and is about 1.96 × 10⁻⁴ M_⊙ yr⁻¹ deg⁻² in the ‘intermediate’ and ‘outer’ bulge fields of sources with mass‐loss rates, M > 3×10⁻⁷ M_⊙ yr⁻¹. The corresponding integrated mass‐loss rate per unit stellar mass is 0.48 × 10⁻¹¹ yr⁻¹. 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.