The Galactic distribution of asymptotic giant branch stars

Abstract

We study the Galactic distribution of ∼10 000 candidate asymptotic giant branch (AGB) stars selected by IRAS colours and variability index. The distance to each star is estimated by assuming a narrow luminosity function and a model-derived bolometric correction. The characteristic AGB star luminosity, L AGB, is determined from the condition that the highest number density must coincide with the Galactic bulge. Assuming a bulge distance of 8 kpc, we determine L AGB ∼ 3500 L⊙, in close agreement with values obtained for nearby AGB stars using the Hipparcos data, and those obtained by other methods. We find that there are no statistically significant differences in the Galactic distribution of AGB stars with different IRAS colours, implying a universal density distribution. The direct determination of this distribution shows that it is separable in the radial, R, and vertical, z, directions. Perpendicular to the Galactic plane, the number density of AGB stars is well described by an exponential function with a vertical scaleheight of 300 pc. In the radial direction the number density of AGB stars is constant up to R ∼ 5 kpc, and then it decreases exponentially with a scalelength of ∼1.6 kpc. This fall-off extends to at least 12 kpc, where the sample becomes too small. The overall normalization implies that there are ∼200 000 AGB stars in the Galaxy. We estimate the [25]‐[12] colour distribution of AGB stars for an unbiased volume-limited sample. By using a model-dependent transformation between the colour and mass-loss rate, ˙ M, we constrain the time dependence of ˙ M. The results suggest that for 10 −6 < ˙ M < 10 −5 M⊙ yr −1 the mass-loss rate increases exponentially with time. We find only marginal evidence that