Asymptotic giant branch stars in the GLIMPSE database

I. Inventory and luminosity function

Leiden Observatory, Leiden, The Netherlands

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Received: 1 August 2025
Accepted: 10 February 2026

Abstract

Aims. The GLIMPSE database contains over one hundred million point sources between longitudes of −60° and +60° and most of these are asymptotic giant branch (AGB) stars within 16 kpc from the Sun. In this first paper, we focus on their luminosity function. The second paper will discuss their spatial distribution in the Milky Way.

Methods. We studied a set of color-magnitude diagrams (CMDs) in 31 fields of one square degree along the galactic equator. We estimated the apparent bolometric magnitude, the interstellar extinction, and the optical depth of the circumstellar envelope.

Results. The CMDs in different fields are very similar to each other. All CMDs show a vertical track of stars that decreases in number for brighter magnitudes. After correction for interstellar extinction all stars share the same color, [K] − [8.0] = 0, with a root mean square around the mean of 0.02. We added the GLIMPSE counterparts of known long-period variables (LPVs) and of masers. These populate the upper end of the vertical track, while causing the track to bend toward cooler surfaces. In all directions, |l| ≥ 10°, the counts of stars between magnitudes [8.0] = 5 and [8.0] = 12 grow exponentially with increasing magnitude and always at the same rate; this is explained by the theoretically predicted exponential growth of the luminosity of AGB stars. Applying this result to the integral of the stellar density along a line of sight leads to a first, smeared-out picture of the AGB star distribution in the Milky Way. Its prominent features include the inner Galactic Bulge and the Nuclear Stellar Disk.

Conclusions. The GLIMPSE database offers an underexplored treasure box for the study of AGB stars in the inner Milky Way.