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Context: The environmental conditions for asympotic giant branch (AGB) starsto reach the carbon-rich (C-rich) phase are important to understand theevolutionary process of AGB stars. The difference between the spatialdistributions of C-rich and oxygen-rich (O-rich) AGB stars is essential for thestudy of the Galactic structure and the chemical evolution of the interstellarmedium (ISM). Aims: We quantitatively investigate the spatial distributions ofC-rich and O-rich AGB stars in our Galaxy. We discuss the difference betweenthem and its origin. Methods: We classify a large number of AGB stars newlydetected by the AKARI id-infrared all-sky survey. In the color-color diagrams,we define their occupation zones based on the locations of known objects. Wethen obtain the spatial distributions of C-rich and O-rich AGB stars, assumingthat they have the same luminosity for a given mass-loss rate. Results: We findthat O-rich AGB stars are concentrated toward the Galactic center and that thedensity decreases with Galactocentric distance, whereas C-rich AGB stars show arelatively uniform distribution within about 8kpc of Sun. Conclusion: Ourresult confirms the trends reported in previous studies and extends them to aGalactic scale. We discuss the relations between our result, the Galacticmetallicity gradient, and the chemical evolution of the ISM in our Galaxy.

Galactic distributions of carbon- and oxygen-rich AGB stars revealed by the AKARI mid-infrared all-sky survey