Galactic distributions of carbon- and oxygen-rich AGB stars revealed by   the AKARI mid-infrared all-sky survey
  Context: The environmental conditions for asympotic giant branch (AGB) stars to reach the carbon-rich (C-rich) phase are important to understand the evolutionary process of AGB stars. The difference between the spatial distributions of C-rich and oxygen-rich (O-rich) AGB stars is essential for the study of the Galactic structure and the chemical evolution of the interstellar medium (ISM). Aims: We quantitatively investigate the spatial distributions of C-rich and O-rich AGB stars in our Galaxy. We discuss the difference between them and its origin. Methods: We classify a large number of AGB stars newly detected 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. We then obtain the spatial distributions of C-rich and O-rich AGB stars, assuming that they have the same luminosity for a given mass-loss rate. Results: We find that O-rich AGB stars are concentrated toward the Galactic center and that the density decreases with Galactocentric distance, whereas C-rich AGB stars show a relatively uniform distribution within about 8kpc of Sun. Conclusion: Our result confirms the trends reported in previous studies and extends them to a Galactic scale. We discuss the relations between our result, the Galactic metallicity gradient, and the chemical evolution of the ISM in our Galaxy. 
Daisuke Ishihara (1), Hidehiro Kaneda (1), Takashi Onaka (2),   Yoshifusa Ita (3), Mikako Matsuura (4,5), and Noriyuki Matsu...
astro-ph.GA