The space distribution of AGB stars

The AGB stars can be classified into three main types on the basis of their atmospheric composition: carbon-rich, oxygen-rich, and S-type. The carbon-rich stars typically have 1.5-solar-mass main sequence stars as progenitors. There are about 40 of these stars per sq kpc projected onto the plane of the Milky Way galaxy in the neighborhood of the sun with an exponential scale height above the galactic disk of about 200 pc. Contrary to the general distribution of mass, there is no decrease with galactocentric radius of the surface density of these stars for at least 3 kpc beyond the solar circle. The S-type stars appear to have the same spatial distribution as the carbon stars; there are about 1/3 as many 'pure' S stars as there are luminous carbon stars. One major class of oxygen-rich AGB stars is the Mira variables. There are approximately between 1 and 2 times as many of these stars per sq kpc projected onto the galactic plane of the Milky Way as there are carbon stars.