2012ApJ...759...94C

The J - K_s color distributions (CDs) with a bin size of 0.05 mag has been carried out for the entire Milky Way using the Two Micron All Sky Survey Point Source Catalog (2MASS PSC). The CDs are bimodal, with a red peak at 0.8 < J - K_s< 0.85 and a blue peak at 0.3 < J - K_s< 0.4. The colors of the red peak are more or less the same for the whole sky, but those of the blue peak depend on Galactic latitude (J - K_s∼ 0.35 at low Galactic latitudes and 0.35 < J - K_s< 0.4 for other sky areas). The blue peak dominates the bimodal CDs at low Galactic latitudes and becomes comparable with the red peak in other sky regions. In order to explain the bimodal distribution and the global trend shown by the all-sky 2MASS CDs, we assemble an empirical Hertzsprung-Russell (H-R) diagram, which is composed of observational-based near-infrared H-R diagrams and color-magnitude diagrams, and incorporate a Milky Way model. In the empirical H-R diagram, the main-sequence turn-off for stars in the thin disk is relatively bluer, (J - K_s)₀= 0.31, compared with that of the thick disk which is (J - K_s)₀= 0.39. The age of the thin/thick disk is roughly estimated to be around 4-5/8-9 Gyr according to the color-age relation of the main-sequence turn-off. In general, the 2MASS CDs can be treated as a tool to measure the age of the stellar population of the Milky Way in a statistical manner and to our knowledge it is the first attempt to do so.