The Information Of The Milky Way From 2MASS Whole Sky Star Count: The Bimodal Color Distributions

TL;DR

This study analyzes the bimodal J-Ks color distribution of stars across the Milky Way using 2MASS data, revealing insights into stellar populations and estimating the ages of the thin and thick disks.

Contribution

It presents the first comprehensive analysis of the Milky Way's stellar populations through bimodal color distributions and estimates disk ages using empirical HR diagrams.

Findings

Bimodal J-Ks color distribution with consistent red peak and latitude-dependent blue peak.

The thin disk's main sequence turnoff is bluer, indicating an age of about 4-5 Gyr.

The thick disk's main sequence turnoff suggests an age of approximately 8-9 Gyr.

Abstract

The J-Ks color distribution (CD) with a bin size of 0.05 magnitude for the entire Milky Way has been carried out by using the Two Micron All Sky Survey Point Source Catalog (2MASS PSC). The CDs are bimodal, which has a red peak at 0.8 < J-Ks < 0.85 and a blue peak at 0.3 < J-Ks < 0.4. The colors of the red peak are more or less the same for the whole sky, but that of the blue peak depend on Galactic latitude, (J-Ks ~ 0.35 at low Galactic latitudes and 0.35 < J-Ks < 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 HR diagram, which is composed by observational-based near infrared HR diagrams and color magnitude diagrams, and incorporate a Milky Way model. In the empirical HR diagram, the main sequence stars turnoff the thin disk is relatively bluer, (J-Ks)0 = 0.31, when we compare with the thick disk which is (J-Ks)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 turnoff. In general, the 2MASS CDs can be treated as a tool to census the age of stellar population of the Milky Way in a statistical manner and to our knowledge this is a first attempt to measure the age.