2011ApJ...740...34C

The K_s-band differential star count of the Two Micron All Sky Survey (2MASS) is used to derive the global structure parameters of the smooth components of the Milky Way. To avoid complication introduced by other fine structures and significant extinction near and at the Galactic plane, we only consider Galactic latitude|b| > 30° data. The star count data are fitted with a three-component model: double exponential thin disk and thick disk, and a power-law decay oblate halo. Using maximum likelihood, the best-fit local density of the thin disk is n₀= 0.030±0.002 stars/pc3. The best-fit scale height and length of the thin disk are H_z1= 360±10 pc and H_r1= 3.7 ± 1.0 kpc, and those of the thick disk are H_z2= 1020±30 pc and H_r2 = 5.0±1.0 kpc, the local thick-to-thin disk density ratio is f₂= 7% ±1%. The best-fit axis ratio, power-law index, and local density ratio of the oblate halo are κ = 0.55±0.15, p = 2.6±0.6, and f_h= 0.20% ±0.10%, respectively. Moreover, we find some degeneracy among the key parameters (e.g., n₀, H_z1, f₂, and H_z2). Any pair of these parameters are anti-correlated to each other. The 2MASS data can be well fitted by several possible combinations of these parameters. This is probably the reason why there is a wide range of values for the structure parameters in literature similar to this study. Since only medium and high Galactic latitude data are analyzed, the fitting is insensitive to the scale lengths of the disks.