The Milky Way structure

Bobylev, Mosenkov, Bajkova & Gontcharov (2014) proposed a new method for selecting stars in the Galactic bar based on 2MASS infrared photometry in combination with stellar proper motions from the Kharkiv XPM catalogue (Fedorov et al. 2009, 2011). They preselected red clump and red giant branch stars based on the color – magnitude diagram (see Fig. 1) and calculated their photometric distances.

Fig1. - Example of identifying the red giant clump (RGC) and the red giant branch (RGB) on the color–magnitude diagram for a selected sky region.

Since the stellar proper motions are indicators of a larger velocity dispersion
toward the bar and the spiral arms compared to the stars with circular orbits, applying
the constraints on the proper motions of the preselected stars that take into account the
Galactic rotation has allowed the background stars to be eliminated. Based on a joint
analysis of the velocities of the selected stars and their distribution on the Galactic plane,
they confidently identified the segment of the Galactic bar nearest to the Sun with an
orientation of 20º–25º with respect to the Galactic center-Sun direction and a semimajor
axis of no more than 3 kpc.

Fig2. - Example of identifying the red giant branch (RGB) with the selected bar (1.3 million stars)
Fig3. - Example of identifying the red giant clump (RGC) with the selected bar (10 million stars)