Constraining the Galactic Bar Parameters with Red Clump Giants

Red clump giants in the Galactic bulge occupy a distinct region in the colour magnitude diagram. They have a very small spread in intrinsic luminosities and their number counts have a well defined peak. We show that these characteristics can be used to constrain the differences in the streaming motions of stars on the near side and those on the far side. We propose two methods to select two samples with one preferentially on the near side and the other on the far side. In the first method, we divide red clump giants into a bright sample and a faint one; stars in the bright sample will be on average more on the near side and vice versa. The second method relies on the fact that lensed bulge stars lie preferentially on the far side due to the enhanced lensing probability by the stars on the near side and in the disk. If the radial streaming motion is ≈ 50 km s, we find the difference in the average radial velocity between the bright and faint samples can reach ≈ 33 km s while the corresponding difference is about ≈ 10 km s between the lensed stars and all observed stars. The difference in the average proper motion between the bright and faint samples is about ≈ 1.6mas yr if there is a tangential streaming motion of 100 km s; the corresponding shift between the lensed stars and all observed stars is approximately ≈ 1mas yr. To observe the shifts in the radial velocity and proper motion, roughly one hundred microlensing events, and/or bright/faint red clump giants, need to be observed either spectroscopically or astrometrically. The spectroscopic observations can be performed efficiently using multi-object spectrographs already available. The proper motion signature of microlensed objects can be studied using ground-based telescopes