Constraining the Galactic Bar Parameters with Red Clump Giants

Abstract

We show that the small intrinsic spread in luminosities of red clump giants can be used to constrain the differences in the streaming motions of Galatic bulge 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 ~50km/s, we find the difference in the average radial velocity between the bright and faint samples can reach ~33km/s while the corresponding difference is about ~10km/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 100km/s; the corresponding shift between the lensed stars and all observed stars is ~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 and HST. These observations will provide strong constraints on the Galactic bar parameters (abridged).