Measurements of streaming motions of the Galactic bar with red clump giants

We report a measurement of the streaming motion of the stars in the Galactic bar with the red clump giants (RCGs) using the data of the Optical Gravitational Lensing Experiment II (OGLE-II). We measure the proper motion of 46961 stars and divide RCGs into bright and faint subsamples that on average will be closer to the near and far sides of the bar, respectively. We find that the far-side RCGs (4979 stars) have a proper motion of Δ<μ>~ 1.5 +/- 0.11 masyr^-1 toward the negative l relative to the near-side RCGs (3610 stars). This result can be explained by stars in the bar rotating around the Galactic Centre in the same direction as the Sun with v_b~ 100 kms^-1. In the disc star (DS) and red giant (RG) samples, we do not find significant difference between bright and faint subsamples. For those samples Δ<μ>~ 0.3 +/- 0.14 masyr^-1 and ~0.03 +/- 0.14 masyr^-1, respectively. It is likely that the average proper motion of RG stars is the same as that of the Galactic Centre. The proper motion of DSs with respect to RGs is ~3.3 masyr^-1 toward positive l. This value is consistent with the expectations for a flat rotation curve and solar motion with respect to local standard of rest. RGs have proper motion approximately equal to the average of bright and faint RCGs, which implies that they are on average near the centre of the bar. This pilot project demonstrates that OGLE-II data may be used to study streaming motions of stars in the Galactic bar. We intend to extend this work to all 49 OGLE-II fields in the Galactic bulge region.