We present the peculiar in-plane velocities derived from the LAMOST red clump stars .
From the variations of the in-plane velocity with the Galactocentric radius for the young and old red clump stars , we are able to identify two types of peculiar velocities : 1 ) both the two red clump populations show that the radial velocity is negative within R = 9.0 kpc and becomes positive beyond ( denoted as the long-wave mode ) ; and 2 ) the young red clump stars show larger mean radial velocity than the old population by about 3 km s ^ { -1 } between R = 9 and 12 kpc ( denoted as the short-wave mode ) .
We find that the elliptical disk induced by the rotating bar can well explain the long-wave mode peculiar velocity .
The axis ratio of the elliptical disk is around 0.8-0.95 and the disk keeps circular at R = 9.24 \pm 0.2 kpc , which should be the location of the outer Lindblad resonance radius ( OLR ) .
Adopting the circular speed of 238 km s ^ { -1 } , the pattern speed of the bar is then derived as 48 \pm 3 km s ^ { -1 } kpc ^ { -1 } from the location of OLR .
On the other hand , the short-wave mode is likely the perturbation of the spiral arms as density waves .