Non-axisymmetries in the Galactic potential ( spiral arms and bar ) induce kinematic groups such as the Hercules stream .
Assuming that Hercules is caused by the effects of the Outer Lindblad Resonance of the Galactic bar , we model analytically its properties as a function of position in the Galaxy and its dependence on the bar ’ s pattern speed and orientation .
Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius , in a manner consistent with our analytical model .
This allows us to obtain new estimates of the parameters of the Milky Way ’ s bar .
The combined likelihood function of the bar ’ s pattern speed and angle has its maximum for a pattern speed of \Omega _ { b } = ( 1.89 \pm 0.08 ) \times \Omega _ { 0 } , where \Omega _ { 0 } is the local circular frequency .
Assuming a Solar radius of 8.05 { kpc } and a local circular velocity of 238 km s ^ { -1 } , this corresponds to \Omega _ { b } = 56 \pm 2 { km s ^ { -1 } { kpc } ^ { -1 } } .
On the other hand , the bar ’ s orientation \phi _ { b } can not be constrained with the available data .
In fact , the likelihood function shows that a tight correlation exists between the pattern speed and the orientation , implying that a better description of our best fit results is given by the linear relation \Omega _ { b } / \Omega _ { 0 } = 1.91 + 0.0044 \left ( { \phi _ { b } } ( \deg ) -48 \right ) , with standard deviation of 0.02 .
For example , for an angle of \phi _ { b } = 30 \deg the pattern speed is 54.0 \pm 0.5 { km s ^ { -1 } { kpc } ^ { -1 } } .
These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun , and are robust to biases in distance .