Constraints on the Galactic Bar from the Hercules stream as traced with
  RAVE across the Galaxy
  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,
this corresponds to $\Omega_b=56\pm 2$ km/s/kpc. On the other hand, the bar's
orientation $\phi_b$ cannot 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 $\Omega_b/\Omega_0=1.91 +
0.0044[\phi_b(\deg)-48]$, with standard deviation of 0.02. For example, for an
angle of $\phi_b=30\deg$ the pattern speed is $54.0\pm0.5$ km/s/kpc. 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.
