Updated Kinematic Constraints on a Dark Disk
  We update the method of the Holmberg & Flynn (2000) study, including an
updated model of the Milky Way's interstellar gas, radial velocities, an
updated reddening map, and a careful statistical analysis, to bound the allowed
surface density and scale height of a dark disk. We pay careful attention to
the self-consistency of the model, including the gravitational influence of the
dark disk on other disk components, and to the net velocity of the tracer
stars. We find that the data set exhibits a non-zero bulk velocity in the
vertical direction as well as a displacement from the expected location at the
Galactic midplane. If not properly accounted for, these features would bias the
bound toward low dark disk mass. We therefore perform our analysis two ways. In
the first, traditional method, we subtract the mean velocity and displacement
from the tracers' phase space distributions. In the second method, we perform a
non-equilibrium version of the HF method to derive a bound on the dark disk
parameters for an oscillating tracer distribution. Despite updates in the mass
model and reddening map, the traditional method results remain consistent with
those of HF2000. The second, non-equilibrium technique, however, allows a
surface density as large as $14\, M_\odot\,{\rm pc}^{-2}$ (and as small as 0),
demonstrating much weaker constraints. For both techniques, the bound on
surface density is weaker for larger scale height. In future analyses of Gaia
data, it will be important to verify whether the tracer populations are in
equilibrium.
