CO emission tracing a warp or radial flow within $\lesssim$ 100 au in
  the HD 100546 protoplanetary disk
  We present spatially resolved ALMA images of CO J=3-2 emission from the
protoplanetary disk around HD100546. We model the spatially-resolved kinematic
structure of the CO emission. Assuming a velocity profile which prescribes a
flat or flared emitting surface in Keplerian rotation, we uncover significant
residuals with a peak of $\approx7\delta v$, where $\delta v = 0.21$ km
s$^{-1}$ is the width of a spectral resolution element. The residuals reveal
the possible presence of a severely warped and twisted inner disk extending to
at most 100au. Adapting the model to include a misaligned inner gas disk with
(i) an inclination almost edge-on to the line of sight, and (ii) a position
angle almost orthogonal to that of the outer disk reduces the residuals to $<
3\delta v$. However, these findings are contrasted by recent VLT/SPHERE,
MagAO/GPI, and VLTI/PIONIER observations of HD100546 that show no evidence of a
severely misaligned inner dust disk down to spatial scales of $\sim 1$au. An
alternative explanation for the observed kinematics are fast radial flows
mediated by (proto)planets. Inclusion of a radial velocity component at close
to free-fall speeds and inwards of $\approx 50$au results in residuals of
$\approx 4 \delta v$. Hence, the model including a radial velocity component
only does not reproduce the data as well as that including a twisted and
misaligned inner gas disk. Molecular emission data at a higher spatial
resolution (of order 10au) are required to further constrain the kinematics
within $\lesssim 100$au. HD100546 joins several other protoplanetary disks for
which high spectral resolution molecular emission shows that the gas velocity
structure cannot be described by a purely Keplerian velocity profile with a
universal inclination and position angle. Regardless of the process, the most
likely cause is the presence of an unseen planetary companion. (Abridged)
