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)