On the Rotation Speed of the Milky Way Determined from HI Emission
  The circular rotation speed of the Milky Way at the solar radius, Theta_o,
has been estimated to be 220 km/s by fitting the maximum velocity of HI
emission as a function of Galactic longitude. This result is in tension with a
recent estimate of Theta_o=240 km/s, based on VLBI parallaxes and proper
motions from the BeSSeL and VERA surveys for large numbers of high-mass star
forming regions across the Milky Way. We find that the rotation curve best
fitted to the VLBI data is slightly curved, and that this curvature results in
a biased estimate of Theta_o from the HI data when a flat rotation curve is
assumed. This relieves the tension between the methods and favors Theta_o=240
km/s.
