Offsets of masers with respect to the middle of the Perseus arm, and the
  co-rotation radius in the Milky Way
  The radial distance to the co-rotation radius Rcoro (where the angular speed
of the gas and stars in orbit around the Galactic Centre is equal to the
angular speed of the spiral arm pattern) has often been predicted (at various
places), but not measured with a high precision.
  Here we test the locations of masers with respect to the Perseus arm (Table
1). Our analysis of the masers and HII regions near the Perseus arm (mostly
located on the inner arm side, by about 0.4 kpc from the cold CO mid-arm) shows
that the co-rotation Rcoro must be > 10.8 kpc from the Galactic Center (Figure
1). This implies that the angular rotation speed of the spiral pattern < 21.3
km/s/kpc.
  Another test in galactic quadrant II shows that the radial velocity of the
masers are generally more negative than that of the CO mid-arm (Figure 2),
indicating a deceleration with respect to the CO mid-arm, by about 9 km/s. This
implies that a spiral pattern angular rotation < 20.7 km/s/kpc, and thus Rcoro
> 11.1 kpc.
  Finally, comparing our results with other published results (Table 2), we
find a statistical mean co-rotation radius Rcoro predicted to be near 12 kpc
from the Galactic Center (beyond the Perseus arm; before the Cygnus arm), and a
mean angular spiral pattern angular rotation predicted to be near 19 km/s/kpc.
