The spiral potential of the Milky Way
  Radial velocities of 1726 candidate B- and A-type stars within 3deg of the
Galactic center (GC) were estimated from FLAMES/VLT spectra in the range of
396-457 nm. The final sample was limited to 1507 stars with either Gaia DR2
parallaxes or main-sequence B-type stars having reliable spectroscopic
distances. The solar peculiar motion in the direction of the GC relative to the
local standard of rest (LSR) was estimated to Uo = 10.7+-1.3 km/s. The
variation in the median radial velocity relative to the LSR as a function of
distance from the sun shows a gradual increase from slightly negative values
near the sun to almost 5 km/s at a distance of around 4 kpc. A sinusoidal
function with an amplitude of 3.4+-1.3 km/s and a maximum at 4.0+-0.6 kpc
inside the sun is the best fit to the data. A positive median radial velocity
relative to the LSR around 1.8 kpc, the expected distance to the Sagittarius
arm, can be excluded at a 99% level of confidence. A marginal peak detected at
this distance may be associated with stellar streams in the star-forming
regions, but it is too narrow to be associated with a major arm feature. A
comparison with test-particle simulations in a fixed galactic potential with an
imposed spiral pattern shows the best agreement with a two-armed spiral
potential having the Scutum-Crux arm as the next major inner arm. A relative
radial forcing of ~1.5% and a pattern speed in the range of 20-30 km/s/kpc
yield the best fit. The lack of a positive velocity perturbation in the region
around the Sagittarius arm excludes it from being a major arm. Thus, the main
spiral potential of the Galaxy is two-armed, while the Sagittarius arm is an
inter-arm feature with only a small mass perturbation associated with it.
