The observed spiral structure of the Milky Way
  The spiral structure of the Milky Way is not yet well determined. The keys to
understanding this structure are to increase the number of reliable spiral
tracers and to determine their distances as accurately as possible. HII
regions, giant molecular clouds (GMCs), and 6.7-GHz methanol masers are closely
related to high mass star formation, and hence they are excellent spiral
tracers. We update the catalogs of Galactic HII regions, GMCs, and 6.7-GHz
methanol masers, and then outline the spiral structure of the Milky Way. We
collected data for more than 2500 known HII regions, 1300 GMCs, and 900 6.7-GHz
methanol masers. If the photometric or trigonometric distance was not yet
available, we determined the kinematic distance using a Galaxy rotation curve
with the current IAU standard, $R_0$ = 8.5 kpc and $\Theta_0$ = 220 km
s$^{-1}$, and the most recent updated values of $R_0$ = 8.3 kpc and $\Theta_0$
= 239 km s$^{-1}$, after we modified the velocities of tracers with the adopted
solar motions. With the weight factors based on the excitation parameters of
HII regions or the masses of GMCs, we get the distributions of these spiral
tracers. The distribution of tracers shows at least four segments of arms in
the first Galactic quadrant, and three segments in the fourth quadrant. The
Perseus Arm and the Local Arm are also delineated by many bright HII regions.
The arm segments traced by massive star forming regions and GMCs are able to
match the HI arms in the outer Galaxy. We found that the models of three-arm
and four-arm logarithmic spirals are able to connect most spiral tracers. A
model of polynomial-logarithmic spirals is also proposed, which not only
delineates the tracer distribution, but also matches the observed tangential
directions.
