The spiral structure of our Milky Way Galaxy
  The spiral structure of our Milky Way Galaxy is not yet known. HII regions
and giant molecular clouds are the most prominent spiral tracers. We collected
the spiral tracer data of our Milky Way from the literature, namely, HII
regions and giant molecular clouds (GMCs). With weighting factors based on the
excitation parameters of HII regions or the masses of GMCs, we fitted the
distribution of these tracers with models of two, three, four spiral-arms or
polynomial spiral arms. The distances of tracers, if not available from stellar
or direct measurements, were estimated kinetically from the standard rotation
curve of Brand & Blitz (1993) with $R_0$=8.5 kpc, and $\Theta_0$=220 km
s$^{-1}$ or the newly fitted rotation curves with $R_0$=8.0 kpc and
$\Theta_0$=220 km s$^{-1}$ or $R_0$=8.4 kpc and $\Theta_0$=254 km s$^{-1}$. We
found that the two-arm logarithmic model cannot fit the data in many regions.
The three- and the four-arm logarithmic models are able to connect most
tracers. However, at least two observed tangential directions cannot be matched
by the three- or four-arm model. We composed a polynomial spiral arm model,
which can not only fit the tracer distribution but also match observed
tangential directions. Using new rotation curves with $R_0$=8.0 kpc and
$\Theta_0$=220 km s$^{-1}$ and $R_0$=8.4 kpc and $\Theta_0$=254 km s$^{-1}$ for
the estimation of kinematic distances, we found that the distribution of HII
regions and GMCs can fit the models well, although the results do not change
significantly compared to the parameters with the standard $R_0$ and
$\Theta_0$.
