Segments of spiral arms of the Galaxy traced by classical Cepheids:
  Effects of age heterogeneity
  We investigated the dependence of the parameters of the segments of spiral
arms of the Galaxy on the age of classical Cepheids. The catalog of Cepheids
(Mel'nik et al. 2015) was divided into two samples$-$relatively young
($P>9^\text{d}$) and relatively old ($P<9^\text{d}$) objects. The parameters of
the spiral structure were determined both for two samples separately and
jointly for the combination of two systems of segments traced by young and old
objects. For most of the segments, their parameters for young and old objects
differ significantly. Taking into account the difference between the two
segment systems, we obtained the estimate $R_0$ equal to $7.23^{+0.19}_{-0.18}$
kpc, which in the modern LMC calibration corresponds to the value of
$R_0={8.08^{+0.21}_{-0.20}}|_{\text{stat.}} {}^{+0.38}_{-0.36}|_{\text{cal.}}$
kpc. It is shown that the displacement between the segments is not reduced to
the effect of differential rotation only. To interpret this displacement for
objects of Perseus and Sagittarius-2 segments we carried out a dynamic modeling
of the change in the position of the segment points when moving in the smooth
gravitational field of the Galaxy. At the angular velocity of rotation of the
spiral pattern $\Omega_{\text{p}} = 25.2 \pm 0.5$ km s$^{-1}$ kpc$^{-1}$
(Dambis et al. 2015) the observed displacement between segments on young and
old objects can be explained by the amplitude of spiral perturbations of the
radial velocity of $u = 10 \pm1.5$ km s$^{-1}$. For the constructed double
system of spiral segments, it is demonstrated that the assumption of constancy
of the pitch angles within each segment and the assumption that the pole of the
spiral pattern is in the direction of the nominal center of the Galaxy do not
contradict the data within the range of uncertainty.
