Stellar Overdensity in the Local Arm in Gaia DR2
  Using the cross-matched data of Gaia DR2 and 2MASS Point Source Catalog, we
investigated the surface density distribution of stars aged ~1 Gyr in the thin
disk in the range of 90{\deg} <= l <= 270{\deg}. We selected 4,654 stars above
the turnoff corresponding to the age ~1 Gyr, that fall within a small box
region in the color-magnitude diagram, (J-Ks)0 versus M(Ks), for which the
distance and reddening are corrected. The selected sample shows an arm-like
overdensity at 90{\deg} <= l <= 190{\deg}.This overdensity is located close to
the Local arm traced by high-mass star forming regions (HMSFRs), but its pitch
angle is slightly larger than that of the HMSFR-defined arm. Although the
significance of the overdensity we report is marginal, its structure poses
questions concerning both of the competing scenarios of spiral arms, the
density-wave theory and the dynamic spiral arm model. The offset between the
arms traced by stars and HMSFRs, i.e., gas, is difficult to be explained by the
dynamic arm scenario. On the other hand, the pitch angle of the stellar Local
arm, if confirmed, larger than that of the Perseus arm is difficult to be
explained with the classical density-wave scenario. The dynamic arm scenario
can explain it if the Local arm is in a growing up phase, while the Perseus arm
is in a disrupting phase. Our result provides a new and complex picture of the
Galactic spiral arms, and encourages further studies.
