The Local Spiral Arm in the LAMOST-Gaia Common Stars?

Abstract

Abstract Using the LAMOST-Gaia common stars, we demonstrate that the in-plane velocity fields for the nearby young stars are significantly different from those for the old ones. For the young stars, the probably perturbed velocities that are similar to the old population are mostly removed from the velocity maps in the X–Y plane. The residual velocity field shows that the young stars consistently move along Y with faster v ϕ at the trailing side of the local arm, while at the leading side, they move slower in the azimuth direction. At both sides, on average the young stars move inward with a v R of km s−1. The divergence of the velocity in the Y direction implies that the young stars are associated with a density wave near the local arm. We therefore suggest that the young stars may reflect the formation of the local spiral arm by correlating themselves with a density wave. The range of the age for the young stars is around 2 Gyr, which is sensible since the transient spiral arm can persist for that long. We also point out that alternative explanations of the peculiar velocity field for the young population cannot be ruled out if solely using this observed data.