Characterizing the spiral Lin–Shu-type density-wave structure of the Milky Way in the 3-kpc-scale solar neighborhood: Astrophysical conclusions

Abstract

In the concluding work of a series of studies, the disk’s spiral structure of the Milky Way is anew considered as a Lin–Shu-type density-wave pattern. As a step forward, a sample of $\sim 500000$ stars from a total of 656161 sources within 3 kpc from the Sun and $\pm 250$ pc from the disk’s mean plane identified in the Gaia EDR3 is examined. The parameters of Galactic rotation and solar peculiar motion corrected for the effects of 3D density waves, and the radial, azimuthal, and vertical components of the streaming motion of stars due to the spiral arms are derived from the measured velocities of objects. The parameters of the waves are also estimated. Two almost equal scales of periodic compression/rarefaction irregularity of the streaming velocity field with the radial and vertical wavelengths of $\lesssim 2$ kpc in the form of a spiral density wave propagating in the disk at the Sun are revealed, which are reasonably related to the global spiral pattern. The Sun is located between the central part and the inner edge of the Orion (Local) wave arm. The spiral pattern in the kinematics of stars is consistent with the spatial distribution of the nearby arm segments traced by young populations. In agreement with the original Lin and Shu proposal, the gravitational field associated with the arms is a small deviation from the total gravitational field of the Galaxy that is axisymmetric in the mean. The very existence of the spiral arrangement in the velocity field suggests that the localized disk is gravitationally unstable.