The nearby spiral density-wave structure of the Galaxy: line-of-sight velocities of the Gaia DR2 main-sequence A, F, G, and K stars

Abstract

Distances and velocities of ≈ 2400 000 main-sequence A, F, G, and K stars are collected from the second data release of ESA's Gaia astrometric mission. This material is analysed to find evidence of radial and azimuthal systematic non-circular motions of stars in the solar neighbourhood on the assumption that the system is subject to spiral density waves (those produced by a spontaneous disturbance, a central bar, or an external companion), developing in the Galactic disc. Data analysis of line-of-sight velocities of ≈ 1500 000 stars selected within 2 kpc from the Sun and 500 pc from the Galactic mid-plane with distance accuracies of <10 per cent makes evident that a radial wavelength of the wave pattern is 1.1-1.6 kpc and a phase of the wave at the Sun's location in the Galaxy is 55°-95°. Respectively, the Sun is situated at the inner edge of the nearest Orion spiral arm segment. Thus, the local Orion arm is a part of a predominant density-wave structure of the system. The spiral structure of the Galaxy has an oscillating nature corresponding to a concept of the Lin-Shu-type moderately growing in amplitude, tightly wound, and rigidly rotating density waves.