Parameters of the Galactic density-wave spiral structure: Line-of-sight velocities of 156 star-forming regions

•The line-of-sight velocities of HII regions in the solar vicinity are studied.•The minor effects caused by the Lin–Shu density waves are taken into account.•Several spiral patterns with different number of arms m may coexist in the Galaxy.•The Sun lies between the main trailing spirals in Sagittarius and Perseus.•The single-armed m=1m=1 spiral mode dominates the pattern.

This paper is the third in a series of articles devoted to the determination of the Milky Way’s spiral density-wave structure from the kinematics of the Galactic objects, using the ordinary Oort equation. The minor effects caused by the Lin–Shu type two-dimensional density waves are also taken into account. The results obtained allow us to conclude that (a) several low-m spiral patterns with different number of spiral arms m   and amplitudes of the perturbed gravitational potential may coexist in the Galaxy. (b) the single-armed m=1m=1 spiral mode dominates the pattern. The distribution of stars and gas in outer parts of the system is thus asymmetric (“lopsided”), (c) the phase velocity of spiral waves (or the pattern rotation speed) does not depend on m   and therefore each Fourier component of a gravity perturbation rotates with the same angular velocity, (d) the Sun lies between the main trailing spiral-arm segments in Carina–Sagittarius and Perseus, closer to the inner Carina–Sagittarius one. The local Cygnus–Orion arm in which the Sun is located is thus not a part of the main spiral pattern, and finally (e) the Galaxy seems to be more homogeneous and rotating more like a solid body than in the standard m=0m=0 model (i.e. spiral arms not included in the model).