The Lin–Shu type density wave structure of our Galaxy: line-of-sight velocities of 396 HII regions

Abstract

In this fifth paper in a series, we check again the Lin–Shu idea of small-amplitude density waves in our Galaxy. The updated catalogue of Galactic objects of Hou and Han (Hou and Han in Astron Astrophys 569:125–146, 2014), available in the literature, is used. The line-of-sight velocity field of 396 HII regions with known trigonometric or photometric distances and their uncertainties within 4 kpc from the Sun is investigated in terms of a wave perturbation. Our previous study is enlarged by examining the secondary minima of the residual sum of squares of the predicted and measured velocities of a sample of objects in a considerable region surrounding the Sun. Both the geometrical and physical parameters of the wave pattern are evaluated. Here we show, for the first time, that the local Cygnus–Orion spiral arm, where the Sun is located, does form a part of the major density wave structure of the system under the question but is not a spur originating at the inner Carina–Sagittarius arm or at the outer Perseus arm.