The Milky Way spiral structure parameters from data on masers and selected open clusters

To estimate the parameters of the Galactic spiral structure - namely the pitch angle i and the number of spiral arms - data on Galactic masers with known trigonometric parallaxes were used. We applied the well-known method based on analysis of the 'position angle - distance logarithm' diagram. Estimates of the pitch angle i obtained from four segments of different arms belonging to the global Galactic structure are self-consistent and close to i = -13 degrees +/- 1 degrees. The segment which is most interesting is that of the Outer arm. It contains only three masers. Hence, in order to obtain correct estimates, we also used the data on 12 very young star clusters with distances determined by Camargo et al. from infrared photometry. The estimates obtained allow us to conclude in favour of the four-armed model of the Galactic spiral structure.