Estimation of the pitch angle of the Galactic spiral pattern

To estimate the pitch angle i of the Galactic spiral arms, we have used data on Galactic masers with known trigonometric parallaxes. These masers are associated with very young objects located in active star-forming regions. The well-knownmethod of analyzing the "position angle-distance logarithm" diagram has been applied. Our estimates of the angle i obtained from four segments of different arms belonging to the grand-design structure are in satisfactory agreement between themselves and are close to i = -13A degrees. The segment of the outer arm is of greatest interest. It contains only three masers, but we have additionally invoked data on 12 very young star clusters with their distances estimated by Camargo et al. from infrared photometry. Using such a combination of data, we have found i = -13.3A degrees +/- 1.3A degrees. Comparison of this value with other spiral density wave parameters obtained from a kinematic analysis of masers leads us to conclude that the model of a four-armed spiral pattern is most likely realized in our Galaxy.