TRIGONOMETRIC PARALLAXES OF HIGH MASS STAR FORMING REGIONS: THE STRUCTURE AND KINEMATICS OF THE MILKY WAY

Abstract

Over 100 trigonometric parallaxes and proper motions for masers associated\nwith young, high-mass stars have been measured with the BeSSeL Survey, a VLBA\nkey science project, the EVN, and the Japanese VERA project. These measurements\nprovide strong evidence for the existence of spiral arms in the Milky Way,\naccurately locating many arm segments and yielding spiral pitch angles ranging\nfrom 7 to 20 degrees. The widths of spiral arms increase with distance from the\nGalactic center. Fitting axially symmetric models of the Milky Way with the 3-D\nposition and velocity information and conservative priors for the solar and\naverage source peculiar motions, we estimate the distance to the Galactic\ncenter, Ro, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, To,\nto be 240 +/- 8 km/s, and a rotation curve that is nearly flat (a slope of -0.2\n+/- 0.4 km/s/kpc) between Galactocentric radii of 5 and 16 kpc. Assuming a\n"universal" spiral galaxy form for the rotation curve, we estimate the thin\ndisk scale length to be 2.44 +/- 0.16 kpc. The parameters Ro and To are not\nhighly correlated and are relatively insensitive to different forms of the\nrotation curve. Adopting a theoretically motivated prior that high-mass star\nforming regions are in nearly circular Galactic orbits, we estimate a global\nsolar motion component in the direction of Galactic rotation, Vsun = 14.6 +/-\n5.0 km/s. While To and Vsun are significantly correlated, the sum of these\nparameters is well constrained, To + Vsun = 255.2 +/- 5.1 km/s, as is the\nangular speed of the Sun in its orbit about the Galactic center, (To + Vsun)/Ro\n= 30.57 +/- 0.43 km/s/kpc. These parameters improve the accuracy of estimates\nof the accelerations of the Sun and the Hulse-Taylor binary pulsar in their\nGalactic orbits, significantly reducing the uncertainty in tests of\ngravitational radiation predicted by general relativity.\n