Project Goals

The goal of this project is to reinforce the foundations of models of the spiral structure of the Milky Way by measuring distances directly to regions of massive star formation across a large portion of the Milky Way. We propose to determine trigonometric parallaxes to strong methanol maser sources, which are associated with regions of massive star formation and their attendant HII regions. Since we cannot determine distances to all known methanol masers in the time frame of this project, we will select a sample of a dozen methanol masers, within approximately 8 kpc of the Sun, that should trace spiral arms in the first and second quadrants of the Milky Way.

Sampling spiral arms roughly every 1-2 kpc should outline the true of the locations of arms. One can then use the large data base in the Georgelin study to "interpolate" between the star forming regions measured with methanol masers. With accurate distances to some of the largest star forming regions, we should be able to verify the existence and determine the locations of the postulated Perseus, Sagittarius, Scutum, and Norma spiral arms. Ultimately, we plan to extend these measurements with a larger sample, including a similar study in the southern hemisphere, and produce an extensive map of the 3-dimensional structure of bright material associated with massive young stars that defines spiral structure. In addition to distances, the observations used to determine trigonometric parallaxes also yield excellent measurements of secular proper motions, with accuracies of about 1 km/s (see W3OH results section). Combining radial velocity measurements with proper motions (and distances) yields full 3-dimensional velocities, relative to the motion of the Sun. Since the Sun's motion in the Milky Way is known with an accuracy approaching 1 km/s, the true space velocity vector of the star forming regions will be accurately determined. Thus, through this project and future extensions, we will also determine the full kinematics of massive star forming regions in the Milky Way, which will accurately define the rotation curve of the Milky Way and, in turn, its enclosed mass as a function of Galactocentric radius. Finally, we should be able to show how material in spiral arms actually moves, to characterize the occasional large kinematic anomalies (such as in the Perseus Arm), and hopefully to understand why these anomalies occur.