The BeSSeL Project
The BeSSeL Survey (Bar and Spiral Structure Legacy Survey) is a VLBA Key Science project to study the spiral structure and kinematics of the Milky Way. The Survey is named in honor of Friedrich Wilhelm Bessel who measured the first stellar parallax in 1838. We will accomplish this by determining distances, via trigonometric parallax, and proper motions of star forming regions in the Milky Way.
The target sources are methanol and water masers that are associated with young massive stars and compact HII regions that trace spiral structure. With accurate distance measurements we will locate spiral arms, and with absolute proper motions we can determine the 3-dimensional motions of these massive young stars.
Over the past decade, we have developed and tested the techniques needed to achieve better than 0.03 mas relative positional accuracy between maser sources and background QSOs at each epoch. Before beginning this project, we determined trigonometric parallaxes to 11 star forming regions with accuracies of order 0.01 mas. We also measured absolute proper motions with accuracies of order 1 km/s. These results have been published in a series of papers in the Astrophyscial Journal