Astrometry of star-forming regions in the Sagittarius spiral arm
We are now outlining the basic spiral structure of our Galaxy. This is not a trivial matter due to our location within the Galactic disk that causes any spiral structure to be superimposed within the line of sight. While radial velocity information can largely disentangle the various spiral arms, kinematic distances are not accurate enough for precise spiral structure studies. We measure the parallaxes and proper motions of water and 6.7 GHz methanol masers associated with star-forming regions in the Sagittarius spiral arm, up to the distance of the tangential point of 5.4 kpc at a Galactic longitude of 49 degree, as part of the BeSSeL survey program. By combining these new astrometric data with previous measurements, we can study the distribution of high-mass star-forming regions in this spiral arm. As the Sagittarius arm is one of the closest spiral arms to the Sun, the fractional parallax uncertainties are relatively small, permitting one to trace the three dimensional structure and velocities of the star forming regions in the arm to ultimately compare these to spiral arm models and observations of other (face-on) galaxies.