Trigonometric Parallaxes of Star Forming Regions Beyond the Tangent
  Point of the Sagittarius Spiral Arm
  As part of the BeSSeL Survey, we report trigonometric parallaxes and proper
motions of molecular maser sources associated with 13 distant high mass star
forming regions in the Sagittarius spiral arm of the Milky Way. In particular,
we obtain improved parallax distance estimates for three well studied regions:
1.9 +0.1/-0.1 kpc for M17, 5.3 +1.3/-0.9 kpc for W51, and 7.9 +0.9/-0.7 kpc for
GAL 045.5+00.0. Peculiar motions for all but one source are less than 20 km/s.
We fit a log-periodic spiral to the locations and estimate an average pitch
angle of 7.2+-1.9 deg. We find that the section of the arm beyond the tangent
point in the first quadrant of the Milky Way appears 15 pc below the
IAU-defined Galactic plane.
