2014ApJ...781..108S

We report four trigonometric parallaxes for high-mass star-forming regions within 4 kpc of the Galactic center. These measurements were made with the Very Long Baseline Array as part of the BeSSeL Survey. By associating these sources kinematically with large-scale features in CO and H I longitude-velocity diagrams, we begin to outline some major features of the inner Milky Way: the Connecting arm, the near and far 3 kpc arms, and the Norma arm. The Connecting arm in the first Galactic quadrant lies closer to the Galactic center than the far 3 kpc arm and is offset by the long-bar's major axis near its leading edge, supporting the presence of an inner Lindblad resonance. Assuming the 3 kpc arms are a continuous physical structure, the relative Galactocentric distance of its near and far sides suggests highly elliptical streamlines of gas around the bar(s) and a bar corotation radius, r_CR ≳ 3.6 kpc. At a Galactic longitude near 10° and a heliocentric distance of about 5 kpc, the near 3 kpc arm and the Norma arm intersect on a face-on view of our Galaxy, while passing at different Galactic latitudes. We provide an accurate distance measurement to the W 31 star-forming complex of 4.95_–0.43^+0.51 kpc from the Sun, which associates it with a bright CO feature belonging to the near 3 kpc arm.