We compile and analyze approximately 200 trigonometric parallaxes and proper motions of molecular masers associated with very young high-mass stars .
Most of the measurements come from the BeSSeL Survey using the VLBA and the Japanese VERA project .
These measurements strongly suggest that the Milky Way is a four-arm spiral , with some extra arm segments and spurs .
Fitting log-periodic spirals to the locations of the masers , allowing for “ kinks ” in the spirals and using well-established arm tangencies in the 4 ^ { th } Galactic quadrant , allows us to significantly expand our view of the structure of the Milky Way .
We present an updated model for its spiral structure and incorporate it into our previously published parallax-based distance-estimation program for sources associated with spiral arms .
Modeling the three-dimensional space motions yields estimates of the distance to the Galactic center , { R _ { 0 } } = 8.15 \pm 0.15 kpc , the circular rotation speed at the Sun ’ s position , { \Theta _ { 0 } } = 236 \pm 7 km s ^ { -1 } , and the nature of the rotation curve .
Our data strongly constrain the full circular velocity of the Sun , { \Theta _ { 0 } } + { V _ { \odot } } = 247 \pm 4 km s ^ { -1 } , and its angular velocity , ( { \Theta _ { 0 } } + { V _ { \odot } } ) / { R _ { 0 } } = 30.32 \pm 0.27 km s ^ { -1 } kpc ^ { -1 } .
Transforming the measured space motions to a Galactocentric frame which rotates with the Galaxy , we find non-circular velocity components typically \mathrel { \hbox { \raise 2.15 pt \hbox { $ < $ } \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \sim$% } } } } 10 km s ^ { -1 } .
However , near the Galactic bar and in a portion of the Perseus arm , we find significantly larger non-circular motions .
Young high-mass stars within 7 kpc of the Galactic center have a scale height of only 19 pc and , thus , are well suited to define the Galactic plane .
We find that the orientation of the plane is consistent with the IAU-defined plane to within \pm 0 { \hbox to 0.0 pt { . } } ^ { \circ } 1 , and that the Sun is offset toward the north Galactic pole by { Z _ { \odot } } = 5.5 \pm 5.8 pc .
Accounting for this offset places the central supermassive black hole , Sgr A* , in the midplane of the Galaxy .
The measured motions perpendicular to the plane of the Galaxy limit precession of the plane to \mathrel { \hbox { \raise 2.15 pt \hbox { $ < $ } \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \sim$% } } } } 4 km s ^ { -1 } at the radius of the Sun .
Using our improved Galactic parameters , we predict the Hulse-Taylor binary pulsar to be at a distance of 6.54 \pm 0.24 kpc , assuming its orbital decay from gravitational radiation follows general relativity .