Science with an ngVLA: A six-dimensional tomographic view of Galactic
  star-formation
  Various sign-posts of recent star-formation activity, such as water and
methanol maser emission or magnetically active low-mass young stars, can be
detected with Very Long Baseline Interferometry (VLBI) radio arrays. The
extremely accurate astrometry already attainable with VLBI instruments implies
that the trigonometric parallax and the proper motion of these objects can be
measured to better than 1% for sources within a few hundred parsecs of the Sun,
and better than 10% for objects at a few kiloparsecs. An ngVLA with baselines
extending to several thousand km will have a sensitivity more than one order of
magnitude better than current VLBI instruments, and will enable such highly
accurate astrometric measurements to be performed throughout the Milky Way.
This will provide a full six-dimensional view (three spatial and three velocity
coordinates) of star-formation in the Galactic disk, and have a transformative
impact on our understanding of both star-formation processes and Galactic
structure.
