  Kinematic distances to 750 molecular clouds identified in the 13CO J=1-0
Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
(BU-FCRAO GRS) are derived assuming the Clemens rotation curve of the Galaxy.
The kinematic distance ambiguity is resolved by examining the presence of HI
self-absorption toward the 13CO emission peak of each cloud using the Very
Large Array Galactic Plane Survey (VGPS). We also identify 21 cm continuum
sources embedded in the GRS clouds in order to use absorption features in the
HI 21 cm continuum to distinguish between near and far kinematic distances. The
Galactic distribution of GRS clouds is consistent with a four-arm model of the
Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS
clouds match star count data from the Galactic Legacy Infrared Mid-Plane Survey
Extraordinaire (GLIMPSE) star-count data. We conclude that molecular clouds
must form in spiral arms and be short-lived (lifetimes < 10 Myr) in order to
explain the absence of massive, 13CO bright molecular clouds in the inter-arm
space.
