The Gould's Belt Distances Survey (GOBELINS) I. Trigonometric parallax
  distances and depth of the Ophiuchus complex
  We present the first results of the Gould's Belt Distances Survey (GOBELINS),
a project aimed at measuring the proper motion and trigonometric parallax of a
large sample of young stars in nearby regions using multi-epoch Very Long
Baseline Array (VLBA) radio observations. Enough VLBA detections have now been
obtained for 16 stellar systems in Ophiuchus to derive their parallax and
proper motion. This leads to distance determinations for individual stars with
an accuracy of 0.3 to a few percent. In addition, the orbits of 6 multiple
systems were modelled by combining absolute positions with VLBA (and in some
cases, near infrared) angular separations. Twelve stellar systems are located
in the dark cloud Lynds 1688, the individual distances for this sample are
highly consistent with one another, and yield a mean parallax for Lynds 1688 of
$\varpi=7.28\pm0.06$ mas, corresponding to a distance $d=137.3\pm1.2$ pc. This
represents an accuracy better than 1%. Three systems for which astrometric
elements could be measured are located in the eastern streamer (Lynds 1689) and
yield an estimate of $\varpi=6.79\pm0.16$ mas, corresponding to a distance
$d=147.3\pm3.4$ pc. This suggests that the eastern streamer is located about 10
pc farther than the core, but this conclusion needs to be confirmed by
observations (currently being collected) of additional sources in the eastern
streamer. From the measured proper motions, we estimate the one-dimensional
velocity dispersion in Lynds 1688 to be 2.8$\pm$1.8 and 3.0$\pm$2.0 ${\rm
km~s}^{-1}$, in R.A. and DEC., respectively, these are larger than, but still
consistent within $1\sigma$, with those found in other studies.
