An application of Galactic parallax: the distance to the tidal stream
  GD-1
  We assess the practicality of computing the distance to stellar streams in
our Galaxy, using the method of Galactic parallax suggested by Eyre & Binney
(2009). We find that the uncertainty in Galactic parallax is dependent upon the
specific geometry of the problem in question. In the case of the tidal stream
GD-1, the problem geometry indicates that available proper motion data, with
individual accuracy ~4 mas/yr, should allow estimation of its distance with
about 50 percent uncertainty. Proper motions accurate to ~1 mas/yr, which are
expected from the forthcoming Pan-STARRS PS-1 survey, will allow estimation of
its distance to about 10 percent uncertainty. Proper motions from the future
LSST and Gaia projects will be more accurate still, and will allow the parallax
for a stream 30 kpc distant to be measured with ~14 percent uncertainty.
  We demonstrate the feasibility of the method and show that our uncertainty
estimates are accurate by computing Galactic parallax using simulated data for
the GD-1 stream. We also apply the method to actual data for the GD-1 stream,
published by Koposov et al. (2009). With the exception of one datum, the
distances estimated using Galactic parallax match photometric estimates with
less than 1 kpc discrepancy. The scatter in the distances recovered using
Galactic parallax is very low, suggesting that the proper motion uncertainty
reported by Koposov et al. (2009) is in fact over-estimated.
  We conclude that the GD-1 stream is (8 +/- 1) kpc distant, on a retrograde
orbit inclined 37 deg to the plane, and that the visible portion of the stream
is likely to be near pericentre.
