VLBA determination of the distance to nearby star-forming regions III.
  HP Tau/G2 and the three-dimensional structure of Taurus
  Using multi-epoch Very Long Baseline Array observations, we have measured the
trigonometric parallax of the weak-line T Tauri star HP Tau/G2 in Taurus. The
best fit yields a distance of 161.2 $\pm$ 0.9 pc, suggesting that the eastern
portion of Taurus (where HP Tau/G2 is located) corresponds to the far side of
the complex. Previous VLBA observations have shown that T Tau, to the South of
the complex, is at an intermediate distance of about 147 pc, whereas the region
around L1495 corresponds to the near side at roughly 130 pc. Our observations
of only four sources are still too coarse to enable a reliable determination of
the three-dimensional structure of the entire Taurus star-forming complex. They
do demonstrate, however, that VLBA observations of multiple sources in a given
star-forming region have the potential not only to provide a very accurate
estimate of its mean distance, but also to reveal its internal structure. The
proper motion measurements obtained simultaneously with the parallax allowed us
to study the kinematics of the young stars in Taurus. Combining the four
observations available so far, we estimate the peculiar velocity of Taurus to
be about 10.6 km s-1 almost completely in a direction parallel to the Galactic
plane. Using our improved distance measurement, we have refined the
determination of the position on the HR diagram of HP Tau/G2, and of two other
members of the HP Tau group (HP Tau itself and HP Tau/G3). Most pre-main
sequence evolutionary models predict significantly discrepant ages (by 5 Myr)
for those three stars -expected to be coeval. Only in the models of Palla &
Stahler (1999) do they fall on a single isochrone (at 3 Myr).
