Hu1-2: a metal-poor bipolar planetary nebula with fast collimated
  outflows
  We present narrow-band optical and near-IR imaging and optical long-slit
spectroscopic observations of Hu1-2, a Galactic planetary nebula (PN) with a
pair of [N II]-bright, fast-moving (> 340 km/s) bipolar knots.
Intermediate-dispersion spectra are used to derive physical conditions and
abundances across the nebula, and high-dispersion spectra to study the
spatio-kinematical structure. Generally Hu1-2 has high He/H (~0.14) and N/O
ratios (~0.9), typical of Type I PNe. On the other hand, its abundances of O,
Ne, S, and Ar are low as compared with the average abundances of Galactic bulge
and disc PNe. The position-velocity maps can be generally described as an
hour-glass shaped nebula with bipolar expansion, although the morphology and
kinematics of the innermost regions cannot be satisfactorily explained with a
simple, tilted equatorial torus. The spatio-kinematical study confines the
inclination angle of its major axis to be within 10 degrees of the plane of
sky. As in the irradiated bow-shocks of IC4634 and NGC7009, there is a clear
stratification in the emission peaks of [O III], H_alpha, and [N II] in the
northwest (NW) knot of Hu1-2. Fast collimated outflows in PNe exhibit higher
excitation than other low-ionization structures. This is particularly the case
for the bipolar knots of Hu1-2, with He II emission levels above those of
collimated outflows in other Galactic PNe. The excitation of the knots in Hu1-2
is consistent with the combined effects of shocks and UV radiation from the
central star. The mechanical energy and luminosity of the knots are similar to
those observed in the PNe known to harbor a post-common envelope (post-CE)
close binary central star.
