  Using the Very Long Baseline Array and the European VLBI Network, we have
observed 22.2 GHz H_2O and 1612 MHz OH masers in the "water fountain" source
IRAS 18460-0151. The H_2O maser spectrum has a very wide line-of-sight velocity
range (~310 km/s) and consists of three groups of emission features at the
blue-shifted (-68 km/s <~ V_LSR <~ -17 km/s) and red-shifted (V_LSR ~= 240
km/s) edges as well as around the systemic velocity (112 km/s <~ V_LSR <~ 133
km/s). The first two H_2O spectral components exhibit a highly-collimated
high-velocity bipolar jet on the sky, with an angular separation of ~120
milliarcseconds (mas) (240 AU in linear length) and a three-dimensional flow
velocity of ~160 km/s. The flow dynamical age is estimated to be only ~6 yr (at
the time of the observation epochs of 2006--2007). Interestingly, the systemic
velocity component clearly exhibits a spherically-expanding outflow with a
radius of ~36 AU and a flow velocity of ~9 km/s. On the other hand, the OH
maser spectrum shows double peaks with a velocity separation of ~25 km/s
(V_LSR=$111--116 and 138--141 km/s), as typically seen in circumstellar
envelopes of OH/IR stars. The angular offset between the velocity-integrated
brightness peaks of the two high-velocity H_2O components is ~25 mas (50 AU).
The offset direction and the alignment of the red-shifted maser spots are
roughly perpendicular to the axis of the H_2O maser flow. High-accuracy
astrometry for the H_2O and OH masers demonstrates that the collimated fast jet
and the slowly expanding outflow originate from a single or multiple sources
which are located within 15 mas (30 AU). On the other hand, the estimated
systemic velocity of the collimated jet (V_sys ~87--113 km/s) has a large
uncertainty. This makes it difficult to provide strong constraints on models of
the central stellar system of IRAS 18460-0151.

Solar and Stellar Astrophysics