THE SPATIOKINEMATICAL STRUCTURE OF H2O AND OH MASERS IN THE "WATER FOUNTAIN" SOURCE IRAS 18460–0151.

Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network, we have observed 22.2 GHz H2O and 1612 MHz OH masers in the water fountain source IRAS 18460−0151. The H2O maser spectrum has a very wide line-of-sight velocity range (≈310 km s−1) and consists of three groups of emission features at the blueshifted (−68 km s−1≲ VLSR ≲ −17 km s−1) and redshifted (VLSR ≃240 km s−1) edges as well as around the systemic velocity (112 km s−1 ≲ VLSR ≲ 133 km s−1). The first two H2O spectral components exhibit a highly collimated high-velocity bipolar jet on the sky, with an angular separation of ≈120 milliarcsec (mas) (240 AU in linear length) and a three-dimensional flow velocity of ≈160 km s−1. 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−1. On the other hand, the OH maser spectrum shows double peaks with a velocity separation of ≈25 km s−1 (VLSR = 111–116 and 138–141 km s−1), as typically seen in circumstellar envelopes of OH/IR stars. The angular offset between the velocity-integrated brightness peaks of the two high-velocity H2O components is ≈25 mas (50 AU). The offset direction and the alignment of the redshifted maser spots are roughly perpendicular to the axis of the H2O maser flow. High-accuracy astrometry for the H2O 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 (Vsys ≈ 87–113 km s−1) has a large uncertainty. This makes it difficult to provide strong constraints on models of the central stellar system of IRAS 18460−0151. [ABSTRACT FROM AUTHOR]