The spatiokinematical structure of H2 O and OH masers in the "Water fountain" source IRAS 18460-0151.
IMAI H., DEGUCHI S., NAKASHIMA J.-I., KWOK S. and DIAMOND P.J.
Abstract (from CDS):
Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network, we have observed 22.2 GHz H2 O and 1612 MHz OH masers in the water fountain source IRAS 18460-0151. The H2 O maser spectrum has a very wide line-of-sight velocity range (~310 km/s) and consists of three groups of emission features at the blueshifted (-68 km/s ≲ VLSR ≲ -17 km/s) and redshifted (VLSR≃ 240 km/s) edges as well as around the systemic velocity (112 km/s ≲ VLSR ≲ 133 km/s). The first two H2 O 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. 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 (VLSR= 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 H2 O 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 H2 O 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) has a large uncertainty. This makes it difficult to provide strong constraints on models of the central stellar system of IRAS 18460-0151.