The spatiokinematical structure of H_2O and OH masers in the "water fountain" source IRAS 18460-0151

TL;DR

This study uses VLBI observations to analyze the complex maser emissions in IRAS 18460-0151, revealing a fast bipolar jet, a spherical outflow, and their spatial relationships, providing insights into the source's dynamic structure.

Contribution

First detailed VLBI imaging of both H_2O and OH masers in IRAS 18460-0151, revealing the coexistence of a high-velocity bipolar jet and a spherical outflow, and their spatial and kinematic relationship.

Findings

H_2O masers trace a high-velocity bipolar jet (~160 km/s) with a dynamical age of ~6 years.

OH masers show a double-peaked spectrum indicating a circumstellar envelope.

The jet and outflow originate from a source within 30 AU, but the systemic velocity remains uncertain.

Abstract

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.