A Pole-on Bipolar Outflow from the AGB Star WX Piscium

Abstract

Abstract We report on H$_{2}$O masers around the asymptotic giant branch (AGB) star WX Piscium (WX Psc $=$ IRC $+$10011), observed with the Japanese VLBI Network (JVN) at three epochs during a span of 2 months. The H$_{2}$O maser spectrum of WX Psc consists of double peaks with a separation of $\sim$ 30 km s$^{-1}$ centered at the systemic velocity of the star. The maser features are found to split roughly in two concentrations separated by about 60 mas from north to south. The northern concentration involves both blueshifted and redshifted components from both of the double peaks, suggesting that it is near the star driving the outflow. The maser spatiokinematics is well approximated by a radial expansion of a thick shell, except for the blueshifted features in the southern concentration. The spatiokinematics is compared with a fan-shaped morphology found in near-infrared emission, suggesting the existence of a bipolar outflow. Two different models are proposed for explaining the observed maser proper motions: one is a biconically expanding flow with a large opening angle ($2\theta_{\rm cone}=$70${}^\circ$–120${}^\circ$); the other is a collimated outflow with precession. The complicated spatiokinematics of the H$_{2}$O maser features indicates that the star is currently in the process of launching a bipolar outflow in the final stage of the AGB phase, shaping a planetary nebula in the next stage.