H2O Maser Outflow from the Red Supergiant Star NML Cygni Observed with Japanese VLBI Network

Abstract

Abstract We present the proper motions of H$_2$O masers in NML Cygni, observed with the Japanese VLBI Network at three epochs spanning 455d. We detected about 15 maser features at each epoch. Overall, 13 features that were detected at least twice were tracked by their radial velocities and proper motions. The three-dimensional kinematics of the maser features indicate the presence of an expanding outflow. The major axis of the outflow is estimated to be at a position angle of $\sim\;$108°, and an inclination angle of $\sim\;$8° with respect to the line of sight. The H$_2$O masers are located between an apparent minimum radius of $\sim\;9.6 \times 10^{12}$m (64AU) and a maximum radius of $\sim\;3.0 \times 10^{13}$m (202AU), where the expansion velocity increases from 12 to 27kms$^{-1}$. A comparison with the distributions of SiO, H$_2$O, and OH masers suggests that the outflow of NML Cygni is expanding outside a radius of $\sim\;$1.5$\times$10$^{13}$m (100AU). This radius corresponds to 6 stellar radii, and is consistent with the radius of the inner boundary for the dust shell.