High-velocity Bullets from v Hydrae, an Asymptotic Giant Branch Star in Transition: Ejection History and Spatio-kinematic Modeling

The carbon star V Hydrae (V Hya) provides new insight into the nature of the launching mechanism of jet-like outflows that are believed to be the cause of the poorly understood transition phase of asymptotic giant branch stars into aspherical planetary nebulae. V Hya has been shown to periodically eject collimated gas blobs at high velocities ("bullets"). By analyzing data from Hubble Space Telescope/Space Telescope Imaging Spectrograph 2D spectra, obtained at six epochs spaced over a decade that show four successively ejected bullets with a spacing of 8.5 yr, we have created kinematic models of the dynamical evolution of a specific bullet (#1) for the first three observed epochs (2002, 2003, 2004) using a 3D spatio-kinematic code, SHAPE. Using these models, we fit the observed morphology, line-of-sight velocity, proper motion, and intensity for the extended, gaseous bullet as a function of time over a period of 2 yr, in order to constrain its 3D movement and the evolution of its physical properties over this period. Our results suggest that although bullet #1's motion is predominantly ballistic, there are small but significant changes in the position angle and inclination angle of the long (symmetry) axis of the bullet that tilt it progressively toward the symmetry axis of the bipolar molecular nebula around V Hya. In contrast, bullet #3 shows strong acceleration soon after ejection. We discuss the possibilities that bullet acceleration is caused by a nonradial magnetic field and/or by hydrodynamic interaction with the ambient gas through which the bullet is traveling.