Po-Sheng Huang (ASIAA); Raghvendra Sahai (JPL); Chin-Fei Lee (ASIAA)
In recent observations, the carbon star V Hydrae (V Hya) was periodically ejecting three collimated bullets that may indicate a transition of the star from an asymptotic-giant-branch (AGB) star into a multipolar planetary nebula (PN). Here we present 3-dimensional (3D) hydrodynamic simulations of the first bullet (Bullet 1) ejected from the star. We aim to capture the initial conditions of the surroundings and bullet when it was ejected. The simulations are performed using the grid-based ZEUS-3D hydrodynamic code. The numerical settings are based on the previous optical spectrograph using the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). The observational data showed that the radius of Bullet 1 was increasing from ~15-22 au between 7.6-9.6 yr after the ejection, and the mass derived from the spectroscopic data with a gas temperature of 9500-14000 K was increasing from 1e27 to 3e27 g that may have entrained the gas of the AGB wind. In our simulation results, the bullet radius quickly expands from 10 to ~30 au in the first year and the mass with a temperature of ~11000 K quickly decreases from 1e28 to 1e23 g when it propagates into a cone-like cavity along the polar axis. When the bullet collides with the wall of the AGB wind around the cavity, the radius gradually decreases to 12 au in 9.6 yr, and the mass with a temperature of 11000 K gradually increases to ~3e26 g after 9.6 yr. The simulations and observations are roughly consistent. Given the mass and the period of 8.5 yr, the mass-loss rate is ~1e-6 Msun/yr. In addition, the accretion rate of the disk should be smaller than the mass-loss rate of the AGB wind, i.e., 2e-5 Msun/yr. Therefore, the accretion rate could be ~1e-6-1e-5 Msun/yr.