High-Speed Bullet Ejections during the AGB to Planetary Nebula Transition: HST Observations of the Carbon Star, V Hydrae

TL;DR

This study uses HST observations to reveal that the carbon star V Hya ejects high-speed bullets approximately every 8.5 years, with a precessing, warped accretion disk likely driving the ejection mechanism, shedding light on the AGB to planetary nebula transition.

Contribution

The paper introduces a model linking periodic high-speed bullet ejections to binary interactions and disk precession, explaining observed phenomena in V Hya during the AGB to planetary nebula transition.

Findings

V Hya ejects bullets at 200-250 km/s every ~8.5 years.

The ejection axis flip-flops, indicating precession or warping.

A binary companion with an eccentric orbit likely drives the ejections.

Abstract

The well-studied carbon star, V Hya, showing evidence for high-speed, collimated outflows and dense equatorial structures, is a key object in the study of the poorly understood transition of AGB stars into aspherical planetary nebulae. Using the STIS instrument onboard HST, we have obtained high spatial-resolution long-slit optical spectra of V Hya that show high-velocity emission in [SII] and [FeII] lines. Our dataset, spanning three epochs spaced apart by a year during each of two periods (in 2002-2004 and 2011-2013), shows that V Hya ejects high-speed (about 200-250 km/s) bullets once every ~8.5 yr. The ejection axis flip-flops around a roughly eastern direction, both in and perpendicular to the sky-plane, and the radial velocities of the ejecta also vary in concert between low and high values. We propose a model in which the bullet ejection is associated with the periastron passage of a binary companion in an eccentric orbit around V Hya with an orbital period of ~8.5 yr. The flip-flop phenomenon is likely the result of collimated ejection from an accretion disk (produced by gravitational capture of material from the primary) that is warped and precessing, and/or that has a magnetic field that is misaligned with that of the companion or the primary star. We show how a previously observed 17-year period in V Hya's light-cycle can also be explained in our model. Additionally, we describe how the model proposed here can be extended to account for multipolar nebulae.