HIGH-SPEEDBULLETEJECTIONSDURINGTHEAGB-TO-PLANETARY.pdfVIP

The Astrophysical Journal, 827:92 (13pp), 2016 August 20 ? 2016. The American Astronomical Society. All rights reserved. doi:10.3847/0004-637X/827/2/92 HIGH-SPEED BULLET EJECTIONS DURING THE AGB-TO-PLANETARY NEBULA TRANSITION: HST OBSERVATIONS OF THE CARBON STAR, V HYDRAE 1 1,2 3 R. Sahai , S. Scibelli , and M. R. Morris 1 Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, Pasadena, CA 91109, USA; raghvendra.sahai@jpl.nasa.gov 2 Stony Brook University, Stony Brook, NY 11794-3800, USA 3 Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1547, USA Received 2016 March 22; revised 2016 May 18; accepted 2016 May 19; published 2016 August 11 ABSTRACT The well-studied carbon star, V Hya, showing evidence for high-speed, collimated out?ows 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 Space Telescope Imaging Spectrograph instrument on board the Hubble Space Telescope, we have obtained high spatial-resolution long-slit optical spectra of V Hya that show high-velocity emission in [S II] and [Fe II] lines. Our data set, 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 (～200–250 km s-1) bullets once every ～8.5 years. The ejection axis ?ip–?ops around a roughly eastern direction, both in and perpendicular to the skyplane, 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 years. The ?ip–?op 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 ?eld that is misaligned with that of the compan