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V471 Tauri and SuWt 2: The Exotic Descendants of Triple Systems?

Published online by Cambridge University Press:  12 April 2016

Howard E. Bond ,
M. Sean O’Brien ,
Edward M. Sion ,
Dermott J. Mullan ,
Katrina Exter ,
Don L. Pollacco  and
Ronald F. Webbink
Howard E. Bond
Affiliation:
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
M. Sean O’Brien
Affiliation:
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Edward M. Sion
Affiliation:
Department of Astronomy & Astrophysics, Villanova University, Villanova, PA 19085, USA
Dermott J. Mullan
Affiliation:
Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
Katrina Exter
Affiliation:
Department of Pure and Applied Physics, Queen’s University, Belfast BT7 1NN, UK
Don L. Pollacco
Affiliation:
Department of Pure and Applied Physics, Queen’s University, Belfast BT7 1NN, UK
Ronald F. Webbink
Affiliation:
Department of Astronomy, University of Illinois, 1002 W. Green Street, Urbana, IL 61801, USA

Abstract

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V471 Tauri is a short-period eclipsing binary, and a member of the Hyades. It is composed of a hot DA white dwarf (WD) and a cool main-sequence dK2 companion. HST radial velocities of the WD, in combination with the ground-based spectroscopic orbit of the K star, yield dynamical masses of MWD = 0.84 and MdK = 0.93 M⊙. During the UV observations we serendipitously detected coronal mass ejections from the K star, passing in front of the WD and appearing as sudden, transient metallic absorption. Eclipse timings show that the active dK star is 18% larger than a main-sequence star of the same mass, an apparent consequence of its extensive starspot coverage. The high Teff and high mass of the WD are paradoxical: the WD is the most massive in the Hyades, but also the youngest. A plausible scenario is that the progenitor system was a triple, with a close inner pair that merged after several × 108 yr to produce a single blue straggler. When this star evolved to the AGB phase, it underwent a common-envelope interaction with a distant dK companion, which spiraled down to its present separation and ejected the envelope. The common-envelope efficiency parameter, αCE, was of order 0.3–1.0, in good agreement with recent hydrodynamical simulations.

SuWt 2 is a southern-hemisphere planetary nebula (PN) with an unusual ring-shaped morphology. The central star is an eclipsing binary with a period of 4.9 days. Surprisingly, the binary is composed of two main-sequence A-type stars with similar masses of ~ 2.5 M⊙. We discuss scenarios involving a third companion which ejected and ionizes the PN.

WeBo 1 is a northern PN with a ring morphology remarkably similar to that of SuWt 2. Although we hoped that its central star would shed light on the nature of SuWt 2, it has proven instead to be a late-type barium star!

Type
Research Article
Information
International Astronomical Union Colloquium , Volume 187: Exotic Stars as Challenges to Evolution , 2002 , pp. 239 - 243
Copyright
Copyright © Astronomical Society of the Pacific 2002

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