Astron. Astrophys. 361, 991-1000 (2000)

An analysis of the Ap binary HD 81009 ^* **

G.A. Wade ^1,2, Y. Debernardi ³, G. Mathys ⁴, D.A. Bohlender ⁵, G.M. Hill ⁶ and J.D. Landstreet <sup>7

1</sup> Astronomy Department, University of Toronto at Mississauga, Mississauga, Ontario, L5L 1C6, Canada
² Déptartment de Physique, Université de Montréal, CP 6128, succ. Centre-Ville, Montréal, Quebec, H3C 3J7, Canada
³ Institut d'Astronomie, Université de Lausanne, 1290 Chavannes-des-bois, Switzerland
⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁵ Herzberg Institute of Astrophysics, Dominion Astrophysical Observatory, 501 West Saanich Road, Victoria, B.C., V8X 4M6, Canada
⁶ Hobby-Eberly Telescope, McDonald Observatory, P.O. Box 1337, Fort Davis, TX 79734, USA
⁷ Physics and Astronomy Department, University of Western Ontario, London, Ontario, N6A 3K7, Canada

Received 25 April 2000 / Accepted 24 July 2000

Abstract

We present a detailed investigation of the orbit, component characteristics and magnetic field of the single spectrum (SB1), visual Ap binary HD 81009.

By simultaneously modeling new and archival radial velocity measurements and new and archival speckle interferometric measurements (obtained with the CHARA array) we obtain a unique model of the orbital geometry and constraints on the component masses of HD 81009. Additional constraints provided by the Hipparcos parallax and component magnitude difference and the optical spectral energy distribution allow us to determine a self-consistent solution for the basic physical properties of the components. HD 81009 is a highly eccentric (), long-period ( y) binary composed of two main sequence A-type stars. While its presence is required in order to explain the astrometric and photometric observations, the cooler secondary component is never detected spectroscopically, and is therefore inferred to rotate somewhat more rapidly than the hotter component.

The hotter primary component is identified as the slowly-rotating () magnetic Ap star. We have modeled the magnetic field geometry of this star using new and archival longitudinal magnetic field and mean magnetic field modulus observations. The rotational variations of the magnetic quantities are consistent with a decentered dipole surface magnetic field geometry with small magnetic obliquity (). This is consistent with the observation of Landstreet & Mathys (2000), who report that nearly all magnetic Ap stars with periods longer than around 25 days exhibit , implying that their magnetic fields are approximately aligned with their rotational axes.

Key words: polarization stars: binaries: general stars: chemically peculiar stars: individual: HD 81009 stars: magnetic fields

* Based on observations obtained at the European Southern Observatory (La Silla, Chile), the Haute Provence and Pic du Midi Observtories (France), and the Las Campanas Observatory (Chile)
** Table 2 is available only in electronic form from the CDS by anonymous ftp at ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/361/991 (130.79.128.5) or at http://cdsweb.u-strasbg.fr/cti-bin/qcat?J/A+A/361/991

Send offprint requests to: G.A. Wade (wade@astro.umontreal.ca)

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. Magnetic field and polarisation measurements
    • 2.1.1. UWO longitudinal field measurements
    • 2.1.2. ESO CASPEC longitudinal field measurements
    • 2.1.3. High-resolution intensity spectra and mean magnetic field modulus measurements
    • 2.1.4. MuSiCoS spectropolarimetry
  • 2.2. Radial velocity measurements
    • 2.2.1. RVs from the CORAVEL scanner and CORALIE spectrograph
  • 2.3. Astrometric data
    • 2.3.1. Visual observations
    • 2.3.2. Speckle observations
    • 2.3.3. Hipparcos observations
• 3. Orbit modeling
  • 3.1. Previous solutions
  • 3.2. New orbital solution
• 4. Component characteristics
  • 4.1. A search for the spectral lines of the secondary
  • 4.2. Relative and absolute luminosities of the components
  • 4.3. Effective temperatures of the components
• 5. Rotation and magnetic field of the primary
  • 5.1. Rotational period
  • 5.2. Surface magnetic field geometry
• 6. Rotation and spectrum of the secondary
• 7. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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