Astron. Astrophys. 324, 109-120 (1997)

The case of the two-period polar BY Camelopardalis (H0538+608) ^*

M. Mouchet ^{1, 2}, J.M. Bonnet-Bidaud ³, N.N. Somov ⁴ and T.A. Somova <sup>4

1</sup> Observatoire de Paris, DAEC, Unité associée au CNRS et à l'Université Paris 7-Denis Diderot, F-92195 Meudon Cedex, France
² Université Denis Diderot, Place Jussieu, F-75005 Paris Cedex, France
³ Service d'Astrophysique, CE Saclay, CEA/DSM/DAPNIA/SAp, F-91191 Gif-sur-Yvette Cedex, France
⁴ Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz 357147, Russia

Received 26 November 1996 / Accepted 31 January 1997

Abstract

We present the results of fast temporal optical spectroscopy and photometry of the AM Her type system BY Cam obtained in February 1990 and March 1991. Emission line profiles show a complex structure and are strongly variable. The radial velocity studies of a sharp component detected in Balmer lines in March confirms the non-synchronism of the system. Different possible emitting regions are discussed to explain the characteristics of the multiple line components. It is shown from a computation of the line velocities and widths that the 'horizontal' stream cannot explain the lines of intermediate widths observed in this system. A critical review of the different estimations of the periods found in BY Cam is presented. Additional periods are revealed from a re-analysis of previous optical polarimetric and UV spectroscopic data. We show that because of the slight asynchronism between the white dwarf rotation and the orbital period, significant changes in the accretion geometry introduce a bias in the period determination depending on the length of the observations. Large phase variations are shown to exist which are well reproduced by a phase-drift model in which a magnetic dipole rotates in the orbital frame with a period of 14.5 1.5 days.

Key words: accretion stars: individual: BY Cam stars: binaries: general stars: cataclysmic variables X-rays: stars

* Based on observations from State Research Center Special Astrophysical Observatory of Russian Academy of Sciences, Russia.

Send offprint requests to: M. Mouchet (mouchet@obspm.fr)

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Contents

• 1. Introduction
• 2. Observations
• 3. Photometry
• 4. Spectroscopy
  • 4.1. Description of the line profiles
  • 4.2. Radial velocity results
• 5. Discussion
  • 5.1. Emission lines from the horizontal stream
  • 5.2. The heated hemisphere of the secondary
  • 5.3. Ephemeris: orbital and spin periods
• 6. Open questions and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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