Does GD 356 have a terrestrial planetary companion?
Version of Record online: 16 MAR 2010
© 2010 The Authors. Journal compilation © 2010 RAS
Monthly Notices of the Royal Astronomical Society
Volume 404, Issue 4, pages 1984–1991, June 2010
How to Cite
Wickramasinghe, D. T., Farihi, J., Tout, C. A., Ferrario, L. and Stancliffe, R. J. (2010), Does GD 356 have a terrestrial planetary companion?. Monthly Notices of the Royal Astronomical Society, 404: 1984–1991. doi: 10.1111/j.1365-2966.2010.16417.x
- Issue online: 21 MAY 2010
- Version of Record online: 16 MAR 2010
- Accepted 2010 January 25. Received 2010 January 20; in original form 2009 November 2
- planetary systems;
- white dwarfs
GD 356 is unique among magnetic white dwarfs because it shows Zeeman-split Balmer lines in pure emission. The lines originate from a region of nearly uniform field strength (δB/B≈ 0.1) that covers 10 per cent of the stellar surface in which there is a temperature inversion. The energy source that heats the photosphere remains a mystery but it is likely to be associated with the presence of a companion. Based on current models, we use archival Spitzer Infrared Array Camera (IRAC) observations to place a new and stringent upper limit of 12 MJ for the mass of such a companion. In the light of this result and the recent discovery of a 115-min photometric period for GD 356, we exclude previous models that invoke accretion and revisit the unipolar inductor model that has been proposed for this system. In this model, a highly conducting planet with a metallic core orbits the magnetic white dwarf and, as it cuts through field lines, a current is set flowing between the two bodies. This current dissipates in the photosphere of the white dwarf and causes a temperature inversion. Such a planet is unlikely to have survived both the red and asymptotic giant branch phases of evolution so we argue that it may have formed from the circumstellar disc of a disrupted He or CO core during a rare merger of two white dwarfs. GD 356 would then be a white dwarf counterpart of the millisecond binary pulsar PSR 1257+12 which is known to host a planetary system.