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A "magnetic syphon" between stars in a pre-cataclysmic binary?

posted May 24, 2013, 2:14 PM by Jeremy Drake   [ updated Jun 2, 2013, 7:24 AM ]
The "cataclysmic variables" are close binary stars with orbital periods of generally only a few hours.  One of the stars is a very compact white dwarf, and the other is usually a red dwarf - a K-type or M-type main sequence star a bit less massive than the Sun.  The stars are sufficiently close to each other that the edge of the red dwarf facing its white dwarf companion lies at the very point at which the gravitational fields of the two stars are balanced. This results in an overflow of gas from the red dwarf to the white dwarf.  (Under certain conditions, this material can build up until temperatures and densities are high enough to trigger a thermonuclear runaway resulting in a giant explosion and a nova event - see previous posts on U Scorpii. and V407 Cygni.)
The overflowing material forms a disk of hot gas that emits strongly in the optical and ultraviolet as it spirals in toward the white dwarf.  Variations and instabilities in this process cause the violently variable brightness fluctuations that give rise to the "cataclysmic" monicker. 

Before becoming "cataclysmic", there is no overflow, at least not from gravity alone.  To touch gravitationally, the stars must first get really close to each other.  This happens from an initially wider binary very gradually by angular momentum loss through the wind - very much like the solar wind - of the red dwarf star.  Some of this wind can fall onto the white dwarf, and we can in principle this this in X-rays or in the UV-optical spectrum of the white dwarf.

We have been trying to better understand the process of angular momentum loss that forms cataclysmic binaries from close binaries that do not yet touch. The solar wind is known to be driven by the Sun's magnetic field at the solar surface, and red dwarf winds, and associated angular momentum loss, are driven by the same process.  Current models of this process ignore the white dwarf, and perhaps more importantly, its magnetic field. Using magnetohydrodynamic supercomputer simulations of the wind from a close "pre-cataclysmic" red dwarf-white dwarf binary, we have found that the white dwarf magnetic field can influence both the mass loss in the form of a wind, and the angular momentum loss.  Under certain conditions, we find that the magnetic fields of the two stars interlock, opening an efficient syphon flow from the red dwarf to its companion.  This process might help explain some observations of accretion in pre-cataclysmic systems that are otherwise very difficult to understand unless the wind mass loss rates from the red dwarfs are much higher than we think. This work was lead by postdoc Ofer Cohen and was published in the 2012 February 10 edition of the Astrophysical Journal Letters.