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The winds of change on a pre-cataclysmic binary

posted Jan 26, 2014, 9:37 PM by Jeremy Drake   [ updated Feb 4, 2014, 8:40 AM ]
In a post from May 2013, I described research on the magnetosphere of a "pre-cataclysmic binary" (illustrated in the figure) - a very close pair comprising a white dwarf and red dwarf in an orbit with a period of only a few hours.  These systems are destined to become cataclysmic variables, novae and, perhaps, Type 1a supernovae.  This change from a fairly well-behaved and quiescent, albeit very close, binary to a system exhibiting outbursts of radiation and enormous thermonuclear explosions occurs because the red dwarf eventually gets sufficiently close to the white dwarf that it begins to lose its atmosphere to the strong gravity of the degenerate star.  It is the accretion of this gas onto the surface of the white dwarf that causes the "cataclysmic" behaviour - either through energy released in hitting the white dwarf surface and heating surrounding material, or through sufficient build-up of matter to reach temperatures and densities high enough to engender thermonuclear runaway.  

We caught one such pre-cataclysmic system on the very brink of the so-called mass-transfer stage of evolution and reported on the discovery in the post "Surprising X-rays from a pre-cataclysmic binary".  The system is known as QS Virginis, or EC 13471-1258, and it was seen in X-rays to be transferring mass at a rate much lower than normally seen in cataclysmic variables - about ten thousand billionths of a solar mass per year.  It looks like it was accreting from either the upper chromosphere or from the stellar wind.  The latter is more likely, but the accretion rate was about ten times higher than we think is probable.  We speculated that this was because of the wind being effectively channeled to the white dwarf by the interaction of the stellar magnetic fields.  

X-rays provided a direct means of measuring accretion, through seeing the gas itself heat up in a shock as it landed on the white dwarf surface.  There is another way to estimate the rate, based on seeing the amount of "metals" - elements heavier than helium -  in the white dwarf atmosphere.  The metals settle out of the hydrogen atmosphere by gravity in a day or so, and the amount seen represents freshly accreted matter.  We discovered a measurement of carbon and silicon abundances on the white dwarf in a paper on a HST spectrum obtained in 1999 - about 6 1/2 years earlier than the XMM-Newton X-ray observation.  The abundances implied a much lower accretion rate, by a factor of a thousand.  It is not easy to explain such a change in a pre-cataclysmic system, but it does fit in with the magnetic channeling idea.  If the red dwarf has a magnetic cycle, like the Sun, its change in magnetic polarity over the cycle will act like an accretion switch, effectively turning the stream on and off.  The test of this magnetic accretion switch is to observe the system repeatedly over a period of years to look for the cycle.  This work was published in the 2013 December 10 edition of Monthly Notices of the Royal Astronomical Society.