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Mind the Gap

posted Mar 3, 2020, 7:08 PM by Jeremy Drake   [ updated Mar 3, 2020, 7:15 PM ]
One of the most challenging puzzles in stellar evolution that emerged in the 1970s and early 1980s was the "cataclysmic variable period gap". Cataclysmic variables (CVs) are interacting binary stars comprising a white dwarf accreting from a more normal companion star. As the population of known CVs grew, it became clear that systems with periods between 2 and 3.12 hr were rarely observed compared with those with shorter and longer periods.

CV orbits generally shrink over time as angular momentum is lost to the stellar wind of the secondary companion. It was realised that if this angular momentum loss abruptly changed at a period of about 3 hours, then the accretion would stop: voila, the CV would detach and not be observed as an accreting object, explaining the period gap. The theory was called "interrupted magnetic breaking".

The underlying motivation for a fairly abrupt angular momentum loss reduction stems from arguments that magnetic dynamo action in Sun-like stars that powers stellar winds and angular momentum loss occurs at the interface between the convection zone and the radiative interior — the “tachocline”. For typical CVs, the orbital period of three hours corresponds to the secondary becoming fully-convective - the tachocline disappears, along with winds and angular momentum loss. However, an earlier post describes work that indicates no such change in magnetic activity in fully convective stars occurs: the tachocline is apparently not important: interrupted magnetic braking theory is broken.

A paper lead by Smithsonian and Harvard researcher Cecilia Garraffo published in the 2018 November 20 edition of the Astrophysical Journal rescues the theory by explaining the stellar wind and angular momentum loss reduction instead in terms of the growing complexity of the surface magnetic field. As noted in an earlier post, a more complex field shuts down the wind and drastically reduces the magnetic braking. Interrupted magnetic braking is mended.