Journal Article

Possible solution to the problem of the extreme mass ratio W UMa-type binaries

B. Arbutina

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 394, issue 1, pages 501-509
Published in print March 2009 | ISSN: 0035-8711
Published online March 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.14332.x
Possible solution to the problem of the extreme mass ratio W UMa-type binaries

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When the total angular momentum of a binary system is at a critical (minimum) value, a tidal instability occurs (Darwin's instability), eventually forcing the stars to merge into a single, rapidly rotating object. The instability sets in at some critical separation which in the case of contact binaries corresponds to a minimum mass ratio depending on dimensionless gyration radius k1. If one considers n= 3 polytrope (fully radiative primary with Γ1= 4/3), k21= 0.075 and qmin≈ 0.085–0.095. There appears to be, however, some W UMa-type binaries with q values very close, if not below these theoretical limits, implying that primary in these systems is probably more centrally condensed. We try to solve the discrepancy between theory and observations by considering rotating polytropes. We show by deriving and solving a modified Lane–Emden equation for n= 3 polytrope that including the effects of rotation does increase the central concentration and could reduce qmin to as low as 0.070–0.074, more consistent with the observed population.

Keywords: instabilities; binaries: close; blue stragglers; stars: late-type

Journal Article.  2776 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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