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Abstract:
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The main research topic of this dissertation are extreme mass ratio contact
close binary systems, q 0.1, of W Ursae Majoris (W UMa) type. These
close binaries (CBs) represent an interesting class of objects in which ”normal”,
approximately one solar mass main-sequence star is in contact with
a significantly less massive companion, M2 ∼ 0.1 M . Earlier theoretical
investigations of these systems found that there is a minimum mass ratio
qmin = M2/M1 ≈ 0.085 − 0.095 (obtained for n = 3 polytrope - fully radiative
primary) above which these CBs are stable and could be observed. If
the mass ratio is lower than qmin, or, equivalently, if orbital angular momentum
is only about three times larger than the spin angular momentum of
a massive primary, a tidal instability develops (Darwin’s instability) forcing
eventually the stars to merge into a single, rapidly rotating object (such as
FK Com-type stars or blue stragglers). However, there appear to be some
W UMa-type CBs with empirically obtained values for the mass ratio below
the theoretical limit for stability. The aim of this dissertation is to try
to resolve the discrepancy between theory and observations by considering
rotating polytropes. By including in theory the effects of higher central condensation
due to rotation we were able to reduce qmin to the new theoretical
value qmin = 0.070 − 0.074, for the overcontact degree f = 0 − 1, which is
more consistent with the observed population. Other candidate systems for
stellar mergers such as AM CVn-type stars have also been discussed in the
dissertation. |