Observational Constraints on Close Binary Star Evolution. II. Extreme Mass Ratios

Abstract W UMa systems become tidally unstable when their mass ratios ( q ≡ M 2 / M 1 ) dip below some minimum value ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">min</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) used for identifying stellar merger candidates. Recent fitting of photometric lightcurves (LCs) has identified dozens of putative extreme- q systems ( q &lt; 0.15). In this paper, we combine high-resolution echelle spectroscopy ( R ∼ 31,000) with high-precision Kepler and TESS photometry to examine 18 putative low- q and extreme- q systems, most notably V857 Her, V1187 Her, and NSVS 2569022. The spectroscopic mass ratios are consistently larger than previously published photometric values, sometimes dramatically. In particular, we find large, previously unidentified third light ( l 3) in V1187 Her, NSVS 2569022, and VSX J082700.8+462850, yielding revised mass ratios q &gt; 0.12. We demonstrate that photometry alone cannot reliably determine q , even for totally eclipsing systems; constraints on l 3 are needed. The mass ratios, which range from q = 0.063 to q = 0.234, violate several flat <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">min</mml:mi> </mml:mrow> </mml:msub> </mml:math> predictions, but are consistent with newer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">min</mml:mi> </mml:mrow> </mml:msub> </mml:math> predictions that are based on modeling the interior of the primary star. The RV profiles of at least five of the systems show significant departures from the conventional model for CBs. The clear anticorrelation between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>f</mml:mi> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>q</mml:mi> </mml:math> for CBs remains largely unexplained, but an emerging W UMa evolutionary framework may help explain the data. More than half of close detached binaries ( P &lt; 2 days) in our extended sample are Algols, exhibiting evidence of prior mass-ratio reversal.