Which is the most eccentric binary known? Insights from the 2023/4 pericenter passages of Zeta Boötis and Eta Ophiuchi

There is a clear dearth of very eccentric binaries among those for which individual eccentricities can be measured. In this paper we report on observations of the two nearby, bright and very eccentric visual binaries Zeta Bo"otis ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>ζ</mml:mi> </mml:math> Boo) and Eta Ophiuchi ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>η</mml:mi> </mml:math> Oph), for which VLTI/GRAVITY interferometric observations were obtained during their pericenter passages in 2023/4. Previous observations of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>ζ</mml:mi> </mml:math> Boo suggest an eccentricity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>e</mml:mi> <mml:mo>&gt;</mml:mo> <mml:mn>0.99</mml:mn> </mml:mrow> </mml:math> with high significance, implying that it has the highest eccentricity of any known binary. However, our interferometric measurements near periastron passage reveal that the eccentricity is actually <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.980450</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.000064</mml:mn> </mml:mrow> </mml:math> (second highest well constrained eccentricity) with a pericenter distance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>a</mml:mi> <mml:mi>p</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.818</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.009</mml:mn> <mml:mrow> <mml:mspace width="0.333em"/> <mml:mtext mathvariant="normal">au</mml:mtext> </mml:mrow> </mml:mrow> </mml:math> . We attribute the previous over-estimation to a degeneracy that plagues very eccentric visual binary orbital solutions. For <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>η</mml:mi> </mml:math> Oph we find an eccentricity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.93077</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.00013</mml:mn> </mml:mrow> </mml:math> (compared to previous estimates of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.95</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> </mml:math> ), a pericenter distance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>a</mml:mi> <mml:mi>p</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2.15</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.10</mml:mn> <mml:mrow> <mml:mspace width="0.333em"/> <mml:mtext mathvariant="normal">au</mml:mtext> </mml:mrow> </mml:mrow> </mml:math> and attribute the over-estimated dynamical mass in the previous solution to an underestimated error in the semi-major axis. In both systems full circularization is expected as the stars evolve and expand, ultimately leading to close binaries with no memory of their very eccentric past.