THE BANANA PROJECT. VI. Close double stars are well aligned with noticeable exceptions; results from an ensemble study using apsidal motion and Rossiter-McLaughlin measurements

TL;DR

This study analyzes the spin-orbit alignment of 51 close double star systems, finding most are well aligned with notable exceptions, using improved measurements of apsidal motion and Rossiter-McLaughlin effects.

Contribution

It provides the first comprehensive ensemble analysis of spin-orbit alignment in close double stars using updated apsidal motion data and statistical modeling.

Findings

48 out of 51 systems are consistent with alignment.

Significant misalignments confirmed in two eccentric systems.

Most systems show alignment, with some notable exceptions.

Abstract

Here we present an ensemble study of spin-orbit alignment in 51 close double star systems. We determine spin-orbit angles, obliquities, in 39 of these systems making use of recently improved apsidal motion rate measurements and apsidal motion constants. In the remaining 12 systems researchers have constrained spin-orbit alignment by different combinations of measurements of apsidal motion rates, projected obliquities and stellar inclinations. Of the 51 systems 48 are consistent with alignment albeit with some measurements having large uncertainties. A Fisher distribution with mean zero and a concentration factor $\kappa = 6.7$ represents this ensemble well. Indeed employing a bootstrapping resampling technique we find our data on these 48 systems is consistent with perfect alignment. We also confirm significant misalignment in two systems which travel on eccentric orbits and where misalignments have been reported on before, namely DI Her and AS Cam. The third misaligned system CV Vel orbits on a circular orbit. So while there are some glaring exceptions, the majority of close double star systems for which data are available appear to be well aligned.