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

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Here we present an ensemble study of spin–orbit alignment in 43 close double star systems. We determine spin–orbit angles, obliquities, in 31 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 43 systems 40 are consistent with alignment albeit with some measurements having large uncertainties. A Fisher distribution with mean zero and a concentration factor κ = 6.1 represents this ensemble well. Indeed employing a bootstrapping resampling technique we find our data on these 40 systems are consistent with perfect alignment. We also confirm significant misalignment in two systems that 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.