The Banana Project. III. Spin-orbit Alignment in the Long-period Eclipsing Binary NY Cephei

TL;DR

This study measures the spin-orbit alignment of the primary star in the NY Cephei binary system, revealing a well-aligned configuration unlike the previously observed misaligned DI Her system, thus indicating diverse evolutionary outcomes.

Contribution

It provides the first measurement of the spin-axis orientation in NY Cephei, demonstrating that spin-orbit misalignment is not universal among similar binary systems.

Findings

NY Cephei's primary star has a well-aligned spin axis (beta_p = 2 +- 4 degrees).

The result contrasts with the misaligned DI Her system, suggesting different evolutionary paths.

The study highlights the need for theories to explain why some binaries are aligned and others are not.

Abstract

Binaries are not always neatly aligned. Previous observations of the DI Her system showed that the spin axes of both stars are highly inclined with respect to one another and the orbital axis. Here we report on a measurement of the spin-axis orientation of the primary star of the NY Cep system, which is similar to DI Her in many respects: it features two young early-type stars (~6 Myr, B0.5V+B2V), in an eccentric and relatively long-period orbit (e=0.48, P=15.d3). The sky projections of the rotation vector and the spin vector are well-aligned (beta_p = 2 +- 4 degrees), in strong contrast to DI Her. Although no convincing explanation has yet been given for the misalignment of DI Her, our results show that the phenomenon is not universal, and that a successful theory will need to account for the different outcome in the case of NY Cep.