BANANA IV: Two aligned stellar rotation axes in the young eccentric binary system EP Crucis: primordial orientation and tidal alignment

TL;DR

This study measures the spin-orbit alignment in the young binary system EP Cru, finding close alignment despite its non-synchronized stellar spins, suggesting some binaries form with aligned axes rather than misaligned ones.

Contribution

It provides the first detailed measurement of spin-orbit angles in EP Cru, showing primordial alignment rather than tidal realignment as the cause.

Findings

EP Cru has nearly aligned stellar rotation and orbital axes.

The stars are spinning faster than synchronized with the orbit.

Tidal dissipation is unlikely to explain the current alignment.

Abstract

With observations of the EP Cru system, we continue our series of measurements of spin-orbit angles in eclipsing binary star systems, the BANANA project (Binaries Are Not Always Neatly Aligned). We find a close alignment between the sky projections of the rotational and orbital angular momentum vectors for both stars (beta_p = -1.8+-1.6 deg and |beta_s|<17 deg). We also derive precise absolute dimensions and stellar ages for this system. The EP Cru and DI Her systems provide an interesting comparison: they have similar stellar types and orbital properties, but DI Her is younger and has major spin-orbit misalignments, raising the question of whether EP Cru also had a large misalignment at an earlier phase of evolution. We show that tidal dissipation is an unlikely explanation for the good alignment observed today, because realignment happens on the same timescale as spin-orbit synchronization, and the stars in EP Cru are far from syncrhonization (they are spinning 9 times too quickly). Therefore it seems that some binaries form with aligned axes, while other superficially similar binaries are formed with misaligned axes.