Misaligned spin and orbital axes cause the anomalous precession of DI Herculis

TL;DR

This paper explains the long-standing discrepancy in DI Herculis's precession rate by showing that misaligned stellar spin axes cause precession opposite to relativistic effects, resolving the 30-year puzzle.

Contribution

It demonstrates that misaligned stellar spin axes cause precession opposite to relativistic effects, explaining the anomalous precession rate of DI Herculis.

Findings

Both stars have nearly perpendicular spin axes to the orbital plane.

Misaligned spins induce stellar oblateness, affecting precession.

The combined effect reconciles observed and theoretical precession rates.

Abstract

The orbits of binary stars precess as a result of general relativistic effects, forces arising from the asphericity of the stars, and forces from additional stars or planets in the system. For most binaries, the theoretical and observed precession rates are in agreement. One system, however -- DI Herculis -- has resisted explanation for 30 years. The observed precession rate is a factor of four slower than the theoretical rate, a disagreement that once was interpreted as evidence for a failure of general relativity. Among the contemporary explanations are the existence of a circumbinary planet and a large tilt of the stellar spin axes with respect to the orbit. Here we report that both stars of DI Herculis rotate with their spin axes nearly perpendicular to the orbital axis (contrary to the usual assumption for close binary stars). The rotationally induced stellar oblateness causes precession in the direction opposite to that of relativistic precession, thereby reconciling the theoretical and observed rates.