Discrepant Approaches to Modeling Stellar Tides, and the Blurring of Pseudosynchronization

TL;DR

This paper compares two methods for modeling stellar tides in binary systems, revealing that damping assumptions significantly influence the predicted phenomenon of pseudosynchronization, with implications for understanding stellar rotation dynamics.

Contribution

It identifies a key damping coefficient error in the modal decomposition approach and demonstrates how correcting it aligns the two methods' predictions about pseudosynchronization.

Findings

Correcting the damping coefficient reduces discrepancies between methods.

Both approaches agree that pseudosynchronization is blurred in KOI-54.

The results apply broadly to stars with frequency-dependent tidal damping.

Abstract

We examine the reasons for discrepancies between two alternative approaches to modeling small-amplitude tides in binary systems. The 'direct solution' (DS) approach solves the governing differential equations and boundary conditions directly, while the 'modal decomposition' (MD) approach relies on a normal-mode expansion. Applied to a model for the primary star in the heartbeat system KOI-54, the two approaches predict quite different behavior of the secular tidal torque. The MD approach exhibits the pseudosynchronization phenomenon, where the torque due to the equilibrium tide changes sign at a single, well-defined and theoretically predicted stellar rotation rate. The DS approach instead shows 'blurred' pseudosynchronization, where positive and negative torques intermingle over a range of rotation rates. We trace a major source of these differences to an incorrect damping coefficient in the profile functions describing the frequency dependence of the MD expansion coefficients. With this error corrected some differences between the approaches remain; however, both are in agreement that pseudosynchronization is blurred in the KOI-54 system. Our findings generalize to any type of star for which the tidal damping depends explicitly or implicitly on the forcing frequency.