Tides in Massive Binaries: Numerical Solutions and Semi-Analytical Comparisons

TL;DR

This paper compares numerical and semi-analytical methods for predicting tidal evolution in massive binary systems, finding that numerical solutions better match observed orbital decay in specific cases, especially during mass transfer phases.

Contribution

It provides a systematic comparison between numerical and semi-analytical tidal evolution predictions for massive binaries, highlighting the importance of direct numerical methods for accurate interpretation.

Findings

Numerical methods agree with semi-analytic predictions within two orders of magnitude before mass transfer.

Semi-analytic prescriptions often overestimate orbital decay timescales, especially during mass transfer.

Numerical solutions reveal resonances with gravity waves, aligning closely with observed orbital decay in PSR J0045--7319.

Abstract

We present a systematic comparison between the tidal secular evolution timescales predicted by the direct numerical method and those given by the commonly used semi-analytic prescriptions implemented in 1-D hydrostatic binary evolution codes. Our study focuses on binary systems with intermediate- to high-mass primaries ($M_1 = 5$-$50\,M_\odot$), companion masses between $1.4\,M_\odot$ and $10\,M_\odot$, and orbital periods ranging from 0.5 to 50 days. Before mass transfer, both approaches predict synchronization and orbital decay timescales that agree within $\sim$2 orders of magnitude and typically exceed the stellar main sequence lifetime, implying negligible tidal impact on secular orbital evolution. However, the implied dissipation channels differ, and the differences become more pronounced once mass transfer begins. To test the theoretical predictions against observations, we apply both approaches to the well-characterized PSR J0045--7319 system, which has an orbital decay timescale of 0.5 Myr. The numerical solution reveals strong resonances with internal gravity waves, bringing the predicted orbital period change rate close to the observed value. In contrast, the semi-analytic prescriptions predict orbital decay timescales longer than the Hubble time. These results suggest that for population studies, modestly calibrated parameterized equations may suffice, but for individual systems, reliable interpretation requires direct numerical approaches.