Fast nodal precession of the disc around Pleione requires a broken disc

TL;DR

This paper models the rapid nodal precession of Pleione's circumstellar disc, proposing that a broken disc structure caused by active feeding explains observed variability and matches the precession timescale and disc size.

Contribution

It demonstrates that a broken disc model with a specific binary eccentricity can reproduce the observed precession rate and disc size of Pleione, unlike unbroken disc models.

Findings

Broken disc model fits observed precession timescale

Outer ring precession matches observed disc size

Unbroken disc models cannot simultaneously fit both observations

Abstract

Pleione is a Be star that is in a 218 day orbit with a low-mass binary companion. Recent numerical simulations have shown that a Be star disc can be subject to breaking when material is actively being fed into the inner parts of the disc. After breaking, the disc is composed of two rings: an inner ring that is anchored to the stellar equator and an outer ring that is free to nodally precess. A double ring disc may explain some of the observed variability in Pleione. We model the nodal precession of the outer disc ring that is driven by the companion on an observed timescale of $80.5\,\rm yr$. We find that the outer ring of a broken disc in a binary with an eccentricity of $e_{\rm b}= 0.6$ can precess on the observed timescale and have an outer radius that is in rough agreement with the observed disc size. An unbroken disc model cannot fit both the observed precession rate and disc size. Suppression of Kozai-Lidov driven disc eccentricity is more likely for a high binary eccentricity if the disc extends to the tidal truncation radius.