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

Abstract

ABSTRACT Pleione is a Be star that is in a 218-d 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 time-scale of $80.5\, \rm yr$. We find that the outer ring of a broken disc in a binary with an eccentricity of eb = 0.6 can precess on the observed time-scale 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.