Ellipsoidal Variability and the Difference between Sequence D and E Red Giants

TL;DR

This study analyzes red giant binaries in the Large Magellanic Cloud, demonstrating that sequence E stars exhibit ellipsoidal variability while sequence D stars do not, highlighting key differences in their physical properties.

Contribution

The paper clarifies the nature of variability in sequences D and E, establishing that sequence E stars are ellipsoidal binaries and sequence D stars are not, with detailed comparative analysis.

Findings

Sequence E stars show ellipsoidal variability confirmed by light and velocity curves.

Sequence D stars' Long Secondary Periods are not caused by ellipsoidal variability.

Sequence E stars lack mid-infrared excess, unlike some other red giants.

Abstract

We have studied a sample of Large Magellanic Cloud red giant binaries that lie on sequence E in the period--luminosity plane. We show that their combined light and velocity curves unambiguously demonstrate that they are binaries showing ellipsoidal variability. By comparing the phased light and velocity curves of both sequence D and E variables, we show that the sequence D variation -- the Long Secondary Period -- is not caused by ellipsoidal variability. We also demonstrate several further differences between stars on sequences D and E. These include differences in velocity amplitude, in the distribution of eccentricity, and in the correlations of velocity amplitude with luminosity and period. We also show that the sequence E stars, unlike stars on sequence D, do not show any evidence of a mid-infrared excess that would indicate circumstellar dust.