# The Orbital Nature of 81 Ellipsoidal Red Giant Binaries in the Large Magellanic Cloud

## TL;DR

This study analyzes 81 ellipsoidal red giant binaries in the Large Magellanic Cloud, revealing their orbital characteristics, mass distributions, and eccentricity evolution, with implications for tidal circularization theories.

## Contribution

First comprehensive orbital analysis of a large sample of LMC red giant binaries, including statistical and individual orbital solutions, and comparison with models and solar vicinity data.

## Key findings

- 59 systems with circular orbits
- 20% of binaries have eccentric orbits
- Eccentricity evolution suggests tidal rates are much lower than standard theory

## Abstract

In this paper, we collect a sample of 81 ellipsoidal red giant binaries in the Large Magellanic Cloud (LMC), and we study their orbital natures individually and statistically. The sample contains 59 systems with circular orbits and 22 systems with eccentric orbits. We derive orbital solutions using the 2010 version of the Wilson-Devinney code. The sample is selection-bias corrected, and the orbital parameter distributions are compared to model predictions for the LMC and to observations in the solar vicinity. The masses of the red giant primaries are found to range from about 0.6 to 9 Msun with a peak at around 1.5 Msun, in agreement with studies of the star formation history of the LMC, which find a burst of star formation beginning around 4 Gyr ago. The observed distribution of mass ratios q=m2/m1 is more consistent with the flat q distribution derived for the solar vicinity by Raghavan et al. than it is with the solar vicinity q distribution derived by Duquennoy & Mayor. There is no evidence for an excess number of systems with equal mass components. We find that about 20% of the ellipsoidal binaries have eccentric orbits, twice the fraction estimated by Soszynski et al. Our eccentricity evolution test shows that the existence of eccentric ellipsoidal red giant binaries on the upper parts of the red giant branch (RGB) can only be explained if tidal circularization rates are ~1/100 the rates given by the usual theory of tidal dissipation in convective stars.