The Nature of a Recently Discovered Wolf-Rayet Binary: Archetype of Stripping?

TL;DR

This study investigates a newly discovered Wolf-Rayet binary in the Large Magellanic Cloud, analyzing its spectral and orbital characteristics to understand its nature and formation, challenging the idea that it resulted from binary stripping.

Contribution

The paper provides detailed spectroscopic and photometric analysis of LMCe055-1, revealing its complex multiple-star system nature and arguing against its formation via binary stripping, which is a novel insight.

Findings

The WR emission and He II absorption are from one star.

He I absorption is from a different star in the system.

The system likely contains a 35-day orbit with a highly eccentric orbit.

Abstract

LMCe055-1 was recently discovered in a survey for WRs in the Large Magellanic Cloud, and classified as a WN4/O4, a lower excitation version of the WN3/O3 class discovered as part of the same survey. Its absolute magnitude precluded it from being a WN4+O4 binary. OGLE photometry show shallow primary and secondary eclipses with a 2.2 day period. The spectral characteristics and short period pointed to a possible origin due to binary stripping. Such stripped WR binaries should be common but have proven elusive to identify conclusively. In order to establish its nature, we obtained HST UV and Magellan optical spectra, along with imaging. Our work shows that the WR emission and He II absorption arise in one star, and the He I absorption in another. The He I contributor is the primary of the 2.2-day system and exhibits ~300 km/sed radial velocity variations on that time scale. However, the WR star shows 30-40 km/sed radial velocity variations, with a likely 35-day period and a highly eccentric orbit. Possibly LMCe055-1 is a physical triple, but that would require the 2.2-day pair to have been captured by the WR star. A more likely explanation is that the WR star has an unseen companion in a 35-day orbit and that the 2.2-day pair is in a longer period orbit about the two. Such examples of multiple systems are well known among massive stars, such as HD 5980. Regardless, we argue that it is highly unlikely that the WR component of the LMCe055-1 system resulted from stripping.