The most luminous stars in the Galaxy and the Magellanic Clouds

TL;DR

This study analyzes the luminosities of Wolf-Rayet stars in the Galaxy and Magellanic Clouds using spectral models, revealing very luminous WNL stars and their evolutionary paths, with implications for massive star formation at different metallicities.

Contribution

It provides the first detailed spectral analysis of the most luminous Wolf-Rayet stars in the Galaxy and Magellanic Clouds, highlighting their luminosities and evolutionary stages.

Findings

Very luminous WNL stars exist in the Galaxy and LMC.

Most SMC WR stars are in binary systems and less luminous.

Massive stars above 60 solar masses evolve directly to WNL stage.

Abstract

Some of the Wolf-Rayet stars are found to have very high bolometric luminosities (more than 1000000 solar). We employ the Potsdam Wolf-Rayet (PoWR) model atmospheres for their spectral analysis, which yields the bolometric corrections. Distance and interstellar reddening also enter the luminosity estimates. Among the Galactic stars, there is a group of very luminous WNL stars (i.e. WR stars of late subtype from nitrogen sequence with hydrogen being depleted in their atmospheres, but not absent). Their distances are often the major source of uncertainty. From K-band spectroscopy we found a very luminous star ($\log L/L_\odot = 6.5$) in the Galactic center region, which we termed the Peony Star because of the form of its surrounding dusty nebula. A similar group of very luminous WNL stars is found in the Large Magellanic Cloud (LMC). In the Small Magellanic Cloud (SMC) the majority of WR stars resides in binary systems. The single WNL stars in the SMC are not very luminous. We conclude that a significant number of very luminous WNL stars exist in the Galaxy and the LMC. With initial masses above 60$M_\odot$, they apparently evolved directly to the WNL stage without a prior excursion to the red side of the HRD. At the low metallicity of the SMC, the binary channel may be dominant for the formation of WR stars.