The Onfp Class in the Magellanic Clouds

TL;DR

This study characterizes the spectral features and rotational velocities of Onfp class stars in the Magellanic Clouds, revealing their broad emission profiles, high rotational speeds, and potential evolutionary origins as mergers or gamma-ray burst progenitors.

Contribution

First comprehensive survey of Onfp stars in the Magellanic Clouds, analyzing their spectral morphology, rotational velocities, and variability to understand their nature and evolutionary status.

Findings

Onfp stars have higher rotational velocities than normal O stars.

Spectral profiles show variability and some are spectroscopic binaries.

Onfp stars span from dwarfs to supergiants, challenging existing theories.

Abstract

The Onfp class of rotationally broadened, hot spectra was defined some time ago in the Galaxy, where its membership to date numbers only eight. The principal defining characteristic is a broad, centrally reversed He II $\lambda$4686 emission profile; other emission and absorption lines are also rotationally broadened. Recent surveys in the Magellanic Clouds (MCs) have brought the class membership there, including some related spectra, to 28. We present a survey of the spectral morphology and rotational velocities, as a first step toward elucidating the nature of this class. Evolved, rapidly rotating hot stars are not expected theoretically, because the stellar winds should brake the rotation. Luminosity classification of these spectra is not possible, because the principal criterion (He II $\lambda$4686) is peculiar; however, the MCs provide reliable absolute magnitudes, which show that they span the entire range from dwarfs to supergiants. The Onfp line-broadening distribution is distinct and shifted toward larger values from those of normal O dwarfs and supergiants with >99.99% confidence. All cases with multiple observations show line-profile variations, which even remove some objects from the class temporarily. Some of them are spectroscopic binaries; it is possible that the peculiar profiles may have multiple causes among different objects. The origin and future of these stars are intriguing; for instance, they could be stellar mergers and/or gamma-ray-burst progenitors.