Similar but different: the varied landscape of Onfp/Oef stars variability

TL;DR

This study investigates the variability of Oef stars using TESS photometry and spectroscopy, revealing diverse behaviors and challenging previous assumptions about their pulsations and classifications.

Contribution

It provides new high-cadence photometric and spectroscopic data, analyzing the variability mechanisms of Oef stars and questioning their classification as a homogeneous group.

Findings

Only zeta Pup shows a long-lived periodicity.

Stars exhibit short-lived low-frequency signals, possibly due to transient spots.

No persistent non-radial pulsations are detected.

Abstract

The Oef category gathers rapidly rotating and evolved O-stars displaying a centrally reversed He II 4686 emission line. The origin of the variability of their photospheric and wind spectral lines is debated, with rotational modulation or pulsations as the main contenders. To shed new light on this question, we analysed high-quality and high-cadence TESS photometric time series for five Oef stars. We also collected a new time series of spectra for one target (lambda Cep) which had been the subject of specific debates in the last years. These observations reveal the variety of Oef behaviours. While space-based photometric data reveal substantial red noise components in all targets, only zeta Pup seems to display a long-lived periodicity. In our sample, stars exhibit a dominant signal at low frequencies but it appears relatively short-lived. This is reminiscent of rotational modulations by transient photospheric spots, though this scenario is challenged by the case of HD 14442, whose 1.230 d$^{-1}$ signal significantly exceeds the critical rotational frequency. In parallel, no evidence of persistent p-mode non-radial pulsations is found in either photometry or spectroscopy of the stars, only temporary excitation of g-mode pulsations could offer an alternative explanation for the dominant signals. Finally, the revised luminosities of the stars using GAIA-DR2 show that they are not all supergiants as zeta Pup. The question then arises whether the Oef peculiarity denotes a homogeneous class of objects after all.