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

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.

Accepted for publication in Monthly Notices of the Royal Astronomical Society

References in corpus (26)

• The Transiting Exoplanet Survey Satellite
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• UBVJHK synthetic photometry of Galactic O stars
• The MiMeS survey of magnetism in massive stars: Magnetic analysis of the O-type stars
• Low-frequency gravity waves in blue supergiants revealed by high-precision space photometry
• The spectroscopic Hertzsprung-Russell diagram
• Variability of OB stars from TESS southern Sectors 1-13 and high-resolution IACOB and OWN spectroscopy
• The MiMeS Survey of Magnetism in Massive Stars: CNO surface abundances of Galactic O stars
• Photometric detection of internal gravity waves in upper main-sequence stars. II. Combined TESS photometry and high-resolution spectroscopy
• A FUSE Survey of the Rotation Rates of Very Massive Stars in the Small and Large Magellanic Clouds
• Modeling Ultraviolet Wind Line Variability in Massive Hot Stars
• BRITE-Constellation high-precision time-dependent photometry of the early-O-type supergiant $\zeta$ Puppis unveils the photospheric drivers of its small- and large-scale wind structures
• Direct spectroscopic observations of clumping in O-star winds
• The IACOB project. VI. On the elusive detection of massive O-type stars close to the ZAMS
• Low-frequency variability in massive stars: Core generation or surface phenomenon?
• A method to discriminate solar and antisolar differential rotation in high-precision light curves
• Time-series photometry of the O4 I(n)fp star zeta Puppis
• Asteroseismology of massive stars with the TESS mission: the runaway Beta Cep pulsator PHL 346 = HN Aqr
• Investigating the origin of cyclical wind variability in hot, massive stars - I. On the dipolar magnetic field hypothesis
• TESS lightcurves of gamma-Cas stars
• The distance, rotation, and physical parameters of zeta Pup
• The IACOB project: IV. New predictions for high-degree non-radial mode instability domains in massive stars and connection with macroturbulent broadening
• Amplitude variability in satellite photometry of the non-radially pulsating O9.5V star zeta Oph
• Coordinated UV and X-ray spectroscopic observations of the O-type giant xi Per: the connection between X-rays and large-scale wind structure
• A detailed X-ray investigation of zeta Puppis IV. Further characterization of the variability
• Is λ Cep a pulsating star?

Cited by in corpus (5)

• On the Origin of Stochastic, Low-Frequency Photometric Variability in Massive Stars
• The IACOB project VIII. Searching for empirical signatures of binarity in fast-rotating O-type stars
• Extreme mass ratios and fast rotation in three massive binaries
• The Transformative Journey of HD 93521
• Diving into the magnetosphere of the Of?p star HD108