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 λ 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 (λ 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 ζ 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 14 442, 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 ζ Pup. The question then arises whether the Oef peculiarity denotes a homogeneous class of objects after all.

Orbital and sub-orbital period determination of the candidate high-mass X-ray binary HD 3191

“…In this scenario, the physical cause of the photometric modulation would be due to the presence of starspots, which would also exist in early-type stars and not only in those of latetype, such as RS Canis Venaticorum systems. These structures would arise as a result of magnetic fields in the sub-surface convective zone of hot stars (Cantiello et al 2009), but their lifetime in early-type stars is not expected to last as long as our persistent sub-orbital period (Rauw & Nazé 2021). However, if largescale magnetic fields are generated by extreme dynamo effects, then these magnetic structures could be stable on a much longer timescale, of approximately a year, at least for late B-type stars (Cantiello & Braithwaite 2019).…”

Orbital and sub-orbital period determination of the candidate high-mass X-ray binary HD 3191

“…In this scenario, the physical cause of the photometric modulation would be due to the presence of starspots, which would also exist in early-type stars and not only in those of late-type, such as RS Canis Venaticorum systems. These structures would arise as a result of magnetic fields in the sub-surface convective zone of hot stars (Cantiello et al 2009), but their lifetime in early-type stars is not expected to last as long as our persistent sub-orbital period (Rauw & Nazé 2021). However, if large-scale magnetic fields are generated by extreme dynamo effects, then these magnetic structures could be stable on a much longer timescale, of approximately a year, at least for late B-type stars (Cantiello & Braithwaite 2019).…”

On the Origin of Stochastic, Low-Frequency Photometric Variability in Massive Stars

“…In recent years asteroseismology has opened a new window on these challenging astrophysical environments, with high precision photometry from space delivering many new exciting results (e.g., MOST, CoRoT, BRITE, Kepler/K2 and TESS, see Bowman 2020). The latest discovery is the detection of a new ubiquitous phenomenon in massive stars: stochastic low-frequency photometric variability (SLF variability; Blomme et al 2011;Bowman et al 2019a,b;Pedersen et al 2019;Bowman et al 2020;Rauw & Nazé 2021). This joins a number of other surface and wind phenomena that are routinely observed in early-type stars, including surface velocity fluctuations (Macroturbulence; Simón-Díaz & Herrero 2014), line profile variability (Fullerton et al 1996), and discrete absorption components in UV spectra (Howarth & Prinja 1989;Cranmer & Owocki 1996;Fullerton et al 1997;Kaper et al 1997).…”