Studying the photometric and spectroscopic variability of the magnetic hot supergiant ζ Orionis Aa

Abstract

Massive stars play a significant role in the chemical and dynamical evolution of galaxies. However, much of their variability, particularly during their evolved supergiant stage, is poorly understood. To understand the variability of evolved massive stars in more detail, we present a study of the O9.2Ib supergiant ζ Ori Aa, the only currently confirmed supergiant to host a magnetic field. We have obtained two-color space-based BRIght Target Explorer photometry (BRITE) for ζ Ori Aa during two observing campaigns, as well as simultaneous ground-based, high-resolution optical CHIRON spectroscopy. We perform a detailed frequency analysis to detect and characterize the star's periodic variability. We detect two significant, independent frequencies, their higher harmonics, and combination frequencies: the stellar rotation period P rot = 6.82 ± 0.18 d, most likely related to the presence of the stable magnetic poles, and a variation with a period of 10.0 ± 0.3 d attributed to circumstellar environment, also detected in the Hα and several He I lines, yet absent in the purely photospheric lines. We confirm the variability with P rot /4, likely caused by surface inhomogeneities, being the possible photospheric drivers of the discrete absorption components. No stellar pulsations were detected in the data. The level of circumstellar activity clearly differs between the two BRITE observing campaigns. We demonstrate that ζ Ori Aa is a highly variable star with both periodic and non-periodic variations, as well as episodic events. The rotation period we determined agrees well with the spectropolarimetric value from the literature. The changing activity level observed with BRITE could explain why the rotational modulation of the magnetic measurements was not clearly detected at all epochs.

Asteroseismology of High-Mass Stars: New Insights of Stellar Interiors With Space Telescopes

“…The constellation of nanosatellites from the collaboration of Austria, Poland, and Canada, was originally launched in 2013 and have provided long-term time series photometry of some of the brightest stars in the sky. This includes β Cep, SPB and Be stars (Baade et al, 2016;Pigulski et al, 2016;Daszyńska-Daszkiewicz et al, 2017;Handler et al, 2017;Walczak et al, 2019), but also pulsating stars in multiple systems (Kallinger et al, 2017;Pablo et al, 2017Pablo et al, , 2019 and stochastic variability in O supergiants and Wolf-Rayet stars (Buysschaert et al, 2017b;Ramiaramanantsoa et al, 2018aRamiaramanantsoa et al, ,b, 2019. The multi-color and long-term photometry of BRITE and its observing strategy are naturally complementary to combing BRITE data with simultaneous ground-based photometry and/or spectroscopy (e.g., Handler et al, 2017).…”

BRITE-Constellation reveals evidence for pulsations in the enigmatic binary η Carinae

“…Similar phenomena were recently confirmed with both photometry and spectroscopy for the early-O supergiant ζ Puppis (Ramiaramanantsoa et al 2017). The late-O supergiant ζ Orionis (Buysschaert et al 2017) shows both a rotational modulation from its magnetic field and a secondary component that varies on a longer timescale and may be associated with the stellar wind.…”

BRITE-Constellation high-precision time-dependent photometry of the early O-type supergiant ζ Puppis unveils the photospheric drivers of its small- and large-scale wind structures

“…Raw light curves were extracted using the reduction pipeline for BRITE data which also includes corrections for intra-pixel sensitivity (Popowicz et al 2017). Then postreduction decorrelations with respect to instrumental effects due to CCD temperature variations, centroid position and orbital phase were performed on each observational setup for each satellite according to the method described by Pigulski et al (2016), and flux variations due to changes in point spread function shape as a function of temperature were performed according to the method described by Buysschaert et al (2017). In the resulting final decorrelated light curves, we do not notice any obvious variations on timescales shorter than the BRITE orbital period (t 100 min) that could be qualified as intrinsic to the star rather than pure instrumental noise.…”