A multisite photometric study of two unusual Beta Cep stars: the magnetic V2052 Oph and the massive rapid rotator V986 Oph

TL;DR

This study presents a comprehensive multisite photometric analysis of two Beta Cep stars, revealing their pulsation modes, magnetic properties, and rotational characteristics, advancing understanding of massive star oscillations and magnetic interactions.

Contribution

It provides the first detailed mode identification and magnetic analysis for V2052 Oph and insights into the variability of V986 Oph, highlighting the potential for asteroseismology in rapidly rotating stars.

Findings

V2052 Oph exhibits three pulsation frequencies and a magnetic oblique rotator configuration.

The dominant pulsation mode in V2052 Oph is radial and likely fundamental.

V986 Oph shows uncertain l=3 mode and additional incoherent light variations.

Abstract

We report a multisite photometric campaign for the Beta Cep stars V2052 Oph and V986 Oph. 670 hours of high-quality differential photoelectric Stromgren, Johnson and Geneva time-series photometry were obtained with eight telescopes on five continents during 182 nights. Frequency analyses of the V2052 Oph data enabled the detection of three pulsation frequencies, the first harmonic of the strongest signal, and the rotation frequency with its first harmonic. Pulsational mode identification from analysing the colour amplitude ratios confirms the dominant mode as being radial, whereas the other two oscillations are most likely l=4. Combining seismic constraints on the inclination of the rotation axis with published magnetic field analyses we conclude that the radial mode must be the fundamental. The rotational light modulation is in phase with published spectroscopic variability, and consistent with an oblique rotator for which both magnetic poles pass through the line of sight. The inclination of the rotation axis is 54o <i< 58o and the magnetic obliquity 58o <beta< 66o. The possibility that V2052 Oph has a magnetically confined wind is discussed. The photometric amplitudes of the single oscillation of V986 Oph are most consistent with an l=3 mode, but this identification is uncertain. Additional intrinsic, apparently temporally incoherent, light variations of V986 Oph are reported. Different interpretations thereof cannot be distinguished at this point, but this kind of variability appears to be present in many OB stars. The prospects of obtaining asteroseismic information for more rapidly rotating Beta Cep stars, which appear to prefer modes of higher l, are briefly discussed.