Asteroseismic detection of an internal magnetic field in the B0.5V pulsator HD 192575

TL;DR

This study addresses how internal magnetic fields influence angular momentum transport in massive stars by presenting a direct magneto-gravito-asteroseismic detection of an internal field in the 12 $M_\odot$ star HD 192575. Using 1951.46 days of TESS data, the authors identify 9 frequencies in the quadrupole ($l=2$) low-order $g$-mode multiplet, which rotation alone cannot explain, and apply a perturbative framework that combines Coriolis forces with an oblique, fossil magnetic field to fit both frequencies and amplitudes. The best-fit magneto-gravito-asteroseismic model (Set 5) yields a maximum internal field strength $B_{\max} = 24^{+651}_{-15}$ kG, an obliquity $\beta \approx 65^\circ$, an inclination $i \approx 24^\circ$, and a near-core rotation $f_{\rm rot} \approx 0.19\,\mathrm{d^{-1}}$, with the $g$-mode kernel peaking near the stellar surface at $R_{\rm peak} \approx 0.994\,R_*$. The results are broadly consistent with p-mode analyses under the same framework, though centrifugal deformation is neglected here, highlighting the need for extended theory. Collectively, this work marks the magneto-gravito-asteroseismic detection of a stable internal magnetic field inside an evolved rotating supernova progenitor and demonstrates a powerful method to probe angular momentum transport in massive stars.

Abstract

Internal magnetic fields are an elusive component of stellar structure. However, they can play an important role in stellar structure and evolution models through efficient angular momentum transport and their impact on internal mixing. We strive to explain the 9 components of one frequency multiplet, identified as a low-order quadrupole gravity mode detected in the light curve of the beta Cep pulsator HD 192575 assembled by the Transiting Exoplanet Survey Satellite (TESS). We update the frequencies of the quadrupole mode under investigation using a standard prewhitening method applied to the 1951.46 d TESS light curve, showing that an internal magnetic field is required to simultaneously explain all 9 components. We implement theoretical pulsation computations applicable to the low-order modes of a beta Cep pulsator including the Coriolis force, as well as a magnetic field that is misaligned with respect to the rotation axis. We apply the theoretical description to perform asteroseismic modelling of the amplitudes and frequencies in the multiplet of the quadrupole g-mode of this evolved beta Cep star. Pulsation predictions based on the measured internal rotation frequency of the star cannot explain the observed 9-component frequency splittings of the quadrupole low-order gravity mode. By contrast, we show that the combined effect of the Coriolis force caused by the near-core rotation with a period of about 5.3 d and the Lorentz force due to an internal inclined magnetic field with a maximum strength of around 24 kG does provide a proper explanation of the 9 multiplet frequencies and their relative amplitudes. Given the stellar mass of about 12 solar masses, this work presents the detection and magneto-gravito-asteroseismic modelling of a stable internal magnetic field buried inside an evolved rotating supernova progenitor.

Figures (6)

• Figure 1: Predictions of the best fitting magneto-gravito pulsation model from Set 5 and its uncertainties (dark blue). The black cross designates a predicted frequency that is not detected in the data. The blue shaded area shows all possible locations of unidentified frequencies. The horizontal red dashed line shows the local average noise level in the data. The 95% confidence intervals for the predicted frequency differences are smaller than the size of the markers.
• Figure 2: Lomb-Scargle periodogram of the TESS Cycle 2,4,5, and 6 light curve of HD 192575. The indicated areas are discussed in this Letter.
• Figure 3: Excerpt of the Lomb-Scargle periodogram of HD 192575 zoomed in on the g-mode (left) and p-mode (right) multiplet under study. The dotted lines indicate the frequencies extracted from the data and the mode labels refer to Table \ref{['ChFit-tab:fitting_results']}.
• Figure 4: Magnetic sensitivity kernel of an example g mode with $n=-1$, $l=2$.
• Figure 5: Magnetic stress tensor of an example magnetic field used following the prescription of duezRelaxedEquilibriumConfigurations2010.