@article{20931, abstract = {Context. Asymmetries in the observed rotational splittings of a multiplet contain information about the star’s rotation profile and internal magnetic field. Moreover, the frequency regularities of multiplets can be used for mode identification. However, to exploit this information, highly accurate theoretical predictions are needed. Aims. We aim to quantify the difference in the predicted mode asymmetries between a 1D perturbative method and a 2D method that includes a 2D stellar structure model, which takes rotation into account. We then place these differences between 1D and 2D methods in the context of asteroseismic measurements of internal magnetic fields. We only focus on the asymmetries and not on possible additional frequency peaks that can arise when the magnetic and rotation axis are misaligned. Methods. We coupled the 1D pulsation codes GYRE and StORM to the 2D stellar structure code ESTER and compared the oscillation predictions with the results from the 2D TOP pulsation code. We focused on zero-age main-sequence models representative of rotating β Cephei pulsators spinning at up to 20 per cent of the critical Keplerian rotation rate. Specifically, we investigated low-radial-order gravity and pressure modes. Results. We find a generally good agreement between the oscillation frequencies resulting from the 1D and 2D pulsation codes. We report differences in predicted mode multiplet asymmetries of mostly below 0.06 d−1. Since the magnetic asymmetries are small compared to the differences in the rotational asymmetries resulting from the 1D and 2D predictions, accurate measurements of the magnetic field are in most cases challenging. Conclusions. Differences in the predicted mode asymmetries of a rotating star between 1D perturbative methods and 2D non-perturbative methods can greatly hinder accurate measurements of internal magnetic fields in main-sequence pulsators with low-order modes. Nevertheless, reasonably accurate measurements could be possible with npg ≥ 2 modes if the internal rotation is roughly below 10 per cent of the Keplerian critical rotation frequency for (aligned) magnetic fields of the order of a few hundred kilogauss. While the differences between the 1D and 2D frequency predictions are mostly too large for internal magnetic field detections, the rotational asymmetries predicted by StORM are in general accurate enough for asteroseismic modelling of the stellar rotation in main-sequence stars with identified low-order modes.}, author = {Mombarg, J. S.G. and Vanlaer, V. and Das, Srijan B and Rieutord, M. and Aerts, C. and Bugnet, Lisa Annabelle and Mathis, S. and Reese, D. R. and Ballot, J.}, issn = {1432-0746}, journal = {Astronomy & Astrophysics}, publisher = {EDP Sciences}, title = {{Is a 1D perturbative method sufficient for asteroseismic modelling of β Cephei pulsators? Implications for measurements of rotation and internal magnetic fields}}, doi = {10.1051/0004-6361/202557247}, volume = {704}, year = {2025}, } @misc{20936, abstract = {Supplementary material for Mombarg et al. (2025, A&A). Title: "Is a 1D perturbative method sufficient for asteroseismic modelling of ~Cephei pulsators? Implications for measurements of rotation and internal magnetic fields" Content: - Non-rotating ESTER models and associated .GSM models. (Xini = 0.71, Zini = 0.014, vertical/horizonal viscosity 10^7 cm^2/s, vertical chemical diffusion 10^4 cm^2/s for evolution model. More details on the ESTER models can be found in the ESTER manual. - Rotational asymmetries computed with StORM and TOP in 1/d, and the central m=0 frequency from TOP in 1/d. (all_A*_new.pkl) - Magnetic asymmetries in 1/d for different obliquity angles between 0 and 90 deg for ZAMS and MAMS model, for B_0 = 75 kG. *_nu key gives unperturbed mode frequencies, *_npg the radial order (asym_dict.pkl, asym_dict_evol.pkl)}, author = {Mombarg, Joey and Vanlaer, Vincent and Das, Srijan B and Rieutord, Michel and Aerts, Conny and Bugnet, Lisa Annabelle and Mathis, Stephane and Reese, Daniel and Ballot, Jerome}, publisher = {Zenodo}, title = {{Is a 1D perturbative method sufficient for asteroseismic modelling of β Cephei pulsators?}}, doi = {10.5281/ZENODO.17580178}, year = {2025}, } @article{20350, abstract = {Context. Rotation plays an important role in stellar evolution. However, the mechanisms behind the transport of angular momentum in stars at various stages of their evolution are not well understood. To improve our understanding of these processes, it is necessary to measure and validate the internal rotation profiles of stars across different stages of evolution and mass regimes. Aims. Our aim is to constrain the internal rotation profile of the 12-M⊙ β Cep pulsator HD 192575 from the observed pulsational multiplets and the asymmetries of their component frequencies. Methods. We updated the forward asteroseismic modelling of HD 192575 based on new TESS observations. We inverted the rotation profile from the symmetric part of the splittings and computed the multiplet asymmetries due to the Coriolis force and stellar deformation, which we treated perturbatively. We compared the computed asymmetries with the observed asymmetries. Results. Our new forward asteroseismic modelling is in agreement with previous results but with increased uncertainties, partially due to increased frequency precision, which required us to relax certain constraints. Ambiguity in the mode identification is the main source of the uncertainty, which also affects the inferred rotation profiles. Almost all acceptable rotation profiles occur in the regime below 0.4 d−1 and favour weak radial differential rotation, with a ratio of core to envelope rotation of less than 2. We find that the quality of the match between the observed and theoretically predicted mode asymmetries is strongly dependent on the mode identification and the internal structure of the star. Conclusions. Our results offer the first detailed rotation inversion for a β Cep pulsator. They show that the rotation profile and the mode asymmetries provide a valuable tool for further constraining the evolutionary properties of HD 192575, and in particular the details of angular momentum transport in massive stars.}, author = {Vanlaer, V. and Bowman, D. M. and Burssens, S. and Das, Srijan B and Bugnet, Lisa Annabelle and Mathis, S. and Aerts, C.}, issn = {1432-0746}, journal = {Astronomy & Astrophysics}, publisher = {EDP Sciences}, title = {{Interior rotation modelling of the β Cep pulsator HD 192575 including multiplet asymmetries}}, doi = {10.1051/0004-6361/202452885}, volume = {701}, year = {2025}, } @article{17189, abstract = {Supergranules, which are solar flow features with a lateral scale of 30,000–40,000 km and a lifetime of ~24 h, form a prominent component of the Sun’s convective spectrum. However, their internal flows, which can be probed only by helioseismology, are not well understood. We analyse dopplergrams recorded by the Solar Dynamics Observatory satellite to identify and characterize ~23,000 supergranules. We find that the vertical flows peak at a depth of ~10,000 km, and remain invariant over the full range of lateral supergranular scales, contrary to numerical predictions. We also infer that, within the local seismic resolution (≳5,000 km), downflows are ~40% weaker than upflows, indicating an apparent mass-flux imbalance. This may imply that the descending flows also comprise plumes, which maintain the mass balance but are simply too small to be detected by seismic waves. These results challenge the widely used mixing-length description of solar convection.}, author = {Hanson, Chris S. and Das, Srijan B and Mani, Prasad and Hanasoge, Shravan and Sreenivasan, Katepalli R.}, issn = {2397-3366}, journal = {Nature Astronomy}, pages = {1088--1101}, publisher = {Springer Nature}, title = {{Supergranular-scale solar convection not explained by mixing-length theory}}, doi = {10.1038/s41550-024-02304-w}, volume = {8}, year = {2024}, } @article{17326, abstract = {Magnetic fields in the stellar interiors are key candidates to explain observed core rotation rates inside solar-like stars along their evolution. Recently, asteroseismic estimates of radial magnetic field amplitudes near the hydrogen-burning shell (H-shell) inside about 24 red giants (RGs) have been obtained by measuring frequency splittings from their power spectra. Using general Lorentz-stress (magnetic) kernels, we investigated the potential for detectability of near-surface magnetism in a 1.3 M⊙ star of supersolar metallicity as it evolves from a mid subgiant to a late subgiant into an RG. Based on these sensitivity kernels, we decompose an RG into three zones—deep core, H-shell, and near-surface. The subgiants instead required decomposition into an inner core, an outer core, and a near-surface layer. Additionally, we find that for a low-frequency g-dominated dipolar mode in the presence of a typical stable magnetic field, ∼25% of the frequency shift comes from the H-shell and the remaining from deeper layers. The ratio of the subsurface tangential field to the radial field in the H-burning shell decides if subsurface fields may be potentially detectable. For p-dominated dipole modes close to vmax, this ratio is around two orders of magnitude smaller in subgiant phases than the corresponding RG. Further, with the availability of magnetic kernels, we propose lower limits of field strengths in crucial layers in our stellar model during its evolutionary phases. The theoretical prescription outlined here provides the first formal way to devise inverse problems for stellar magnetism and can be seamlessly employed for slow rotators.}, author = {Bhattacharya, Shatanik and Das, Srijan B and Bugnet, Lisa Annabelle and Panda, Subrata and Hanasoge, Shravan M.}, issn = {1538-4357}, journal = {Astrophysical Journal}, number = {1}, publisher = {IOP Publishing}, title = {{Detectability of axisymmetric magnetic fields from the core to the surface of oscillating post-main-sequence stars}}, doi = {10.3847/1538-4357/ad4708}, volume = {970}, year = {2024}, } @article{18528, abstract = {The recent measurement of magnetic field strength inside the radiative interior of red giant stars has opened the way toward full 3D characterization of the geometry of stable large-scale magnetic fields. However, current measurements, which are limited to dipolar (ℓ = 1) mixed modes, do not properly constrain the topology of magnetic fields due to degeneracies on the observed magnetic field signature on such ℓ = 1 mode frequencies. Efforts focused toward unambiguous detections of magnetic field configurations are now key to better understand angular momentum transport in stars. We investigated the detectability of complex magnetic field topologies (such as the ones observed at the surface of stars with a radiative envelope with spectropolarimetry) inside the radiative interior of red giants. We focused on a field composed of a combination of a dipole and a quadrupole (quadrudipole) and on an offset field. We explored the potential of probing such magnetic field topologies from a combined measurement of magnetic signatures on ℓ = 1 and quadrupolar (ℓ = 2) mixed mode oscillation frequencies. We first derived the asymptotic theoretical formalism for computing the asymmetric signature in the frequency pattern for ℓ = 2 modes due to a quadrudipole magnetic field. To access asymmetry parameters for more complex magnetic field topologies, we numerically performed a grid search over the parameter space to map the degeneracy of the signatures of given topologies. We demonstrate the crucial role played by ℓ = 2 mixed modes in accessing internal magnetic fields with a quadrupolar component. The degeneracy of the quadrudipole compared to pure dipolar fields is lifted when considering magnetic asymmetries in both ℓ = 1 and ℓ = 2 mode frequencies. In addition to the analytical derivation for the quadrudipole, we present the prospect for complex magnetic field inversions using magnetic sensitivity kernels from standard perturbation analysis for forward modeling. Using this method, we explored the detectability of offset magnetic fields from ℓ = 1 and ℓ = 2 frequencies and demonstrate that offset fields may be mistaken for weak and centered magnetic fields, resulting in underestimating the magnetic field strength in stellar cores. We emphasize the need to characterize ℓ = 2 mixed-mode frequencies, (along with the currently characterized ℓ = 1 mixed modes), to unveil the higher-order components of the geometry of buried magnetic fields and to better constrain angular momentum transport inside stars.}, author = {Das, Srijan B and Einramhof, Lukas and Bugnet, Lisa Annabelle}, issn = {1432-0746}, journal = {Astronomy & Astrophysics}, publisher = {EDP Sciences}, title = {{Unveiling complex magnetic field configurations in red giant stars}}, doi = {10.1051/0004-6361/202450918}, volume = {690}, year = {2024}, }