The Stagger Code for Accurate and Efficient, Radiation-coupled Magnetohydrodynamic Simulations

Stein, Robert F.; Nordlund, Åke; Collet, Remo; Trampedach, Regner

doi:10.3847/1538-4357/ad4706

ApJ

2024

DOI: 10.3847/1538-4357/ad4706

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The Stagger Code for Accurate and Efficient, Radiation-coupled Magnetohydrodynamic Simulations

Robert F. Stein,

Åke Nordlund,

Remo Collet

et al.

Abstract: We describe the Stagger code for simulations of magnetohydrodynamic (MHD) systems. This is a modular code with a variety of physics modules that will let the user run simulations of deep stellar atmospheres, sunspot formation, stellar chromospheres and coronae, proto-stellar disks, star formation from giant molecular clouds, and even galaxy formation. The Stagger code is efficiently and highly parallelizable, enabling such simulations with large ranges of both spatial and temporal scales. We describe the metho… Show more

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Cited by 3 publications

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Unveiling complex magnetic field configurations in red giant stars

Das,

Einramhof,

Bugnet

2024

A&A

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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 $( mixed modes, do not properly constrain the topology of magnetic fields due to degeneracies on the observed magnetic field signature on such $ 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 $ and quadrupolar ($ mixed mode oscillation frequencies. We first derived the asymptotic theoretical formalism for computing the asymmetric signature in the frequency pattern for $ 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 $ 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 $ and $ 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 $ and $ 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 $ mixed-mode frequencies (along with the currently characterized $ mixed modes) to unveil the higher-order components of the geometry of buried magnetic fields and to better constrain angular momentum transport inside stars.

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Unveiling complex magnetic field configurations in red giant stars

Das,

Einramhof,

Bugnet

2024

A&A

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Can we rely on EUV emission to identify coronal waveguides?

Kohutova,

Antolin,

Szydlarski

et al. 2024

A&A

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Traditional models of coronal oscillations rely on a modelling of the coronal structures that support them as compact cylindrical waveguides. An alternative model of the structure of the corona has recently been proposed, in which the thin strand-like coronal loops, that are observed in the extreme-UV (EUV) emission are the result of the line-of-sight integration of warps in more complex coronal structures. This is referred to as the coronal veil model. We extend the implications of the coronal veil model of the solar corona to models of coronal oscillations. Using convection-zone-to-corona simulations with the radiation-magnetohydrodynamics (rMHD) code Bifrost, we analysed the structure of the self-consistently formed simulated corona. We focused on the spatial variability of the volumetric emissivity of the Fe IX 171.073 AA EUV line and on the variability of the Alfvén speed, which captures the density and magnetic structuring of the simulated corona. We traced features associated with large magnitudes of the Alfvén speed gradient, which trap MHD waves and act as coronal waveguides. We searched for the correspondence with emitting regions, which appear as strand-like loops in the line-of-sight-integrated EUV emission. We find that the cross sections of the waveguides bounded by large Alfvén speed gradients become less circular and more distorted with increasing height in the solar atmosphere. The waveguide filling factors corresponding to the fraction of the waveguides filled with plasma that emits in the given EUV wavelength range from 0.09 to 0.44. This suggests that we can only observe a small fraction of the waveguide. Similarly, the projected waveguide widths in the plane of the sky are several times larger than the widths of the apparent loops that are observed in the EUV. We conclude that the coronal veil structure is independent of the model. As a result, we find a lack of straightforward correspondence between peaks in the integrated emission profile that constitute apparent coronal loops and regions of plasma bound by a large Alfvén speed gradient that act as waveguides. Coronal waveguides cannot be reliably identified based on emission in a single EUV wavelength is not reliable in the simulated corona formed in convection-zone-to-corona models.

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Probing Na in giant exoplanets with ESPRESSO and 3D NLTE stellar spectra

Canocchi,

Morello,

Lind

et al. 2024

A&A

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Neutral sodium was the first atom that was detected in an exoplanetary atmosphere using the transmission spectroscopy technique. To date, it remains the most successfully detected species due to its strong doublet in the optical at 5890\ A and 5896\ A . However, the center-to-limb variation (CLV) of these lines in the host star can bias the Na I detection. When combined with the Rossiter-McLaughlin (RM) effect, the CLV can mimic or obscure a planetary absorption feature if it is not properly accounted for. This work aims to investigate the impact of three-dimensional (3D) radiation hydrodynamic stellar atmospheres and non-local thermodynamic equilibrium (NLTE) radiative transfer on the modeling of the CLV+RM effect in single-line transmission spectroscopy to improve the detection and characterization of exoplanet atmospheres. We produced a grid of 3D NLTE synthetic spectra for Na I for FGK-type dwarfs within the following parameter space: $T_ eff =4500-6500$\,K, $ g =4.0-5.0$, and Fe/H . This grid was then interpolated to match the stellar parameters of four stars hosting well-known giant exoplanets, generating stellar spectra to correct for the CLV+RM effect in their transmission spectra. We used archival observations taken with the high-resolution ESPRESSO spectrograph. Our work confirms the Na I detections in three systems, namely WASP-52b, WASP-76b, and WASP-127b, also improving the accuracy of the measured absorption depth. Furthermore, we find that 3D NLTE stellar models can explain the spectral features in the transmission spectra of HD 209458b without the need for any planetary absorption. In the grid of stellar synthetic spectra, we observe that the CLV effect is stronger for stars with low $T_ eff $ and high $ g$. However, the combined effect of CLV and RM is highly dependent on the orbital geometry of the planet-star system. With the continuous improvement of instrumentation, it is crucial to use the most accurate stellar models available to correct for the CLV+RM effect in high-resolution transmission spectra to achieve the best possible characterization of exoplanet atmospheres. This will be fundamental in preparation for instruments such as ANDES at the Extremely Large Telescope to fully exploit its capabilities in the near future. We make our grid of 3D NLTE synthetic spectra for Na I publicly available.

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The Stagger Code for Accurate and Efficient, Radiation-coupled Magnetohydrodynamic Simulations

Cited by 3 publications

References 67 publications

Unveiling complex magnetic field configurations in red giant stars

Unveiling complex magnetic field configurations in red giant stars

Can we rely on EUV emission to identify coronal waveguides?

Probing Na in giant exoplanets with ESPRESSO and 3D NLTE stellar spectra

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