arXiv subjects

Lukas Einramhof

Publications and source records attributed to Lukas Einramhof.

4 recordsLinked to original sources

arXiv2026

Magnetic Signatures in Merger Products

Asteroseismic estimates of the magnetic field strength in the radiative interior of red giant stars depend strongly on the internal stellar structure derived from models. Since red giant branch merger products have been shown to be able to possess a different core structure than single stars of the same mass, we investigate how a mass-gain history influences our estimates of an internal magnetic field strength. We construct stellar models with and without a mass-gain event after the onset of the RGB evolutionary phase with masses of $1.1\,M_\odot \le M \le 2\,M_\odot$. First, by assuming a weak magnetic field, we investigate the influence of a mass-gain event on the global sensitivity of the oscillation frequencies to the magnetic field. We find that mass-gain models can be several times more sensitive to the field than single-star models of identical total mass at masses above $1.6\,M_\odot$. Therefore, considering a mass-gain evolutionary history for merger candidates allows a significant correction to the magnetic field strength. In the presence of strong magnetic fields, we also show that the critical field strength needed to suppress mixed dipole modes is significantly lower if a mass-gain event has occurred (for stars with masses $M\gtrsim1.6\,M_\odot$). The massive end of the suppressed stars' distribution is therefore strongly favored by a merger origin. We conclude that properly constraining the stellar evolutionary history is essential when aiming to constrain internal field strength estimates from asteroseismic observations.

astro-ph.SR↗

arXiv2026

Magneto-Archeology of White Dwarfs. Revisiting the fossil field scenario with observational constraints during the red giant branch

The detection of strong, large-scale magnetic fields at the surface of only the oldest population of white dwarfs might point towards a hidden internal magnetic field slowly rising to the surface. In addition, strong magnetic fields have recently been measured through asteroseismology in the radiative interiors of red giant stars, the progenitors of white dwarfs. To investigate the potential connection between these observations, we revisit the fossil field framework by using the asteroseismic detections to constrain the strength of such magnetic fields as they evolve to the white dwarf stage. We assume that the magnetic field was either created during the main sequence core convection or that it fills the radiative interior as the star evolves on the red giant branch. From these, we evolve the magnetic flux, allowing for magnetic diffusion along the evolution of a 1.5Msun modelled star. We find that measured field strengths in red giants attributed to the hydrogen-burning shell are compatible with the field amplitudes and emergence timescales of magnetized white dwarfs. On the contrary, magnetic fields generated solely from a convective-core dynamo on the main-sequence and detectable during the red giant branch would be buried too deep in the star and not match the breakout timescales and the field strengths of magnetic white dwarfs. A broadly magnetized internal radiative zone during the red giant branch is therefore key for the fossil field theory to connect magnetic fields observed along the late evolution of stars.

astro-ph.SR↗

arXiv2024

Unveiling complex magnetic field configurations in red giant stars

Recent measurements of magnetic field strength inside the radiative interior of red giant stars open the way towards the characterization of the geometry of stable large-scale magnetic fields. However, current measurements do not properly constrain the topology of magnetic fields due to degeneracies on the observed magnetic field signature on such $\ell=1$ mode frequencies. Efforts focused towards unambiguous detections of magnetic field configurations are now key to better understand angular momentum transport in stars. We investigate the detectability of complex magnetic field topologies inside the radiative interior of red giants. We focus on a field composed of a combination of a dipole and a quadrupole (quadrudipole), and on an offset field. We explore the potential of probing such magnetic field topologies from a combined measurement of magnetic signatures on $\ell=1$ and quadrupolar ($\ell=2$) mixed mode oscillation frequencies. We first derive the asymptotic theoretical formalism for computing the asymmetric signature in frequency pattern for $\ell=2$ modes due to a quadrudipole magnetic field. The degeneracy of the quadrudipole with a dipole is lifted when considering both $\ell=1$ and $\ell=2$ 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 demonstrate that offset fields may be mistaken for weak and centered magnetic fields, resulting in underestimating magnetic field strength in stellar cores. We emphasize the need to characterize $\ell=2$ mixed-mode frequencies, (along with the currently characterized $\ell=1$ mixed modes), to unveil the higher-order components of the geometry of buried magnetic fields, and better constrain angular momentum transport inside stars.

astro-ph.SR↗

arXiv2021

Efficient Adaptive Computation of the Dynamics of Mott Transistors

We investigate time-adaptive Magnus-type integrators for the numerical approximation of a Mott transistor. The rapidly attenuating electromagnetic field calls for adaptive choice of the time steps. As a basis for step selection, asymptotically correct defect-based estimators of the local error are employed. We analyze the error of the numerical approximation in the presence of the unsmooth external potential and demonstrate the advantages of the adaptive approach.

math.NA↗