Revisiting the emergence of buried fossil fields to the surface of white dwarfs from asteroseismic constraints on the RGB

Einramhof, Lukas; Bugnet, Lisa

doi:10.5281/zenodo.17241586

Revisiting the emergence of buried fossil fields to the surface of white dwarfs from asteroseismic constraints on the RGB

Einramhof, Lukas; Bugnet, Lisa

Publisher: Zenodo
DOI: 10.5281/zenodo.17241586
Repository: zenodo.org
Language: en

Abstract

The origin of magnetic fields at the surface of white dwarfs has been the object of many investigations, resulting in the current picture of competing promising scenarios: fossil fields, crystallization dynamos, and merger products. Constraining the formation scenarios of magnetic fields in white dwarfs is key for a better understanding of low- and intermediate-mass stellar evolution, as magnetic fields are believed to play a critical role in shaping the internal structure and dynamics of stars throughout their evolution.We revisit the fossil field scenario, following two recent observational hints: strong magnetic fields being detected in the core of progenitor red giants with asteroseismology, and young white dwarfs between 0.5-1 solar masses lacking surface magnetism compared to their older counterparts. Under the assumption that internal fields of red giants can remain trapped in the core along the late evolutionary stages, this discrepancy hints at a delay in buried magnetic fields reaching the stellar surface of white dwarfs.We aim at constraining the time needed for stable magnetic fields to emerge at the surface of white dwarfs from given initial configurations in the cores of red giants. For this purpose, we numerically solve the magnetic diffusion equation from initial fields using a varying magnetic diffusivity calculated from MESA models for white dwarfs along their cooling sequence. Key initial parameters we explore are: the depth below which the magnetic field is confined, the central field strength, and the field geometry.This way, we provide a distribution of possible fossil field strengths at the surface of white dwarfs as a function of their age, from asteroseismology constraints and hypotheses on the red giant branch. Additionally, we compare this distribution with existing observations of white dwarf surface fields. Lastly, we give an insight into the possible connection of magnetic fields between white dwarfs and their red giant progenitors.

Full text

in collaboration with Lisa Bugnet Magneto-Archeology of White Dwarfs Revisiting fossil field scenario Lukas Einramhof 1 Lukas Einramhof, TASC9/KASC16 Why are magnetic fields important? 2 Observations CORE ROTATION RATE [μHz] Standard stellar evolution models SURFACE GRAVITY (log) [cgs] Main-sequence stars Red giants Subgiants EVOLUTION OF INTERMEDIATE-MASS STARS Adapted from Aerts+ (2019) e.g. Deheuvels+ (2014, 2015) Moraveji+ (2016) Van Reeth+ (2016) Di Mauro+ (2016) Triana+ (2017) Remember Moyano’s talk yesterday! Lukas Einramhof, TASC9/KASC16 Why are magnetic fields important? 2 Observations CORE ROTATION RATE [μHz] Standard stellar evolution models SURFACE GRAVITY (log) [cgs] Main-sequence stars Red giants Subgiants EVOLUTION OF INTERMEDIATE-MASS STARS Adapted from Aerts+ (2019) e.g. Deheuvels+ (2014, 2015) Moraveji+ (2016) Van Reeth+ (2016) Di Mauro+ (2016) Triana+ (2017) e.g. Spruit+ (2002), Eggenberger+ (2005, 2019) Fuller+ (2019), Takahashi+ (2021), Moyano+ (2023) Magnetic fields might provide the missing angular momentum transport Remember Moyano’s talk yesterday! Lukas Einramhof, TASC9/KASC16 Can we connect magnetic fields across evolution? 3 Red Giant Lukas Einramhof, TASC9/KASC16 Can we connect magnetic fields across evolution? hydrogen burning shell e.g. Bugnet+ 2021 Li+ 2022 Deheuvels+ 2023 Bhattacharya+ 2024 Hatt+ 2024 Das, Einramhof, Bugnet 2024 Li+ 2022 Red Giant 3 Lukas Einramhof, TASC9/KASC16 Can we connect magnetic fields across evolution? ? hydrogen burning shell Li+ 2022 Distribution of magnetic WDs over cooling ages and masses by Bagnulo & Landstreet (2022) white dwarf mass Cooling age [Gyr] Red Giant White dwarf e.g. Bugnet+ 2021 Li+ 2022 Deheuvels+ 2023 Bhattacharya+ 2024 Hatt+ 2024 Das, Einramhof, Bugnet 2024 3 Lukas Einramhof, TASC9/KASC16 Can we connect magnetic fields across evolution? ? hydrogen burning shell Li+ 2022 Distribution of magnetic WDs over cooling ages and masses by Bagnulo & Landstreet (2022) white dwarf mass Cooling age [Gyr] We assume only single star evolution Binary mergers e.g. Pakmor+ (2024) Red Giant White dwarf e.g. Bugnet+ 2021 Li+ 2022 Deheuvels+ 2023 Bhattacharya+ 2024 Hatt+ 2024 Das, Einramhof, Bugnet 2024 3 Lukas Einramhof, TASC9/KASC16 Can we connect magnetic fields across evolution? ? hydrogen burning shell Li+ 2022 Distribution of magnetic WDs over cooling ages and masses by Bagnulo & Landstreet (2022) white dwarf mass Cooling age [Gyr] We assume only single star evolution Crystallization dynamo e.g. Isern+ (2017) Red Giant White dwarf e.g. Bugnet+ 2021 Li+ 2022 Deheuvels+ 2023 Bhattacharya+ 2024 Hatt+ 2024 Das, Einramhof, Bugnet 2024 3 Lukas Einramhof, TASC9/KASC16 Can we connect magnetic fields across evolution? ? hydrogen burning shell Li+ 2022 Distribution of magnetic WDs over cooling ages and masses by Bagnulo & Landstreet (2022) white dwarf mass Cooling age [Gyr] We assume only single star evolution Fossil Fields e.g. Camisassa+ (2024) Red Giant White dwarf e.g. Bugnet+ 2021 Li+ 2022 Deheuvels+ 2023 Bhattacharya+ 2024 Hatt+ 2024 Das, Einramhof, Bugnet 2024 3 Lukas Einramhof, TASC9/KASC16 Calculating the diffusivity Cooling age M = 1.5M , Z=0.02 ⊙ Increases toward the surface (Einramhof & Bugnet in prep) Decreases with age 5 Lukas Einramhof, TASC9/KASC16 Finding an initial Field Geometry Mass evolution of the radiative interior 6 (Einramhof & Bugnet in prep) RGB WDMS M = 1.5M , Z=0.02 ⊙ Lukas Einramhof, TASC9/KASC16 Finding an initial Field Geometry Compression of the magnetic field 7 logT logL M = 1.5M , Z=0.02 ⊙ Realistic profiles from e.g. Broderick & Narayan (2008), Duez & Mathis (2010) Lukas Einramhof, TASC9/KASC16 Finding an initial Field Geometry Compression of the magnetic field Br(Rhburn) = 100kG Constraints on the field strength at the hydrogen burning shell from observations Distribution of the field strength vs mass in Hatt+ (2024) B increases logT logL M = 1.5M , Z=0.02 ⊙ 7 Lukas Einramhof, TASC9/KASC16 Finding an initial Field Geometry Compression of the magnetic field Radiative core during RGB Assuming that the field does not diffuse between RG and WD logT logL M = 1.5M , Z=0.02 ⊙ 7 Lukas Einramhof, TASC9/KASC16 Magnetic relaxation in white dwarfs 8 (Einramhof & Bugnet in prep) logT logL M = 1.5M , Z=0.02 ⊙ Lukas Einramhof, TASC9/KASC16 (Einramhof & Bugnet in prep) logT logL M = 1.5M , Z=0.02 ⊙ The steep part of the field smoothes out fast Magnetic relaxation in white dwarfs 8 Lukas Einramhof, TASC9/KASC16 (Einramhof & Bugnet in prep) logT logL M = 1.5M , Z=0.02 ⊙ The steep part of the field smoothes out fast It takes time until the field becomes detectable at the surface Magnetic relaxation in white dwarfs 8 Lukas Einramhof, TASC9/KASC16 (Einramhof & Bugnet in prep) logT logL M = 1.5M , Z=0.02 ⊙ The steep part of the field smoothes out fast It takes time until the field becomes detectable at the surface The main contributor to the surface field evolution is the mass ratio between the radiative core and the initial white dwarf Magnetic relaxation in white dwarfs 8 Lukas Einramhof, TASC9/KASC16 Detecting fossil fields on white dwarf surfaces 9 (Einramhof & Bugnet in prep) PRELIMINARY Undetectable Lukas Einramhof, TASC9/KASC16 Distribution of magnetic WDs over cooling ages and masses by Bagnulo & Landstreet (2022) Fossil fields can explain the distribution of magnetic fields of white dwarfs Detecting fossil fields on white dwarf surfaces Work in progress Models are currently running 10 Lukas Einramhof, TASC9/KASC16 Distribution of magnetic WDs over cooling ages and masses by Bagnulo & Landstreet (2022) Fossil fields can explain the distribution of magnetic fields of white dwarfs Initial depth of the field is the key parameter Detecting fossil fields on white dwarf surfaces Work in progress Models are currently running 10 Lukas Einramhof, TASC9/KASC16 Distribution of magnetic WDs over cooling ages and masses by Bagnulo & Landstreet (2022) Fossil fields can explain the distribution of magnetic fields of white dwarfs Initial depth of the field is the key parameter Test different scenarios of fossil magnetic field survival between RGB and WD to constrain initial depth in the white dwarf Testing different magnetic geometries Next steps Detecting fossil fields on white dwarf surfaces Work in progress Models are currently running 10 Lukas Einramhof, TASC9/KASC16 Thank you for your interest! I am looking forward to your questions!