Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?

Camisassa, María E.; Raddi, R.; Althaus, L. G.; Isern, J.; Rebassa‐Mansergas, A.; Torres, Santiago; Córsico, A. H.; Korre, Lydia

doi:10.1093/mnrasl/slac078

Cited by 7 publications

(3 citation statements)

References 57 publications

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“…These groups cite Isern et al (2017) and/or Ginzburg et al (2022) as a possible explanation for this. In a novel use of the findings of Isern et al (2017), Camisassa et al (2022) infer the existence of O/Ne cores based on the existence of magnetic fields in white dwarfs that are too hot to be crystallizing if they have C/O cores but are expected to be crystallizing if they are O/Ne.…”

Section: Magnetic Field Generationmentioning

confidence: 99%

Fluid Mixing during Phase Separation in Crystallizing White Dwarfs

Montgomery,

Dunlap

2024

ApJ

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Accurate models of cooling white dwarfs must treat the energy released as their cores crystallize. This phase transition slows the cooling by releasing latent heat and also gravitational energy, which results from phase separation: liquid C is released from the solid C/O core, driving an outward carbon flux. The Gaia color–magnitude diagram provides striking confirmation of this theory by revealing a mass-dependent overdensity of white dwarfs, indicating slowed cooling at the expected location. However, the observed overdensity is enhanced relative to the models. Additionally, it is associated with increased magnetism, suggesting a link between crystallization and magnetic field generation. Recent works aimed at explaining an enhanced cooling delay and magnetic field generation employ a uniform mixing prescription that assumes large-scale turbulent motions; we show here that these calculations are not self-consistent. We also show that thermohaline mixing is most likely efficient enough to provide the required chemical redistribution during C/O phase separation, and that the resulting velocities and mixing lengths are much smaller than previous estimates. These reduced fluid motions cannot generate measurable magnetic fields, suggesting any link with crystallization needs to invoke a separate mechanism. Finally, this mixing alters the chemical profiles, which in turn affects the frequencies of the pulsation modes.

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Section: Magnetic Field Generationmentioning

confidence: 99%

Fluid Mixing during Phase Separation in Crystallizing White Dwarfs

Montgomery,

Dunlap

2024

ApJ

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“…Among these studies, the most promising solutions invoke 22 Ne distillation (Blouin et al 2021) and 22 Ne sedimentation processes (Camisassa et al 2021) occurring in ultramassive white dwarfs with CO cores. All attempts to reproduce the necessary cooling delay in ultramassive white dwarfs with ONe cores have failed, and this is particularly interesting because, to date, there is no clear evidence that ultramassive white dwarfs harbor CO or ONe cores (see e.g., Camisassa et al 2022b, for a thorough discussion).…”

Section: Introductionmentioning

confidence: 99%

Exsolution process in white dwarf stars

Camisassa,

Baiko,

Torres

et al. 2024

A&A

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White dwarf stars are considered to be suitable cosmic laboratories for studying the physics of dense plasma. Furthermore, the use of white dwarf stars as cosmic clocks to date stellar populations and main sequence companions demands an appropriate understanding of the physics of white dwarfs in order to provide precise ages for these stars. We aim to study exsolution in the interior of white dwarf stars, a process in which a crystallized ionic binary mixture separates into two solid solutions with different fractions of the constituents. Depending on the composition of the parent solid mixture, this process can release or absorb heat, thus leading to a delay or a speed-up of white dwarf cooling. Relying on accurate phase diagrams for exsolution, we modeled this process in hydrogen(H)-rich white dwarfs with both carbon--oxygen (CO) and oxygen--neon (ONe) core composition, with masses ranging from 0.53 to 1.29 $M_ and from 1.10 to 1.29 $M_ respectively. Exsolution is a slow process that takes place at low luminosities ($ -2.75$) and effective temperatures eff 18\,000 K$) in white dwarfs. We find that exsolution begins at brighter luminosities in CO than in ONe white dwarfs of the same mass. Massive white dwarfs undergo exsolution at brighter luminosities than their lower-mass counterparts. The net effect of exsolution on white dwarf cooling times depends on the stellar mass and the exact chemical profile. For standard core chemical profiles and preferred assumptions regarding miscibility gap microphysics, the cooling delay can be as large as $ 0.35$ Gyr at $ -5$. We neglect any chemical redistribution possibly associated with this process, which could lead to a further cooling delay. Although the chemical redistribution is known to accompany exsolution in binary solid mixtures on Earth, given the solid state of the matter, it is hard to model in a reliable way, and its effect may be postponed until very low luminosities. Exsolution has a marginal effect on white dwarf cooling times and, accordingly, we find no white dwarf branches associated with it on the Gaia color--magnitude diagram. However, exsolution in massive white dwarfs can alter the faint end of the white dwarf luminosity function, thus impacting white dwarf cosmochronology.

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“…Interest in compositionally driven convection in white dwarfs has also been recently revived with the suggestion of Isern et al (2017) that it leads to a magnetic dynamo in crystallizing white dwarfs (Schreiber et al 2021a(Schreiber et al , 2021b(Schreiber et al , 2022Belloni et al 2021;Camisassa et al 2022;Ginzburg et al 2022). Using the scaling of Christensen et al (2009) for a saturated dynamo, Isern et al (2017) found that fields up to ∼1 MG could be generated.…”

Section: Introductionmentioning

confidence: 99%

Heat Transport and Convective Velocities in Compositionally Driven Convection in Neutron Star and White Dwarf Interiors

Fuentes

Cumming

Castro-Tapia

et al. 2023

ApJ

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We investigate heat transport associated with compositionally driven convection driven by crystallization at the ocean–crust interface in accreting neutron stars, or growth of the solid core in cooling white dwarfs. We study the effect of thermal diffusion and rapid rotation on the convective heat transport, using both mixing length theory and numerical simulations of Boussinesq convection. We determine the heat flux, composition gradient, and Péclet number, Pe (the ratio of thermal diffusion time to convective turnover time) as a function of the composition flux. We find two regimes of convection with a rapid transition between them as the composition flux increases. At small Pe, the ratio between the heat flux and composition flux is independent of Pe, because the loss of heat from convecting fluid elements due to thermal diffusion is offset by the smaller composition gradient needed to overcome the reduced thermal buoyancy. At large Pe, the temperature gradient approaches the adiabatic gradient, saturating the heat flux. We discuss the implications for cooling of neutron stars and white dwarfs. Convection in neutron stars spans both regimes. We find rapid mixing of neutron star oceans, with a convective turnover time of the order of weeks to minutes depending on rotation. Except during the early stages of core crystallization, white dwarf convection is in the thermal-diffusion-dominated fingering regime. We find convective velocities much smaller than recent estimates for crystallization-driven dynamos. The small fraction of energy carried as kinetic energy calls into question the effectiveness of crystallization-driven dynamos as an explanation for observed magnetic fields in white dwarfs.

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Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?

Cited by 7 publications

References 57 publications

Fluid Mixing during Phase Separation in Crystallizing White Dwarfs

Fluid Mixing during Phase Separation in Crystallizing White Dwarfs

Exsolution process in white dwarf stars

Heat Transport and Convective Velocities in Compositionally Driven Convection in Neutron Star and White Dwarf Interiors

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