Current challenges in the physics of white dwarf stars

Saumon, D.; Blouin, Simon; Tremblay, Pier-Emmanuel

doi:10.1016/j.physrep.2022.09.001

Cited by 40 publications

(19 citation statements)

References 347 publications

(571 reference statements)

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“…Initially, the field is trapped in the convection zone, deep inside the core. Only later on in the evolution of the star, when the convection zone has expanded outward closer to the surface, is the magnetic diffusion time (e.g., Cumming 2002;Saumon et al 2022) short enough for the field to break out to the surface and be observed. The extent of the convection zone during the efficient convection phase depends on the initial carbonoxygen abundance profile, which is set during stellar evolution and depends on the star's mass (with the convective mantle of massive WDs being closer to the surface, e.g., Bauer 2023).…”

Section: Discussionmentioning

confidence: 99%

A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

Fuentes,

Castro-Tapia,

Cumming

2024

ApJL

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The origin of large magnetic fields (≳106 G) in isolated white dwarfs is not clear. One possible explanation is that crystallization of the star’s core drives compositional convection, which when combined with the star’s rotation, can drive a dynamo. However, whether convection is efficient enough to explain the large intensity of the observed magnetic fields is still under debate. Recent work has shown that convection in cooling white dwarfs spans two regimes: efficient convection at the onset of crystallization, and thermohaline convection during most of the star’s cooling history. Here, we calculate the properties of crystallization-driven convection for cooling models of several white dwarfs of different masses. We combine mixing-length theory with scalings from magnetorotational convection to estimate the typical magnitude of the convective velocity and induced magnetic field for both scenarios. In the thermohaline regime, we find velocities ∼10−6–10−5 cm s−1, with fields restricted to ≲ 100 G. However, when convection is efficient, the flow velocity can reach magnitudes of ∼102–103 cm s−1, with fields of ∼106–108 G, independent of the star’s rotation rate. Thus, dynamos driven at the onset of crystallization could explain the large intensity magnetic fields measured for single white dwarfs.

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Section: Discussionmentioning

confidence: 99%

A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

Fuentes,

Castro-Tapia,

Cumming

2024

ApJL

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“…Prior to the TP-AGB phase, M core is equal to the mass of the hydrogen-exhausted core. In order to include the possibility that white dwarfs may retain a thin hydrogen atmosphere on top of the He intershell and CO core (Saumon et al 2022), we take…”

Section: Evolutionary Propertiesmentioning

confidence: 99%

The Role of the Third Dredge-up and Mass Loss in Shaping the Initial–Final Mass Relation of White Dwarfs

Addari,

Marigo,

Bressan

et al. 2024

ApJ

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The initial–final mass relation (IFMR) plays a crucial role in understanding stellar structure and evolution by linking a star’s initial mass to the mass of the resulting white dwarf. This study explores the IFMR in the initial mass range 0.8 ≤ M ini/M ⊙ ≤ 4 using full PARSEC evolutionary calculations supplemented with COLIBRI computations to complete the ejection of the envelope and obtain the final core mass. Recent works have shown that the supposed monotonicity of the IFMR is interrupted by a kink in the initial mass range M ini ≈ 1.65–2.10 M ⊙, due to the interaction between recurrent dredge-up episodes and stellar winds in carbon stars evolving on the thermally pulsing asymptotic giant branch phase. To reproduce the IFMR nonmonotonic behavior we investigate the role of convective overshooting efficiency applied to the base of the convective envelope (f env) and to the borders of the pulse-driven convective zone (f pdcz), as well as its interplay with mass loss. We compare our models to observational data and find that f env must vary with initial mass in order to accurately reproduce the IFMR’s observed kink and slopes. We find some degeneracy between the overshooting parameters when only the IFMR information is used. Nonetheless, this analysis provides valuable insights into the internal mixing processes during the TP-AGB phase.

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“…refs. [47,48] and section 3.5 of ref. [49]), dP (r) dr = − GM (r)ρ(r) r 2 and dM (r) dr = 4πr 2 ρ(r), (…”

Section: Energy-loss Ratesmentioning

confidence: 99%

Stellar limits on scalars from electron-nucleus bremsstrahlung

Bottaro,

Caputo,

Raffelt

et al. 2023

J. Cosmol. Astropart. Phys.

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We revisit stellar energy-loss bounds on the Yukawa couplings g B,L of baryophilic and leptophilic scalars ϕ. The white-dwarf luminosity function yields g B ≲ 7 × 10-13 and g L ≲ 4 × 10-16, based on bremsstrahlung from 12C and 16O collisions with electrons. In models with a Higgs portal, this also implies a bound on the scalar-Higgs mixing angle sinθ ≲ 2 × 10-10. Our new bounds apply for mϕ ≲ 1 keV and are among the most restrictive ones, whereas for mϕ ≲ 0.5 eV, long-range force measurements dominate. Besides a detailed calculation of the bremsstrahlung rate for degenerate and semi-relativistic electrons, we prove with a simple argument that non-relativistic bremsstrahlung by the heavy partner is suppressed relative to that by the light one by their squared-mass ratio. This large reduction was overlooked in previous much stronger bounds on g B. In an appendix, we provide fitting formulas (few percent precision) for the bremsstrahlung emission of baryophilic and leptophilic scalars as well as axions for white-dwarf conditions, i.e., degenerate, semi-relativistic electrons and ion-ion correlations in the “liquid” phase.

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Current challenges in the physics of white dwarf stars

Cited by 40 publications

References 347 publications

A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

The Role of the Third Dredge-up and Mass Loss in Shaping the Initial–Final Mass Relation of White Dwarfs

Stellar limits on scalars from electron-nucleus bremsstrahlung

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