arXiv subjects

Valentin A. Skoutnev

Publications and source records attributed to Valentin A. Skoutnev.

6 recordsLinked to original sources

arXiv2026

Unifying Transport Models of Thermohaline Convection in Stars

Thermohaline convection is a standard chemical mixing process in stellar interiors, yet its mixing efficiency is not fully settled. Competing theories predict turbulent diffusion coefficients, $D_μ$, that can differ by orders of magnitude, leading to uncertainties in stellar models and interpretations of observations. This paper explores a potential resolution to existing discrepancies. We first complete the linear stability theory and identify two types of unstable modes: slow growing modes at large length scales and fast growing modes at small length scales. We then reevaluate $D_μ$ considering the full spectrum of unstable modes and find that it can self-consistently interpolate between previously proposed theoretical scalings across the instability parameter space. The question of thermohaline mixing efficiency in stars may be settled by future simulations that quantify the scale-dependent contributions of fast and slow modes to $D_μ$ and determine how the modes dominating the transport change across parameter space.

astro-ph.SR↗

arXiv2026

Inefficiency of chiral dynamos in protoneutron stars and the early universe

The chiral plasma instability (CPI) has been invoked as a possible mechanism for generating primordial magnetic fields in the universe and ultrastrong fields in neutron stars. We investigate chiral dynamos where the chirality imbalance is pumped by a source on a timescale $t_0$ and show that the CPI rate $γ$ is limited to $γ_0/(1+{\cal Q}^2)$, where ${\cal Q}= (γ_0 t_0)^{1/3}$ and $γ_0$ corresponds to models with instantaneously created chirality imbalance $(t_0=0)$. We then find that chiral flipping with rate $Γ_{\mathrm f}$ hinders the chiral dynamo if $Γ_{\mathrm f} >γ_0/(1+{\cal Q}^2)$ and completely suppresses it if $Γ_{\mathrm f} >γ_0/(1+{\cal Q}^{3/2})$. Realistic $t_0$ typically give ${\cal Q}\gg 1$, which makes the dynamo greatly vulnerable to suppression by chiral flipping. The suppression is strong in protoneutron stars and may be (barely) avoided near the electroweak transition in the early universe.

astro-ph.HE↗

arXiv2025

Magnetic Webs in Stellar Radiative Zones

Rotational evolution of stellar radiative zones is an old puzzle. We argue that angular momentum transport by turbulent processes induced by differential rotation is insufficient, and propose that a key role is played by ``magnetic webs." We define magnetic webs as stable magnetic configurations that enforce corotation of their coupled mass shells, and discuss their resistance to differential torques that occur in stars. Magnetic webs are naturally expected in parts of radiative zones that were formerly convective, retaining memory of extinguished dynamos. For instance, red giants with moderate masses $M\gtrsim 1.3M_\odot$ likely contain a magnetic web deposited on the main sequence during the retreat of the central convective zone. The web couples the helium core to the hydrogen envelope of the evolving red giant and thus reduces spin-up of the contracting core. The magnetic field and the resulting slower rotation of the core are both consistent with asteroseismic observations, as we illustrate with a stellar evolution model with mass $1.6M_\odot$. Evolved massive stars host more complicated patterns of convective zones that may leave behind many webs, transporting angular momentum towards the surface. Efficient web formation likely results in most massive stars dying with magnetized and slowly rotating cores.

astro-ph.SR↗

arXiv2025

Zones of Tayler Instability in Stars

The Tayler instability (TI) of toroidal magnetic fields is a candidate mechanism for driving turbulence, angular momentum (AM) transport, and dynamo action in stellar radiative zones. Recently \cite{Skoutnev_2024} revisited the linear stability analysis of a toroidal magnetic field in a rotating and stably stratified fluid. In this paper, we extend the analysis to include both thermal and compositional stratification, allowing for general application to stars. We formulate an analytical instability criterion for use as a ``toggle switch" in stellar evolution codes. It determines when and where in a star the TI develops with a canonical growth rate as assumed in existing prescriptions for AM transport based on Tayler-Spruit dynamo. We implement such a ``toggle switch" in the MESA stellar evolution code and map out the stability of each mode of the TI on a grid of stellar evolution models. In evolved lower mass stars, the TI becomes suppressed in the compositionally stratified layer around the hydrogen burning shell. In higher mass stars, the TI can be active throughout their radiative zones, but at different wavenumbers than previously expected.

astro-ph.SR↗

arXiv2024

Tayler Instability Revisited

Tayler instability of toroidal magnetic fields $B_ϕ$ is broadly invoked as a trigger for turbulence and angular momentum transport in stars. This paper presents a systematic revision of the linear stability analysis for a rotating, magnetized, and stably stratified star. For plausible configurations of $B_ϕ$, instability requires diffusive processes: viscosity, magnetic diffusivity, or thermal/compositional diffusion. Our results reveal a new physical picture, demonstrating how different diffusive effects independently trigger instability of two types of waves in the rotating star: magnetostrophic waves and inertial waves. It develops via overstability of the waves, whose growth rate sharply peaks at some characteristic wavenumbers. We determine instability conditions for each wave branch and find the characteristic wavenumbers. The results are qualitatively different for stars with magnetic Prandtl number $Pm\ll 1$ (e.g. the Sun) and $Pm\gg 1$ (e.g. protoneutron stars). The parameter dependence of unstable modes suggests a non-universal scaling of the possible Tayler-Spruit dynamo.

astro-ph.SR↗

arXiv2022

Critical Balance and Scaling of Strongly Stratified Turbulence at Low Prandtl Number

We extend the scaling relations of stably stratified turbulence from the geophysical regime of unity Prandtl number to the astrophysical regime of extremely small Prandtl number applicable to stably stratified regions of stars and gas giants. A transition to a new turbulent regime is found to occur when the Prandtl number drops below the inverse of the buoyancy Reynolds number, i.e. $PrRb<1$, which signals a shift of the dominant balance in the buoyancy equation. Application of critical balance arguments then derives new predictions for the anisotropic energy spectrum and dominant balance of the Boussinesq equations in the $PrRb\ll1$ regime. We find that all the standard scaling relations from the unity $Pr$ limit of stably stratified turbulence simply carry over if the Froude number, $Fr$, is replaced by a modified Froude number, $Fr_M\equiv Fr/(PrRb)^{1/4}$. The geophysical and astrophysical regimes are thus smoothly connected across the $PrRb=1$ transition. Applications to vertical transport in stellar radiative zones and modification to the instability criterion for the small-scale dynamo are discussed.

physics.flu-dyn↗