arXiv subjects

Praneet Pathak

Publications and source records attributed to Praneet Pathak.

3 recordsLinked to original sources

arXiv2026

Wave-Driven Mixing Enhanced by Rotation in Red Giant Branch Stars

Stars like our Sun expand as they exhaust their core hydrogen fuel, becoming red giants that eventually reach sizes up to 100 times their original radius. These giants have long presented a puzzle: they show systematic changes in their surface chemical composition that can only be explained by the transport of material from their nuclear-burning interior to their surface. The challenge is that this transport must somehow cross a stable layer that acts as a barrier between the star's outer convective envelope and its nuclear-burning interior. The convective motions in the envelope create internal waves that propagate through this barrier layer, but on their own these waves produce very little material transport. Here we show through high-resolution three-dimensional hydrodynamical simulations that stellar rotation dramatically amplifies how effectively these waves can mix material across this barrier. We find that the mixing rates can exceed those in non-rotating stars by over 100 times, increasing with faster rotation rates. This enhanced mixing provides a natural explanation for the observed chemical signatures in typical red giants. The amplification of wave-driven mixing by rotation may have implications beyond red giants to other types of stars.

astro-ph.SR↗

arXiv2025

3D hydrodynamic simulations of massive main-sequence stars -- IV. Internal gravity waves matter for SLF variability

The power spectrum of light curves from satellites like CoRoT and TESS of massive main-sequence stars show stochastic low-frequency (SLF) variability. To investigate the origin of this phenomenon, we conducted high-resolution 3D hydrodynamic \texttt{PPMstar} simulations of a non-rotating \unit{25}{\Msun} zero-age main sequence star, modeling 95\% of the stellar structure with both a core and a thin outer envelope convection zone. The outer envelope convection zone was implemented through modification of the opacity model, shifting the Fe opacity bump inward and enhancing its amplitude for computational feasibility. The luminosity power spectrum from our primary simulation (M424) exhibits qualitative and quantitative characteristics similar to observed SLF variability, with a $\approx2$-dex difference between high- and low-frequency power. The spectrum displays distinct features attributable to internal gravity wave (IGW) eigenmodes. To isolate the contributions of different stellar regions, we performed numerical experiments with suppressed core convection, envelope convection and envelope-only configurations. The comparative analysis demonstrates that outer envelope convection alone produces significantly less low-frequency power than the full-star configuration. In our simulations the outer envelope convection zone excites at its inner boundary a rich IGW eigenmode spectrum in the layer just below. In an otherwise identical simulation where the core convection is not driven by heating, the SLF spectrum is remarkably similar and the integrated power is reduced by only 10\%, suggesting that the envelope convection is the dominant contributor to SLF power spectrum. The IGW spectral characteristics depend on the complete stellar stratification, demonstrating that interior structure could influence observable surface variability.

astro-ph.SR↗

arXiv2024

Quantifying systematic uncertainties in white dwarf cooling age determinations

Cooling ages of white dwarfs are routinely determined by mapping effective temperatures and masses to ages using evolutionary models. Typically, the reported uncertainties on cooling ages only consider the error propagation of the uncertainties on the spectroscopically or photometrically determined $T_{\rm eff}$ and mass. However, cooling models are themselves uncertain, given their dependence on many poorly constrained inputs. This paper estimates these systematic model uncertainties. We use MESA to generate cooling sequences of $0.5-1.0 M_{\odot}$ hydrogen-atmosphere white dwarfs with carbon-oxygen cores under different assumptions regarding the chemical stratification of their core, the thickness of their helium envelope, their hydrogen content, and the conductive opacities employed in the calculations. The parameter space explored is constrained by the range of values predicted by a variety of stellar evolution models and inferred from asteroseismological studies. For a $0.6 M_{\odot}$ white dwarf, we find an uncertainty of 0.03 Gyr at 10,000 K (corresponding to a 5% relative uncertainty) and 0.8 Gyr at 4000 K (9%). This uncertainty is significant, as it is comparable to the age uncertainty obtained by propagating the measurement errors on $T_{\rm eff}$ and mass for a typical white dwarf. We also separately consider the potential impact of $^{22}$Ne shell distillation, which plausibly leads to an additional uncertainty of $\sim 1$ Gyr for crystallized white dwarfs. We provide a table of our simulation results that can be used to evaluate the systematic model uncertainty based on a white dwarf's $T_{\rm eff}$ and mass. We encourage its use in all future studies where white dwarf cooling ages are measured.

astro-ph.SR↗