arXiv subjects

Laura J. A. Scott

Publications and source records attributed to Laura J. A. Scott.

4 recordsLinked to original sources

arXiv2025

New Level Resolved Ground and Excited State Pb III, IV, V & VI Photoionization Cross Sections for Heavy Metal Subdwarf Modeling

High abundances of various lead (Pb) species have been identified in the spectra of many Asymptotic Giant Branch (AGB) stars and O- and B-type subdwarfs (sdO/B). Additional atomic data relating to Pb, and in particular photoionization cross sections, are needed to allow a greater understanding of the origin of these observed Pb abundances, and hence discern the evolutionary pathway of these stars. We have calculated level-resolved photoionization cross sections for Pb III, IV, V and VI. Four new target structures have been developed with the General Relativistic Atomic Structure Package (GRASP0), whose corresponding energy levels, Einstein A-coefficients and oscillator strengths have been found to be in good agreement with previous experimental and theoretical sources. The photoionization cross sections calculated using the Dirac Atomic R-matrix Codes (DARC) are available in TOPBASE format, and follow the trends expected for an isonuclear series. These new Pb data sets will now allow for the modelling of Pb abundances and line opacities under Non-Local Thermodynamic Equilibrium (non-LTE) conditions. Using the helium-rich hot subdwarf EC 22536-5304 as a test case, we show that there are noticeable differences in the Pb line profiles across the ultraviolet and optical wavelength regions under LTE and non-LTE conditions. There is both depletion and enrichment of individual Pb species. This highlights the importance of applying non-LTE conditions when modelling EC 22536-5304, as well as other O/B-type stars.

astro-ph.SR↗

arXiv2021

3D Simulations and MLT: II. Onsager's Ideal Turbulence

We simulate stellar convection at high Reynolds number (Re$\lesssim$7000) with causal time stepping but no explicit viscosity. We use the 3D Euler equations with shock capturing (Colella & Woodward 1984). Anomalous dissipation of turbulent kinetic energy occurs as an emergent feature of advection ("Onsager damping"), caused by the moderate shocks which terminate the turbulent kinetic energy spectrum; see also (Perry 2021). In strongly stratified stellar convection the asymptotic limit for the global damping length of turbulent kinetic energy is $\ell_d \sim \langle u^3 \rangle /\langle ε\rangle$. This "dissipative anomaly" (Onsager 1949) fixes the value of the "mixing length parameter", $α= \ell_{\rm MLT}/H_P =\overline{\langleΓ_1\rangle}$, which is $\sim\, 5/3$ for complete ionization. The estimate is numerically robust, agrees to within 10% with estimates from stellar evolution with constant $α$. For weak stratification $\ell_d$ shrinks to the depth of a thin convective region. Our flows are filamentary, produce surfaces of separation at boundary layers, resolve the energy-containing eddies, and develop a turbulent cascade down to the grid scale which agrees with the $4096^3$ direct numerical simulation of Kaneda (2003). The cascade converges quickly, and satisfies a power-law velocity spectrum similar to Kolmogorov (1941). Our flows exhibit intermittency, anisotropy, and interactions between coherent structures, features missing from K41 theory. We derive a dissipation rate from Reynolds stresses which agrees with (i) our flows, (ii) experiment (Warhaft 2002), and (iii) high Re simulations of the Navier-Stokes equations (Iyer, et al. 2018).

astro-ph.SR↗

arXiv2020

Relative Importance of Convective Uncertainties in Massive Stars

In this work, we investigate the impact of uncertainties due to convective boundary mixing (CBM), commonly called `overshoot', namely the boundary location and the amount of mixing at the convective boundary, on stellar structure and evolution. For this we calculated two grids of stellar evolution models with the MESA code, each with the Ledoux and the Schwarzschild boundary criterion, and vary the amount of CBM. We calculate each grid with the initial masses $15$, $20$ and $25\,\rm{M}_\odot$. We present the stellar structure of the models during the hydrogen and helium burning phases. In the latter, we examine the impact on the nucleosynthesis. We find a broadening of the main-sequence with more CBM, which is more in agreement with observations. Furthermore during the core hydrogen burning phase there is a convergence of the convective boundary location due to CBM. The uncertainties of the intermediate convective zone remove this convergence. The behaviour of this convective zone strongly affects the surface evolution of the model, i.e. how fast it evolves red-wards. The amount of CBM impacts the size of the convective cores and the nucleosynthesis, e.g. the $^{12}$C to $^{16}$O ratio and the weak s-process. Lastly, we determine the uncertainty that the range of parameter values investigated introduce and we find differences of up to $70\%$ for the core masses and the total mass of the star.

astro-ph.SR↗

arXiv2018

Relative Importance of Convective Uncertainties

Convection plays a key role in the evolution of stars due to energy transport and mixing of composition. Despite its importance, this process is still not well understood. One longstanding conundrum in all 1D stellar evolution codes is the treatment of convective boundaries. In this study we compare two convective uncertainties, the boundary location (Ledoux versus Schwarzschild) and the amount of extra mixing, and their impact on the early evolution of massive stars. With increasing convective boundary mixing (CBM), we find a convergence of the two different boundary locations, a decreasing blue to red super giant ratio and a reduced importance of semiconvection.

astro-ph.SR↗