Diffusion coefficients in the envelopes of white dwarfs

Heinonen, R. A.; Saumon, D.; Daligault, J.; Starrett, C. E.; Baalrud, S. D.; Fontaine, G.

doi:10.3847/1538-4357/ab91ad

Title: Diffusion coefficients in the envelopes of white dwarfs

Journal Article · 07 June 2020 · The Astrophysical Journal (Online)

DOI: https://doi.org/10.3847/1538-4357/ab91ad · OSTI ID:1774434

Heinonen, R. A. ^[1];

^[2];

^[2]; Baalrud, S. D. ^[3]; Fontaine, G. ^[4]

Univ. of California San Diego, La Jolla, CA (United States)
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Univ. of Iowa, Iowa City, IA (United States)
Univ. of Montreal, QC (Canada)

The diffusion of elements is a key process in understanding the unusual surface composition of white dwarfs and their spectral evolution. The diffusion coefficients of Paquette et al. have been widely used to model diffusion in white dwarfs. In this work, we perform new calculations of the coefficients of interdiffusion and ionic thermal diffusion with (1) a more advanced model that uses a recent modification of the calculation of the collision integrals that is more suitable for the partially ionized, partially degenerate, and moderately coupled plasma and (2) classical molecular dynamics. The coefficients are evaluated for silicon and calcium in white dwarf envelopes of hydrogen and helium. A comparison of our results with Paquette et al. shows that the latter systematically underestimates the coefficient of interdiffusion yet provides reliable estimates for the relatively weakly coupled plasmas found in nearly all types of stars, as well as in white dwarfs with hydrogen envelopes. In white dwarfs with cool helium envelopes ($${T}_{\mathrm{eff}}$$ < 15,000 K), the difference grows to more than a factor of two. We also explored the effect of the ionization model used to determine the charges of the ions and found that it can be a substantial source of discrepancy between different calculations. Finally, we consider the relative diffusion timescales of Si and Ca in the context of the pollution of white dwarf photospheres by accreted planetesimals and find factor of ≳3 differences between calculations based on Paquette et al. and our model.

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Research Organization:: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Sponsoring Organization:: USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE National Nuclear Security Administration (NNSA)

Grant/Contract Number:: 89233218CNA000001

OSTI ID:: 1774434

Report Number(s):: LA-UR-20-21671

Journal Information:: The Astrophysical Journal (Online), Journal Name: The Astrophysical Journal (Online) Journal Issue: 1 Vol. 896; ISSN 1538-4357

Publisher:: Institute of Physics (IOP)Copyright Statement

Country of Publication:: United States

Language:: English

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