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Justin M. Brown is the first author, with Pascale Garaud and Stephan Stellmach, of a study of chemical mixing by fingering convection in stars. The 2013 paper developed a physically motivated transport model, tested it with three-dimensional direct numerical simulation, and supplied a method for incorporating the results into one-dimensional stellar calculations.^[ref]

Fingering convection is a form of double-diffusive convection: heat and composition affect buoyancy but spread at different rates. A background that resists ordinary overturning can therefore admit smaller-scale mixing when the compositional arrangement is destabilizing. The paper also investigated when layered structures could form, rather than assuming that every unstable region develops a staircase.^[ref]

A simple scaling comparison helps explain why unequal diffusion matters. In time $t$, a diffusing disturbance spreads a distance proportional to $\sqrt{Dt}$. If the thermal diffusivity is one hundred times the diffusion coefficient for composition, thermal changes spread roughly ten times farther in the same time. The calculation is illustrative, not a measured universal ratio for stars. Brown's coauthored work connects such local processes to the much larger problem of predicting chemical transport through a stellar interior.

Pascale Garaud is an applied mathematician and astrophysicist whose work connects fluid dynamics, turbulence and the evolution of stars. Her institutional biography identifies research on stellar flows, magnetohydrodynamics and double-diffusive convection, using both analytical and computational methods.^[ref]

With collaborators, she developed a model for mixing by fingering convection and tested it against three-dimensional direct numerical simulations. A stellar layer can resist ordinary overturning while still mixing because heat and composition diffuse at different rates. Predicting the resulting transport matters when a one-dimensional stellar calculation cannot resolve the individual turbulent motions.^[ref]

Her lecture notes explain how competing thermal and compositional density effects lead to double-diffusive convection. The connection is scientifically useful: a process familiar from salt water becomes relevant to material redistribution inside stars, with very different diffusivity ratios.^[ref]

Portrait: Simons Foundation speaker profile, identified portrait of Pascale Garaud.