Snow-lines as probes of turbulent diffusion in protoplanetary discs

TL;DR

This paper proposes using the sharp chemical discontinuities at snow-lines in protoplanetary discs as a method to measure turbulent diffusion levels, specifically the radial Schmidt number, through ALMA observations.

Contribution

It introduces a novel approach to directly measure the radial Schmidt number in protoplanetary discs using chemical gradients at snow-lines, linking turbulence properties to observable features.

Findings

Chemical gradients at snow-lines are sensitive to turbulent diffusion levels.

ALMA can constrain the Schmidt number by observing species like N$_2$H$^+$.

Different turbulence mechanisms predict different Schmidt numbers.

Abstract

Sharp chemical discontinuities can occur in protoplanetary discs, particularly at `snow-lines' where a gas-phase species freezes out to form ice grains. Such sharp discontinuities will diffuse out due to the turbulence suspected to drive angular momentum transport in accretion discs. We demonstrate that the concentration gradient - in the vicinity of the snow-line - of a species present outside a snow-line but destroyed inside is strongly sensitive to the level of turbulent diffusion (provided the chemical and transport time-scales are decoupled) and provides a direct measurement of the radial `Schmidt number' (the ratio of the angular momentum transport to radial turbulent diffusion). Taking as an example the tracer species N$_2$H$^+$, which is expected to be destroyed inside the CO snow-line (as recently observed in TW Hya) we show that ALMA observations possess significant angular resolution to constrain the Schmidt number. Since different turbulent driving mechanisms predict different Schmidt numbers, a direct measurement of the Schmidt number in accretion discs would allow inferences about the nature of the turbulence to be made.