Razor-thin dust layers in protoplanetary disks: Limits on the vertical shear instability

TL;DR

This study investigates whether the vertical shear instability (VSI) can explain the observed thin dust layers in protoplanetary disks, finding that VSI likely cannot operate in such thin layers due to dust stirring effects.

Contribution

The paper demonstrates that VSI cannot sustain in geometrically thin dust layers, suggesting the need for reduced small dust grain populations to inhibit VSI in disks.

Findings

Large dust aggregates are stirred up by VSI, conflicting with observed thin layers.

Thin dust layers with large grains are hard to produce optically thick at millimeter wavelengths.

Reducing small dust grains by a factor of 10-100 can suppress VSI, aligning with observations.

Abstract

Context: Recent observations with the Atacama Large Millimeter Array (ALMA) have shown that the large dust aggregates observed at millimeter wavelengths settle to the midplane into a remarkably thin layer. Aims: We intend to find out if the geometric thinness of these layers is evidence against the vertical shear instability (VSI) operating in these disks. Methods: We performed hydrodynamic simulations of a protoplanetary disk with a locally isothermal equation of state, and let the VSI fully develop. We sprinkled dust particles and followed their motion as they got stirred up by the VSI. We determined for which grain size the layer becomes geometrically thin enough to be consistent with ALMA observations. We then verified if, with these grain sizes, it is still possible to generate a moderately optically thick layer at millimeter wavelengths, as observations appear to indicate. Results: We found that even very large dust aggregates with Stokes numbers close to unity get stirred up to relatively large heights above the midplane by the VSI, which is in conflict with the observed geometric thinness. For grains so large that the Stokes number exceeds unity, the layer can be made to remain thin, but we show that it is hard to make dust layers optically thick at ALMA wavelengths (e.g., tau(1.3mm)>=1) with such large dust aggregates. Conclusions: We conclude that protoplanetary disks with geometrically thin midplane dust layers cannot be VSI unstable, at least not down to the disk midplane. Explanations for the inhibition of the VSI include a reduced dust-to-gas ratio of the small dust grains that are responsible for the radiative cooling of the disk. A reduction of small grains by a factor of between 10 and 100 is sufficient to quench the VSI. Such a reduction is plausible in dust growth models, and still consistent with observations at optical and infrared wavelengths.