The effect of the streaming instability on protoplanetary disc emission at millimetre wavelengths

In this paper, we investigate whether overdensity formation via streaming instability is consistent with recent multiwavelength Atacama Large Millimeter Array (ALMA) observations in the Lupus star-forming region. We simulate the local action of streaming instability in 2D using the code athena, and examine the radiative properties at mm wavelengths of the resulting clumpy dust distribution by focusing on two observable quantities: the optically thick fraction ff (in ALMA band 6) and the spectral index alpha (in bands 3-7). By comparing the simulated distribution in the ff-alpha plane before and after the action of streaming instability, we observe that clump formation causes ff to drop, because of the suppression of emission from grains that end up in optically thick clumps. alpha, instead, can either increase or decline after the action of streaming instability; we use a simple toy model to demonstrate that this behaviour depends on the sizes of the grains whose emission is suppressed by being incorporated in optically thick clumps. In particular, the sign of evolution of alpha depends on whether grains near the opacity maximum at a few tenths of a mm end up in clumps. By comparing the simulation distributions before/after clump formation to the data distribution, we note that the action of streaming instability drives simulations towards the area of the plane where the data are located. We furthermore demonstrate that this behaviour is replicated in integrated disc models provided that the instability is operative over a region of the disc that contributes significantly to the total mm flux.

Automated summary

This paper investigates whether overdensity formation via streaming instability is consistent with recent observations in the Lupus star-forming region using simulations and comparisons with data. The study finds that clump formation leads to a decrease in the optically thick fraction ff and that the spectral index alpha may increase or decrease after the action of streaming instability. This behavior is observed to drive simulations towards the region where the data are located, especially when the instability is operative over a region of the disc that contributes significantly to the total mm flux.