Abstract
ABSTRACT 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 α (in bands 3–7). By comparing the simulated distribution in the ff–α 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. α, 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 α 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.
Highlights
According to core accretion theory, planets form through dust growth from initial μm-sized grains up to the size of a planet (Safronov & Zvjagina 1969)
Our method can be described as a four-step process: (1) We perform hydrodynamics simulations of systems where streaming instability takes place; (2) we define a physical disc model, which allows us to translate the simulation results to physical systems; (3) we compute the radiative properties of these systems; and (4) we compare two observable properties to those derived from Atacama Large Millimeter Array (ALMA) observations by Tazzari et al (2020a,b)
In our models, the streaming instability was successful in improving the match to the properties of observed discs on account of the changes in optical depth and spectral index effected when the maximum grain size is close to the opacity resonance
Summary
According to core accretion theory, planets form through dust growth from initial μm-sized grains up to the size of a planet (Safronov & Zvjagina 1969). The planetesimal formation stage represents a critical step in the growth process because the formation of km-sized objects is hampered by the so-called ‘metre-sized barrier’ or ‘radial drift barrier’ Unless other mechanisms interfere with the inward drift, the ∼cm– m-sized grains are soon lost and they are no longer available to form planetesimals
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