The Impact of Silicate Grain Coagulation on Millimeter Emission from Massive Protostellar Disks

TL;DR

This study models how silicate grain growth affects millimeter emission in massive protostellar disks, revealing that larger grains cause scattering dimming and constraining grain fragmentation velocities through ALMA observations.

Contribution

It provides a self-consistent radiative transfer model including dust scattering and temperature gradients, linking grain size evolution to observable millimeter emission in massive protostellar disks.

Findings

Grain growth beyond the observing wavelength dims disk emission by 20-30%.

Fragmentation velocity of silicate grains is about 15 m/s, lower than in low-mass star disks.

Two scenarios proposed to explain bright inner-disk emission: limited grain growth or higher stellar luminosity.

Abstract

Hot accretion disks around massive protostars provide a unique opportunity to study ice-free silicate grains that cannot be investigated in protoplanetary disks. We conduct a self-consistent investigation into grain-size evolution and its impact on (sub)millimeter-wave emission from massive protostellar disks. Our radiative transfer modeling accounts for dust self-scattering and includes vertical temperature gradients in the disk structure. The results show that once silicate grains grow to sizes exceeding the observing wavelength, enhanced scattering dims the disk emission by 20\%--30\% relative to the blackbody emission expected at the disk surface temperature. By comparing our model with Atacama Large Millimeter/submillimeter Array 1.14 mm observations of the disk around the massive protostar GGD27-MM1, we constrain the threshold velocity for collisional fragmentation of silicate grains to approximately 15 m s-1. This fragmentation velocity is lower than the typical maximum collisional velocities in protoplanetary disks around low-mass stars, suggesting that collisional coagulation alone is insufficient for silicate dust to form rocky planetesimals in such environments. Furthermore, our analysis identifies two potential scenarios to better reproduce the bright inner-disk emission of GGD27-MM1. One possibility is that the grain growth is limited to 160 mum by another growth barrier (e.g., collisional bouncing), reducing scattering dimming. Alternatively, the stellar luminosity may be as much as five times higher than current estimates, compensating for the reduced brightness. Future multiwavelength observations, particularly at shorter submillimeter wavelengths, will be crucial to distinguish between these scenarios and further constrain silicate grain coagulation processes in massive protostellar disks.