Understanding the formation of massive stars is crucial due to their pivotal roles in galaxy evolution and the formation of stars and planets. Observations and theoretical studies (Girart et al. 2018, Yamamuro et al. 2023) suggest that dust grains in a massive protostellar disk grow larger than those in the interstellar medium. This grain growth can skew estimates of fundamental disk properties, such as mass and temperature, impacting our understanding of star formation.
To investigate dust coagulation, we first developed a steady-state, axisymmetric analytical disk model for a massive protostar. The results revealed that dust grains could grow under typical gas and temperature conditions in the disk. Many massive protostellar disks, however, exhibit spiral arms. To explore dust coagulation in such non-axisymmetric and steady-state environments, we conducted two-dimensional radiation hydrodynamics simulations. These simulations showed that, even in disks with spiral arms, dust growth occurs symmetrically in the azimuthal direction. This symmetry results from dust collision timescales being longer than Keplerian timescales, leading to averaged growth across azimuthal variations.
Finally, we compared our steady-state axisymmetric disk model with ALMA Band 6 observations of the GGD27-MM1 disk (at the base of the HH80-81 jet). We examined which disk parameters and dust coagulation mechanisms could better match the observed continuum emission. We found that the mm-continuum emission can be explained if the stellar luminosity is about five times higher than previously estimated from infrared observations, which becomes plausible when we take the flashlight effect into account.