Importance of source structure on complex organics emission III. Effect of disks around massive protostars

TL;DR

This study investigates how disks around massive protostars influence methanol emission, finding that disks have limited impact on temperature and emission, and that low methanol emission in high-luminosity sources is likely due to HII regions.

Contribution

It demonstrates that disks around high-mass protostars do not significantly alter methanol emission, and explains the observed emission scatter through envelope-plus-disk models and HII region effects.

Findings

Disks do not significantly affect temperature structure or methanol emission in high-mass protostars.

Vertical temperature inversion occurs at radii < 50au in certain models.

Low methanol emission in high-luminosity sources is likely due to HII regions.

Abstract

Complex organic molecules are only detected toward a fraction of high-mass protostars. The goal of this work is to investigate whether high-mass disks can explain the lack of methanol emission from some massive protostellar systems. We consider an envelope-only and an envelope-plus-disk model and use RADMC-3D to calculate the methanol emission. High and low millimeter (mm) opacity dust are considered for both models separately and the methanol abundance is parameterized. Viscous heating is included due to the high accretion rates of these objects in the disk. In contrast with low-mass protostars, the presence of a disk does not significantly affect the temperature structure and methanol emission. The shadowing effect of the disk is not as important for high-mass objects and the disk mid-plane is hot because of viscous heating, which is effective due to the high accretion rates. Consistent with observations of infrared absorption lines toward high-mass protostars, we find a vertical temperature inversion, i.e. higher temperatures in the disk mid-plane than the disk surface, at radii < 50au for the models with $L=10^4$ L$_{\odot}$ and large mm opacity dust as long as the envelope mass is >550 M$_{\odot}$. The large observed scatter in methanol emission from massive protostars can be mostly explained toward lower luminosity objects with the envelope-plus-disk models including low and high mm opacity dust. The methanol emission variation toward sources with high luminosities cannot be explained by models with or without a disk. However, the $L/M$ of these objects suggest that they could be associated with hypercompact/ultracompact HII regions. Therefore, the low methanol emission toward the high-luminosity sources can be explained by them hosting an HII region where methanol is absent.