Importance of source structure on complex organics emission II. Can disks explain lack of methanol emission from some low-mass protostars?

TL;DR

This study investigates how the presence of disks and dust optical depth affect methanol emission in protostars, showing that disks can obscure emission even when methanol is abundant, explaining observations of low methanol emission.

Contribution

It demonstrates through radiative transfer modeling that disks and dust optical depth significantly influence methanol emission, providing a new explanation for low emission in some protostars.

Findings

Disks reduce methanol emission by lowering temperatures and causing continuum over-subtraction.

Envelope-only models overpredict methanol emission compared to observations.

Envelope-plus-disk models align better with observed low methanol emission in certain protostars.

Abstract

Some protostellar systems show little or no millimetre line emission of complex organics. This can be interpreted as a low abundance of these molecules, alternatively they could be present in the system but are not seen in the gas. The goal is to investigate the latter hypothesis for methanol. We will attempt to answer the question: Does the presence of a disk and optically thick dust reduce methanol emission even if methanol is abundant in the ices and gas? Using the radiative transfer code RADMC-3D, methanol emission lines from an envelope-only model and an envelope-plus-disk model are calculated and compared with each other and the observations. Methanol emission from the envelope-only model is always stronger than from the envelope-plus-disk model by at least a factor 2 as long as the disk radius is larger than 30 au (for L=8 L$_{\odot}$). In most cases, this is due to lower temperatures and, hence, the smaller amount of warm methanol inside the snow surface of the envelope-plus-disk model. The intensities drop by more than an order of magnitude for models including high mm opacity dust grains and disk radii of at least 50 au (for L=8 L$_{\odot}$) due to continuum over-subtraction. The line intensities from the envelope-only models overproduce the observations of protostars with lower methanol emission even with large dust optical depth effects. The envelope-plus-disk models can explain the bulk of the observations. However, they can only reproduce the observations of sources with high luminosities and low methanol emission when dust optical depth effects especially continuum over-subtraction in the disk becomes effective. Therefore, both the effects of disk and dust optical depth should be considered to explain the observations. In conclusion: Absence of methanol emission does not imply absence of methanol molecules in either gas or ice.