SiO Outflow Signatures Toward Massive Young Stellar Objects with Linearly Distributed Methanol Masers

TL;DR

This study investigates whether linearly distributed methanol masers around massive young stellar objects trace circumstellar disks or outflows by analyzing SiO emission, revealing outflows inconsistent with the disk hypothesis.

Contribution

The paper provides observational evidence using SiO emission to test the methanol maser-disk hypothesis, challenging the idea that linear maser distributions trace circumstellar disks.

Findings

Most sources show SiO outflows with broad line wings.

Methanol maser velocities often offset from cloud velocities.

Outflow orientations do not align with maser distributions.

Abstract

Methanol masers are often found in linear distributions, and it has been hypothesized that these masers are tracing circumstellar accretion disks around young massive stars. However, recent observations in H2 emission have shown what appear to be outflows at similar angles to the maser distribution angles, not perpendicular as expected in the maser-disk scenario. The main motivation behind the observations presented here is to determine from the presence and morphology of an independent outflow tracer, namely SiO, if there are indeed outflows present in these regions and if they are consistent or inconsistent with the maser-disk hypothesis. For ten sources with H2 emission we obtained JCMT single dish SiO (6-5) observations to search for the presence of this outflow indicator. We followed up those observations with ATCA interferometric mapping of the SiO emission in the (2-1) line in six sources. The JCMT observations yielded a detection in the SiO (6-5) line in nine of the ten sources. All of the sources with bright SiO lines display broad line wings indicative of outflow. A subset of the sources observed with the JCMT have methanol maser velocities significantly offset from their parent cloud velocities, supporting the idea that the masers in these sources are likely not associated with circumstellar disks. The ATCA maps of the SiO emission show five of the six sources do indeed have SiO outflows. The spatial orientations of the outflows are not consistent with the methanol masers delineating disk orientations. Overall, the observations presented here seem to provide further evidence against the hypothesis that linearly distributed methanol masers generally trace the orientations of circumstellar disks around massive young stars.