Resolving distance ambiguities towards 6.7 GHz methanol masers

TL;DR

This study resolves the kinematic distance ambiguity for 6.7 GHz methanol masers using HI absorption spectra, improving distance estimates to star-forming regions and assessing the reliability of different resolution methods.

Contribution

It introduces a method combining HI absorption spectra with velocity comparisons to accurately resolve distance ambiguities for methanol masers.

Findings

Successfully resolved distances for 41 sources.

Identified conflicts with previous measurements and discussed their implications.

Showed that vertical scale height arguments can lead to errors.

Abstract

Distances to most star forming regions are determined using kinematics, through the assumption that the observed radial velocity arises from the motion of the source with respect to the Sun resulting from the differential rotation of Galaxy. The primary challenge associated with the application of this technique in the inner Galaxy is the kinematic distance ambiguity. In this work, we aim to resolve the kinematic distance ambiguity towards a sample of 6.7 GHz methanol masers, which are signposts of the early stages of massive star formation. We measured 21 cm HI absorption spectra using the Very Large Array in C and CnB configurations. A comparison of the maximum velocity of HI absorption with the source velocity and tangent point velocity was used to resolve the kinematic distance ambiguity. We resolved the distance ambiguity towards 41 sources. Distance determinations that are in conflict with previous measurements are discussed. The NE2001 spiral arm model is broadly consistent with the locations of the star forming complexes. We find that the use of vertical scale height arguments to resolve the distance ambiguity can lead to erroneous classifications for a significant fraction of sources.