European VLBI Network observations of 6.7-GHz methanol masers in a candidate circumstellar disc

TL;DR

This study presents high-resolution VLBI observations of 6.7-GHz methanol masers in a candidate circumstellar disc, confirming the disc's shape and rotation, and discusses implications for massive star formation models.

Contribution

First high-resolution VLBI maps of methanol masers in a candidate circumstellar disc, testing the disc hypothesis and the effectiveness of Gaussian centroiding techniques.

Findings

Confirmed the disc shape and Keplerian rotation curve

Resolved out extended emission with EVN

Linked the maser disc to a massive protostar and outflow

Abstract

The first high-resolution (5 mas) VLBI observations of 6.7-GHz methanol masers in DR21(OH)N, a candidate circumstellar disc around a very young massive star, are presented. Previous observations of these masers at 50 mas angular resolution revealed a rotating structure at the position of a candidate massive protostar, with a well-sampled position-velocity diagram suggesting Keplerian rotation. Observations presented here using the European VLBI Network (EVN) have provided the first high angular resolution maps of the masers, providing a test for the disc hypothesis and the Gaussian centroiding technique. The EVN maps have confirmed the shape of the disc and its rotation curve. Weaker maser emission seen previously with MERLIN between the two main spectral peaks is seen in the EVN total power spectrum, but is absent in the cross-power spectrum. This suggests that the spatially extended emission is resolved out by the EVN. The rotating disc is coincident with a Class I massive (proto)star and at the implied centre of an outflow traced by two bow shocks. We discuss the impact of this result on the massive stellar accretion disc hypothesis and on the validity of the centroiding technique to determine the structures of unresolved masers using compact radio interferometric arrays.