Modelling a Circumstellar Disc Traced by Methanol Masers.

We present the accurate modelling of one methanol maser feature in NGC7538-IRS1. The aim of this work is to test whether this particular maser feature traces a disc or an outflow in the immediate surroundings of a massive protostar. From our data it is clear that we are in the presence of probably the most smooth and regular maser emission discovered until now. The disc model seems to fit remarkably well the data, while a bi-cone jet model fails to reproduce most of the features seen in the maps and position-velocity diagrams. NGC7538 IRS1 N is a furnace of star formation in which many methanol masers have been detected and mark different objects (see Poster by V. Minier et al.). Methanol maser emission is detected all over the region and seems to be cospatial with the very young protostellar objects or cluster of protostellar objects in the region: IRS 1, IRS 9, NGC7538 S. All these sources possibly host discs and outflows at different scales and traced by different types of emission (e.g. [7, 9, 8, 2, 1]). The masers in IRS 9 and NGC7538 S have been recently positioned (features G and F, see Fig. 2, [6]) and cannot yet be identified as having a clear shape. The maser emission toward IRS 1 comprises features A, B, C, D, E which spread over an area of approximately 600 x 400 mas. The most prominent feature is feature A, which was suggested to be a Keplerian rotating disc in previous works [3]. Feature A shows an extremely smooth and linear structure both in space and velocity (Fig. 1). Moreover, the same structure is marked by maser emission at two frequencies (6.7 and 12.2 GHz), which appear to be perfectly cospatial (within VLBI position accuracy, 1-2 mas). For the model we consider the 12.2 GHz mainly as this data shows the highest spatial resolution. A general assumption for all modelling is that the maser amplifies the background (HII region) continuum radiation. Its intensity is then: