10 years of 12.2 GHz methanol maser VLBI observations towards NGC 7538 IRS1 N: proper motions and maser saturation

Paper in proceeding, 2012

We present the outcomes of the consistent analysis of 6 epochs of VLBA 12.2 GHz data obtained between 1995 and 2005 towards the known high-mass star formation reigon NGC7538 IRS1 N. Our analysis concentrates on the study of the main spectral/spatial feature, which is 20 VLBA synthesized beams in size with a distinct velocity gradient. We looked for proper motion signals relative to the central peak which, in an edge-on disc framework, is expected to be stationary. We also study the peak flux and the spatial brightness profile of the main maser feature searching for maser variability. Our results are twofold: we detect a clear proper motion signal of three spatial features (0.21, 0.1, 0.65 mas yr−1) and conclude that these can be made consistent with previous modelling of a Keplerian disc seen edge-on around a high-mass protostar. We further detect a consistent decrease of the peak flux over the time-span 1995-2005 (~ 5.4 Jy yr−1), confirmed when taking into account earlier data (1986, 1987) as well as by the 6.7 GHz maser emission. Also, the width of the spatial brightness profile of the main feature seems to decrease between 1995 and 2005 by some 50%. We consider these observables as clear signs of partial maser saturation.