Class II Methanol Masers


Michele Pestalozzi
Moshe Elitzur
Vincent Minier
John Conway
Roy Booth

VLBA Experiment number BM104, November 1998
EVN experiment number EP039b, February 2001

Class II Methanol Masers
Class II methanol masers seem to be uniquely associated with massive star formation. Their extraordinary high brightness makes them a unique tool to study the smallest spatial and dynamical scales around a massive protostar at great distances, using VLBI. Thanks to the presence of methanol masing at two frequencies detected toward the 2.7 kpc distant high-mass forming region NGC7538-IRS1 N and beautifully imaged by the VLBA at 12 GHz (VLBA image by Minier et al. 2000) and by the European VLBI Network (EVN) at 6.7 GHz we were able to model for the first time a circumstellar Keplerian disc seen edge-on around a massive protostar. Our model is able to reproduce all dynamical features visible in the displacement-LOS-velocity diagram (lower right) and fits the optical depth profile as well as the spectrum (upper panels) over a wide range of magnitudes. The final result is that for a central object of about 30 solar masses the masing flat disc seen edge-on has an outer radius of 750 AU and an inner radius of 290 AU.


Pestalozzi, Elitzur, Conway & Booth, 2004, ApJ Letters, 603, L113

Minier, Booth & Conway, 2000, A&A, 362, 1093

Minier, Booth & Conway, 2002, A&A, 383, 614