Dr Maxim Voronkov (ATNF)

Methanol masers: theory and observations - Dr Maxim Voronkov Colloquium

ATNF Marsfield Lecture Theatre

Abstract

Methanol molecule is very rich for observable transitions and
has by far the largest number of detected transitions, both in thermal
and maser emission. This fact makes masers on this molecule very attractive
for theoretical researches. Even the simplest models can
explain two classes of methanol masers for bright lines. It was found by
K.Menten that methanol maser transitions were divided into two classes.
Masers in Class I transitions are usually seen apart from infrared sources
and ultra-compact HII regions, while Class II masers tend to associate with
them. The simulations of masers show that the strong radiation from a
nearby source quenches the Class I maser transitions and produces
the Class II masers. Due to this strong masers in transitions of the
different classes should never originate in the same volume of space.
However, it might be possible for weak masers. A weak line at 6.7 GHz
(Class II) was found in OMC-1 towards the cluster of 25 GHz maser
spots (Class I). Various interferometric studies of the most widespread
maser transition at 6.7 GHz (Class II) revealed that the brightness
temperature reaches 10^12 for some sources.
It is difficult to explain such high brightnesses in the model involving
only rotational levels of the molecule. Sobolev & Deguchi proposed the
pumping model for the Class II methanol masers involving the torsionally
excited levels of the molecule.
This model is able to explain high observed brightnesses of the 6.7 GHz
masers. The models can be tested observationally by search for newly predicted
maser transitions. The model of Sobolev & Deguchi predicts a weak
torsionally excited maser at 44.9 GHz, which was found in W3(OH).
The talk will also include recent results on Class I methanol maser
lines at 25 GHz and 104.3 GHz.