Sharmila Goedhart (Hartebeesthoek Radio Astronomy Observatory, South Africa.)

Variability in Class II methanol masers - Sharmila Goedhart colloquium

ATNF Marsfield Lecture Theatre

Abstract

There is an increasing body of evidence that class II methanol masers are
closely associated with high mass star formation regions. However, the
exact nature of the association is not clear. It appears that the masers
are linked to an early evolutionary stage but there is some debate as to
whether they form in disks, on the edges of outflows or in some other
location in the protostellar envelope. The 6.7 and 12.2 GHz maser
transitions are particularly intense, making them suitable for monitoring
with a modest-sized telescope. Masers arise from very compact volumes of
gas, thus monitoring of changes in flux density has the potential to probe
changes at small scales in the immediate vicinity of high-mass protostars.

An intensive monitoring program of 53 6.7-GHz methanol maser sources was
carried out at the Hartebeesthoek Radio Astronomy Observatory from January
1999 to April 2003. A wide range of behaviours was seen, including no
variability, isolated flares, quasi-periodic and possibly periodic flares.
The monitoring program was subsequently continued on 19 sources of
interest. Analysis of the resulting time-series stretching over seven
years shows that six of the sources show extremely regular variations,
with periods ranging from 133 days to 505 days. The waveforms in
individual sources range from sinusoidal to sharp flares and, in addition,
there can be other long term trends in the time-series. The amplitudes of
the variations can also change from cycle to cycle and there are
indications that the flaring may cease after several cycles. I will
present the time-series for sources of interest, with emphasis on the
regular variables, and discuss the implications of these results.