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Class II methanol masers are believed to be associated with an early evolutionary stage of massive star formation. The 6.7 GHz transition is particularly intense and is a good tool for studying conditions in the vicinity of the protostar, in a region which is highly obscured at optical wavelengths. Temporal variations in the masers can provide clues to conditions deep in the star formation region. A programme to monitor 53 6.7-GHz masers was carried out at Hartebeesthoek Radio Astronomy Observatory from January 1999 to April 2003 and subsequently continued on 19 of the sources. Analysis of the resulting time-series stretching over eight years shows that six of the sources exhibit periodic variations with periods between 133 to 504 days. A few more sources show what may be classified as quasi-periodic or regular variations. This is the first reported detection of periodic phenomena in massive star forming regions and the cause of the periodicity is not known at this stage. The waveforms in individual sources range from sinusoidal to sharp flares and there can be other long term trends in the time-series. The amplitudes of the variations can also change from cycle to cycle. The masers are believed to be pumped by mid-infrared radiation, so they will be sensitive to changes in the radiation propagating through the dust surrounding the young star. The variability could also be modulated by the seed photons, which probably originate from a hypercompact HII region. In either case, the variability of the masers is pointing to a periodic process associated with the massive star. The challenge is to find out what the masers are telling us about the process of massive star formation.
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