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Paper: Periodic Variations in Class II Methanol Masers
Volume: 387, Massive Star Formation: Observations Confront Theory
Page: 124
Authors: Goedhart, S.; Gaylard, M.; van der Walt, J.; Elitzur, M.
Abstract: Time variations in masers can provide clues to conditions deep in the star formation region with which they are associated. A programme to monitor 53 6.7-GHz methanol 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 regular variations with periods between 133 to 504 days. The data span now covers six to twenty-four cycles, depending on the period. This is the first definitive detection of periodic phenomena in massive star forming regions and the cause 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 background radiation, which probably originates 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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