Periodic flares of methanol masers

Observations with the Torun 32-m radio telescope have revealed periodic flares of 6.7GHz methanol maser line in four sites of high-mass star formation.

The simplest alcohol, methanol, is widely produced in the environment of high-mass star formation regions (HMSFRs). This molecule emits a huge amount of radio lines and some of them arise due to natural maser process.  The 6.7 GHz transition is a very strong and extremely useful signpost of sites where high-mass stars are born.

Thanks to the use of the Torun 32-m radio telescope four new periodic sources were detected and periodic variability was confirmed in the previously known object. The flux density of individual maser features or the entire spectrum varies with period of 120 to 245 days. For two HMSFRs mapped with a few milliarcsecond resolution the periodic activity of the maser is confined to a region of size less than 500-750 au which constitutes only 20-50% of the overall size of maser distribution. This outcome strongly constrains plausible causes of periodic variability. First of all, it implies that the flares cannot be derived from/driven by large-scale variations in either the background seed photon flux or the luminosity of the powering star. The most probable mechanism of periodic flares is related to local changes in the pumping conditions that can be induced by a binary system.  Read more  periodic-2015.

The 6.7GHz methanol maser flux density over velocity and time for G22.357+0.066 (left) and G25.411+0.105 (right) presented in the form of false-colour plots. The velocity scale is relative to the Local Standard of Rest. The dates of observations in the Modified Julian Days (MJD) are marked as the horizontal bars in the left coordinate.