Understanding the origin of Bunny-Hoplight curves of periodic methanol masers

Abstract

Class II 6.7 GHz methanol maser emission, first discovered by Menten (1991), is exclusively associated with high-mass star-forming regions. Long-term observational studies have shown that masers are variable on numerous timescales; a surprising follow-up finding was that of the periodic methanol masers originally reported by Goedhart et al. (2003) and Goedhart et al. (2004). Maser source G358.460 - 0.391 was previously identified as periodic by Maswan ganye et al. (2015); this research involved long-term monitoring of this 6.7 GHz methanol maser source using the 26-metre HartRAO telescope. This extended monitoring, which in cluded high cadence observations, was necessary to characterise the behaviour of this source fully. The maser source displayed distinct bunny-hop shape periodicity in the primary and secondary features and velocity drifting in the primary feature. Furthermore, archival maser sources were studied for evidence of periodicity and velocity drifting; 19 of the archival sources were identified as good candidates for velocity drifting. A large portion of the archival sources also displayed velocity drifting leading to the conclusion that velocity drifting may be more common of a phenomena in maser sources. It is not yet clear what the underlying cause of the bunny-hop periodicity is, it may be linked to the face-on orientation of the maser region and periodic changes in seed photon density. Velocity drifting may be due to spectral blending of features, motion of gas, or disc precession. Extended observations if maser sources are essential in understanding the nature of masers and their star forming environments