New Periodic 6.7 G Hz Class Ii Methanol Maser Associated With G358.460 0.391

1525 words - 7 pages

\section{Introduction}

The two brightest class II methanol masers (6.7 and 12.2 GHz) are found in massive star forming regions. Some of these methanol masers show variability but the most noticeable ones are regularly varying behaviour which was first noted by \citet{goedhart2003} in G9.621+0.196E. This behaviour was later reported and confirmed to the eight more sources \citep{goedhart2003,goedhart2004,goedhart2009,araya2010,szymczak2011}. The variations in these masers reflect the dynamics in massive star forming regions which are still poorly understood \citep{zinnecker2007,mckee2007}. The origin of the observed periodicity in class II methanol masers is also not confirmed.

Currently there are three competing models which attempt to explain the origin of the quasi-periodic and periodic variation in the class II methanol masers. These models use the changes in pumping mechanism as the origin of maser variability. The first model was proposed by \citet{sobolev2007}. In this model, the variation in dust temperature results in pumping mechanism variation; this will have an effect on the masing population level. The fluctuations in dust temperature could be due to the formation of the filaments in the circumstellar disk of a protostar. In contrast, \citet{vanderwalt2011} proposed the Colliding Wind Binary (CWB) model as the source of the observed periodicity. The CWB is a second model. In this model, the colliding winds from a binary system in massive star forming region produce the photon ionisation shock. The photon ionisation shock is absorbed when it reaches a partially ionised or unionised region. The absorbed photon produces infrared radiation which is the pumping mechanism for class II methanol masers. This explains, the steep rise in each waveform for G9.621+0.196E \citep{goedhart2003,goedhart2004,goedhart2013}. The exponential decay in each waveform is explained by the hydrogen recombination lines. \citet{inayoshi2013} proposed the third model. This model uses pulsation of the Young Stellar Object (YSO) due to rapid accretion rate ($\dot{M_{\ast}}$ $\gtrsim$ ${10}^{-3}$ $M_{\odot}$yr$^{-1}$) to explain the observed periodicity. There has been no scientific observation in massive star forming regions with class II methanol masers which confirm any of the above models.

The number of periodic class II methanol masers is very small compared to the number of known massive star forming regions. They also show different waveforms which lead to diverse models in explaining the origin of periodic variability. Increasing the number of these types of masers will improve the maser model to explain the periodicity phenomena.

This article provides the monitoring report for the eight class II methanol maser from the 6.7 GHz Methanol MultiBeam (MMB) survey catalogues I \citep{caswell2010} and II \citep{green2010}. These sources were monitored for 2 years and 11 months (from February 2011 to January 2014) with the 26m Hartebeesthoek Radio...

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