Variability of methanol and OH masers associated with the star forming region G339.62-0.12
Abstract
Astrophysical maser emission from various molecules and in a number of astrophysical environments is now a well-known phenomenon. Since the first discovery of hydroxyl (OH) maser emission by Weinreb et al. (1963), the use of masers to obtain information of the kinematics and physical properties of the environments where they occur, have expanded significantly. Masers occur in compact dusty dense regions and are associated with various environments such as high-mass star forming regions (SFRs), circumstellar
envelopes around late-type stars, comets and even extragalactic galaxies. The physical conditions of these environments can be investigated through maser indications such as variability (i.e. the variations in the maser flux density). Periodic variability was first evident in the brightest class II methanol (CH3OH) masers
associated in G9.62+0.19E (Goedhart et al., 2003). The discovery has led to several more studies of periodic variable masers in various high-mass SFRs. In various high-mass SFRs, the class II methanol masers are known to spatially coincide with OH masers and have a similar pumping mechanism (Cragg et al., 1992). It is, therefore, necessary to also investigate the OH masers for periodic behaviour other than just that of methanol. This work presents the parallel observation of the 6.7 GHz CH3OH and the mainline (1665 and 1667 MHz) OH masers in G339.62-0.12 carried out from February 2013 to July 2015. The 26m single-dish telescope at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) and the Karoo Array Telescope (KAT-7) are used respectively. The radio continuum at 18 cm observed with the KAT-7 is also presented.
The findings of this work show strong evidence of variability as well as periodicity of the 6.7 GHz CH3OH and the mainline OH masers associated with G339.62-0.12. From the time-series analysis, the maser features are seen to be blue- and red-shifted from the systemic velocity (V sys = -34.2kms−1). One interesting aspect is that periodic variability is clearly visible for the blue-shifted masers with velocities ranging between -37.6kms−1 to -35.5kms−1. Using the Lomb-Scargle (LS) periodogram, the periods for the 6.7 GHz methanol, the 1665 and 1667MHz OH masers are found to be 203 ± 2 days, 208 ± 2 days and 210 ± 2 days respectively. These periods fall within the derived periodicity (23.9-668 days) for all the periodic maser features. The red-shifted masers seem to show uncorrelated variability with some sort of periodicity. The period for the red-shifted masers is found to be 193 days, which is less than the periods of the blue-shifted masers. This
may be an indication that the red-shifted masers are possibly associated with a different SFR. Although the mainline OH maser emission occurs in the same region as the 6.7 GHz CH3OH, from our results we can deduce that they are not spatially co-located.