An investigation into maser variability in high-mass star forming regions

Abstract

The two brightest class II methanol masers - at 6.7- and 12.2-GHz, are exclusively associated with the early phases of high-mass star formation. This implies that studying these maser species is an indirect probe of the dynamics and physical properties of a specific epoch in high-mass star formation processes. Goedhart, Gaylard & van der Walt (2003) investigated the nature of variability in the 6.7- and 12.2-GHz methanol masers associated with G9.62+0.19E and found that they show periodic variations with a 246 day period. Since then, fourteen new methanol masers have been found to show periodic variability, which is a significantly small number compared to known methanol maser regions. The total known population of the 6.7-GHz methanol masers in our Galaxy is 1032 although the total Galactic population of these masers is expected to ∼ 1290 (Green et al., 2017). However, their periods have been found to range from 29.5 to 509 days. Some of the light curves of these sources show remarkable diversity. Also, the origin of the observed periodicity in the class II methanol masers is not yet confirmed, although some authors have proposed possible hypotheses to explain the periodicity in methanol masers. Considering some of the challenges above, in this thesis, we investigate maser variability in high-mass star-forming regions by searching for more periodic masers, conducting time-dependent numerical modelling of OH masers and investigate the stability of the determined periods. In the first part of our investigation, eighteen methanol masers from the 6.7-GHz MMB survey catalogues I, II, III, and IV were selected for a long-term monitoring programme using the 26m HartRAO radio telescope. Two of the eighteen methanol masers were found to be periodically variable in our monitoring window. These sources are - G358.460-0.391, at 6.7-GHz, and G339.986-0.425, at both 6.7- and 12.2-GHz. The periods were searched using the three standard methods - viz. the Lomb-Scargle, epoch-folding and Jurkevich, and fitting an appropriate analytical function using the Levenberg-Marquardt method. It was the latter method which gave the better estimate of periods and their corresponding uncertainties, which are 242 ± 1 and 221 ± 1 day for G339.986-0.425 and G358.460-0.391, respectively. The time series of G339.986- 0.425 show the strong correlations and time delays. The time delays in G339.986-0.425 across the channels show remarkable structure. However, they could not be correlated with the maser spot map from ATCA interferometric data due to their low spectroscopic and angular resolution. Using the samples from our investigation and Goedhart, Gaylard & van der Walt (2004), the probability of finding a periodic maser in a sample was estimated to be 0.13 ± 0.04. From this estimate, it was proposed that there may be 38 ± 15 periodic masers from the remaining 292 maser sources from the 6.7-GHz MMB catalogues I, II, III, IV and V. In our second investigation, seven periodic methanol masers from Goedhart, Gaylard & van der Walt (2003), Goedhart, Gaylard & van der Walt (2004), Goedhart et al. (2009) and Goedhart et al. (2013) were used to investigate evolution of the spectra, light curves and periods. These class II methanol masers were in a long-term monitoring programme for 17 yr, using the 26m HartRAO radio telescope. These sources are associated with G12.890+0.490, G338.93-0.06, G339.62-0.12, G328.24-0.55, G9.62+0.19E, G188.95+0.89 and G331.13-0.24, and their updated periods are 29.42 ± 0.08, 133 ± 1, 201 ± 2, 221 ± 5, 243 ± 3, 393 ± 11 and 506 ± 11 days, respectively. Over the 17 yr of monitoring these sources, some maser features did turn on and other faded away, meaning there was an evolution of the spectra. On the other hand, the light curves and periods did not show a significant evolution, and we argued that this ties into the stability of the driving mechanism of the observed periodicity and the light curve. In our third investigation, the statistical rate equations with line overlap are used to investigate a one-to-one correspondence of the OH masers variability with the time-dependent variability of dust temperature. The investigation is restricted to only 24 levels of OH. Also, only four dust temperature profiles were considered. It was noted that in some cases, it is possible for masers to follow the dust temperature variability, and in other cases, there was no one-to-one correspondence between dust temperature and maser variability. Therefore, it is concluded from this simple model with 24 eigenstates of OH, that one should be careful in simply assuming that the dust temperature variations will have a one-to-one correspondence with maser brightness variability. It was also noted that different maser transitions of the same molecule could respond differently to the same dust temperature variability. From this, it can be deduced that it is reasonable to argue that different maser species could also respond differently to the same dust temperature changes.