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Spectroscopic Study of a Bright Maser Source – Methanol Maser G9.62 + 0.19E: Theory and Observations

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Thesis(4.13 MB)

Date

2024

Authors

Supervisor

Gulyaev, Sergei
Natusch, Tim

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Thesis

Degree name

Master of Science (Research)

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Auckland University of Technology

Abstract

Astrophysical masers are naturally occurring galactic sources of anomalously intense non-thermal microwave radiation. The study presented in this thesis focuses on the analysis of the Class II methanol maser source G9.62 + 0.19E through theoretical modelling, observations, data processing, and spectral analysis. This active stellar region is of particular interest due to its high, quasiperiodic variability – as well as it being the brightest recorded methanol maser. The study of maser sources is crucial for validating their role as sensitive tracers of celestial objects and facilitating the exploration of their unique properties, aiding in measuring physical parameters and understanding the kinematics of our Galaxy. We review approaches to deriving the radiative transfer equation for unsaturated masers of different geometries, consulting both standard and semi-classical maser theory variations. A simplified form of the radiative transfer equation is derived for approximating an unsaturated spherical maser source and used for numerical modelling of spectral line shapes in active (masing) media. Our findings reveal that the maser spectral line shape deviates from a single Gaussian, comprised of a strong narrow peak on a wide (Gaussian) pedestal. Demonstrating that the ratios of amplitudes and widths between the narrow and wide features are dependent on the maser source’s optical depth. These ratios enable the determination of individual maser source optical depth and, consequently, key astrophysical parameters like density, temperature, and pumping mechanism, given knowledge of the object’s geometry and size. As a case study, observations of the 6.67 GHz methanol maser source G9.62 + 0.19E were conducted using the New Zealand 30-m radio telescope at Warkworth. Implementing multiple frequency switching (MFS) confirmed the presence of a wide pedestal - typically excluded in standard polynomial continuum reduction. Utilizing amplitude and width ratios of the narrow peak and pedestal, we determined the optical depth to be within the range of tau = 4 → 6. To further validate this method, future studies should encompass more refined numerical modelling and observations of other maser sources.

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http://hdl.handle.net/10292/17686

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