Maser Emission in Evolved Stellar Systems
Summary
Maser emission in evolved stellar systems arises when molecules in the expanding envelopes of ageing stars undergo a population inversion, producing intense, coherent microwave radiation. Key maser species include hydroxyl (OH), water (H2O) and silicon monoxide (SiO), each tracing distinct regions of the circumstellar environment. During the asymptotic giant branch (AGB) phase, heavy mass loss creates a dense envelope in which OH and SiO masers probe inner dust formation zones and pulsation-driven shocks. As stars evolve into post-AGB and pre-planetary nebula stages, the emergence of high-velocity H2O maser jets—so-called water fountain sources—marks the development of bipolar outflows that sculpt complex nebular morphologies. In the final planetary nebula phase, remnant OH and H2O masers can reveal magnetically influenced structures through Zeeman splitting. Across these stages, masers serve as powerful kinematic and magnetic probes, enable precise distance measurements via very long baseline interferometry, and illuminate the mechanisms by which evolved stars return enriched material to the interstellar medium.
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Maser Emission in Evolved Stellar Systems publication trend
The graph below shows the total number of articles in maser emission in evolved stellar systems across all publications each year (not limited to Nature Index journals).
Technical terms
Maser: Microwave amplification by stimulated emission of radiation in astrophysical environments, producing narrow, intense spectral features.
Circumstellar envelope: A shell of gas and dust expelled by an evolved star through sustained mass loss.
Asymptotic giant branch (AGB): A late evolutionary stage of low- to intermediate-mass stars characterised by shell burning and heavy mass loss.
Water fountain source: An evolved star exhibiting high-velocity H2O maser jets that signal the onset of bipolar outflows in the post-AGB transition.
Population inversion: A condition in which a higher molecular energy level has greater occupancy than a lower one, enabling maser action.
Zeeman splitting: The magnetic-field-induced separation of spectral lines, used to measure field strengths via maser observations.
References
- Excited-state OH Masers in the Water Fountain Source IRAS 18460-0151. The Astrophysical Journal Letters (2024).
- Systematic Search for Water Fountain Candidates Using the Databases of Circumstellar Maser Sources. The Astrophysical Journal Supplement Series (2024).
- A Search for Asymmetric Kinematic Components in Circumstellar Envelopes Using OH Main Line Masers. The Astrophysical Journal (2024).
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