SiOmaser

SiO masers are a type of astrophysical maser produced by silicon monoxide molecules in the inner regions of some stellar envelopes and, in some cases, in star-forming regions. They arise when a population inversion occurs in the rotational transitions of SiO within vibrationally excited states, leading to stimulated emission at microwave frequencies. The most prominent transitions are the v=1, J=1-0 line near 43 GHz and the v=2, J=1-0 line near 43 GHz, with higher transitions such as J=2-1 around 86 GHz observed in some sources.

SiO masers are most commonly found in oxygen-rich asymptotic giant branch (AGB) stars and red supergiants, where they form in the warm, dense gas of the inner circumstellar envelope, typically a few stellar radii from the photosphere, near the dust formation zone. They have also been detected in a subset of massive star-forming regions, where they trace energetic processes such as shocks in outflows. The emission is typically compact and bright, and it often shows temporal variability tied to the pulsation cycles of the host star.

Excitation of SiO masers is thought to involve a combination of radiative pumping by infrared radiation from the central star and collisional processes. The resulting maser emission is highly beamed, allowing high-resolution techniques such as very long baseline interferometry (VLBI) to map the spatial and velocity structure of the maser spots. Such observations reveal ring-like or clumpy distributions around stars and enable precise measurements of proper motions and parallaxes.

Applications of SiO masers include studying the dynamics and structure of the inner circumstellar environment, contributing to distance measurements in the Galaxy, and providing insights into magnetic fields through polarization studies. They remain a valuable tool in astrophysical maser research.