This diatomic molecule is the simplest oxide of silicon. Silicon monoxide, SiO, has been detected in the gas phase in both interstellar molecular clouds and in the envelopes of evolved stars. In addition to the principal isotopic species, 28Si16O, the rarer forms 29Si16O and 30Si16O are also observed. Maser emission is frequently observed from various transitions of SiO. Not surprisingly, SiO is not detected in cold, dark interstellar clouds or in the diffuse interstellar medium, where the silicon is thought to be primarily in the interstellar dust grains. However, it is detected in the shocks associated to molecular outflows emanating from newly formed stars.
Interstellar SiO was first detected in 1974 by L. Snyder and D. Buhl at millimeter wavelengths. The maser emission from stellar envelopes is often strong enough to provide useful sources for evaluating the pointing accuracy of radio telescopes.