Collisional pumping of SiO masers in evolved stars

Abstract

ALTHOUGH SiO masers (lasing systems at microwave frequencies) have been detected in many old red-giant stars^1,2, several fundamental questions, such as what drives the masers and where in the stars' environment they are located³, remain unresolved. Evolved stars are known to have winds, which distribute enriched material throughout the interstellar medium, but the acceleration mechanism and the point in the stellar atmosphere at which the winds are initiated, are unknown⁴. SiO masers have high brightness temperatures, which allows them to be studied with high spatial and spectral resolution using very-long-baseline interferometry (VLSI) techniques, and thereby potentially determine how the stellar winds are generated. This will only be possible, however, once the physical conditions giving rise to the maser emission are known. Here we present images of positionally coincident maser emission from the J = 1 → 0 rotational transition of the first two vibrational levels in the SiO masers in VY Canis Majoris and W Hydrae. These results clearly demonstrate that the maser emission is collisionally pumped in distinct regions, rather than radiatively pumped. This means that SiO maser emission can be used to follow the clumps of gas as they are accelerated in the stellar atmosphere.