Time Variations of SiO (v=0, J=2-1) Emission from Circumstellar Shells

Abstract

Observations of the SiO (v = 0, J = 2 - 1) emission line towards eight late-type stars are presented, including the first detections in Omicron Ceti, U Ori, and T Cep. Some of the stars have been observed several times during a four-year period and time variations in the line shapes are clearly visible in R Leo and R Cas. This probably indicates maser action. However, there are no significant variations in the integrated intensities during this period. Other sources also show evidence for anomalous excitation; i.e., narrow features are superposed on normal parabolic line profiles or the entire line is narrow and not obviously parabolic. This implies that stellar radial and envelope expansion velocities and SiO abundances may not be correctly determined from ground-state SiO emission. A CO (J = 1 - 0) spectrum obtained towards R Cas shows a clearly asymmetric line shape.