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

Abstract

The SiO (v=0, J=2-1) line has been observed towards several late type stars with the Onsala 20 m telescope over a period of four years. No significant variations in the integrated intensities were detected. However, two of the sources, R Leo and R Cas, show clear changes in their line profiles during this time. Figure 1 presents two of the spectra obtained towards R Leo. The January 1984 spectrum shows a red-shifted feature at ~ 8 kms^-1 which has completely disappeared by April 1984. The time variations and the highly redshifted velocity suggest that this feature was of maser origin. The line profile towards R Cas changed from a roughly parabolic line in 1979, to a flat topped profile with weak wings extending out to ~15 km s^-1 from the stellar velocity in 1984. From a CO (J=1-0) spectrum we estimate the envelope expansion velocity to be ~ 11 kms^-1. χ Cyg and NML Tau were also observed several times, but in these sources no time variations were detected. Their spectra are parabolic, which is the expected line profile for optically thick emission from “thermally” excited molecules in an unresolved cir-cumstellar envelope. R Aql, o Ceti, T Cep, and U Ori were observed on one occasion, in January 1984, and SiO ground state emission was detected in the latter three sources for the first time. All four sources show evidence for anomalous excitation.