OPTICAL ABSORPTION AND EMISSION BANDS OF SiC_2 IN CARBON STARS

摘要

Observations of the Merrill-Sanford bands of SiC_2 in the 4750-5050 A spectral region, seen in absorption in some carbon stars and in emission in IRAS 12311—3509, are reported. Vibrational band assignments based on recent laser-based laboratory electronic spectroscopy of this molecule constitute substantial revisions compared with earlier work. Vibrational hot bands are very strong when present in carbon stars, but were greatly weakened in T Mus for a time in 1994, indicating that the bands were then formed in a cooler region than that typical of carbon star photospheres. Emission bands of SiC_2, with intensities that are consistent with optical excitation of cool gas, appear in IRAS 12311-3509 which is probably a J-type carbon star occulted by a dusty disk so that the star is seen by reflection from material out of the plane of the disk.