Observation of the Spin-Forbidden Cameron Bands of CO in the Ultraviolet Emission Spectrum of the Red Rectangle

The center of the biconical nebula known as the Red Rectangle has been observed by using the ultraviolet spectrographs aboard the Hubble Space Telescope . We report the observation of phosphorescence of the Cameron bands of CO ( a 3 Π → X 1 Σ + ) between 1850 and 2600 Å. These bands are seen for the first time outside of the solar system. The bands are about one-fifth as strong as the accompanying emission bands of the spin-allowed Fourth Positive system. A charged-particle impact excitation mechanism is suggested for the direct production of CO ( a 3 Π) from CO ( X 1 Σ + ). The C I ( 1 D - 1 P o ) line is observed at 1931 Å, suggesting that a large amount of C I ( 1 D ) is also present.