Knowledge Bank

Following the detection of the $000-000$ band of the $A1\Pi u\leftarrow X1\Sigma g+$ transition of $C3$ at $4051.6\backslash AA$ by Maier and $othersa$ toward three diffuse sightlines, we have searched for the spectrum toward more reddened stars. Although the lower resolution (R = 37,500) of our spectrometer (ARCES) did not resolve the rotational structure of the spectrum, it allowed us to observe denser clouds toward fainter stars which are expected to contain more carbon molecules. The $C3$ spectrum has been observed toward 15 stars with color excesses ranging from E(B-V) = 0.33 to 1.12 and the $C3$ column densities were determined by comparing the observed spectra with simulated spectra. The observed $C3$ column densities from $1012$ to $1013cm-2$ are well correlated with the $C2$ column densities indicating their close chemical relation. The formation of $C3$ from $C2$ will be discussed. The studies of $C3$ and $C2$ have led us to the revelation of a group of diffuse interstellar bands whose intensities correlate well with the $C3/C2$ column $densitiesb$. It was also noted that abundances of $C3/C2$ and $H3+$ are completely uncorrelated. Our non-detection of the $020-000$ vibronic band at $3991.6\backslash AA$ set an upper limit to its Franck-Condon factor and that of $C4$ and $C5$ set upper limits of their column densities.