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With a molecular weight of 123 amu and 11 atoms, HC9N (cyano-octatetra-yne) is the heaviest and largest molecule yet detected in interstellar space. The J = 18-17 and J = 25-24 transitions have been observed in Heiles's Cloud 2 by using a molecular constant obtained by extrapolation from the lighter cyanopolyyne molecules. The column density is estimated to be trillion per sq cm, down by a factor of 4 from that of HC9N in the same source.
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Broten et al. (1978) studied this question.
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