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\bibitem{Bujarrabal_etal2010} Bujarrabal, V., Alcolea, J., Soria-Ruiz, R., Planesas, P., Teyssier, D., Marston, A. P., et al. (2010). Herschel/HIFI observations of high-J CO transitions in the protoplanetary nebula CRL 618. \textit{Astron. Astrophys.}, \textit{521}, L3 (1 to 5).

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