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Paper: FUSE Observations of Interstellar CO and H2 toward Stars in Cepheus
Volume: 348, Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE
Page: 433
Authors: Federman, S.R.; Sheffer, Y.; Pan, K.; Andersson, B.-G.
Abstract: Far ultraviolet spectra contain numerous electronic transitions of CO and H2. In the course of analyzing FUSE data on O and B stars in the Cep OB2 and OB3 Associations, we detected absorption features from Rydberg bands of CO toward HD203374A. While the B — X (0—0), C — X (0—0), and E — X (0—0) bands were seen in Copernicus and ORFEUS spectra before, a larger number of higher-energy (shorter-wavelength) bands was observed for the first time. Absorption in these higher-energy bands leads to CO photodissociation. Thus, it is important to measure their oscillator strengths, which are key ingredients in theoretical modeling of CO photochemistry in interstellar environments. A comparison of our astronomically derived oscillator strengths with laboratory measurements is presented. We also compare H2 results with optical observations of CH for all our FUSE targets. While it has been known that CH and H2 abundances track each other very well in diffuse molecular clouds, we show that local conditions, gas density and the strength of the ultraviolet radiation field, control the dispersion in the relationship between CH and H2. Results on the correspondence between CO and H2 for these sight lines are also given.
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