The Far-Ultraviolet Spectral Signatures of Formaldehyde and Carbon Dioxide in Comets

Observations of four comets made with the Far-Ultraviolet Spectroscopic Explorer show the rotational envelope of the (0,0) band of the CO Hopfield–Birge system ( C 1 Σ + – X 1 Σ + ) at 1088 Å to consist of both "cold" and "hot" components, the "cold" component accounting for ∼75% of the flux and with a rotational temperature in the range 55 K–75 K. We identify the "hot" component as coming from the dissociation of CO 2 into rotationally "hot" CO, with electron impact dissociation probably dominant over photodissociation near the nucleus. An additional weak, broad satellite band is seen centered near the position of the P (40) line that we attribute to CO fluorescence from a non-thermal high J rotational population produced by photodissociation of formaldehyde into CO and H 2 . This process also leaves the H 2 preferentially populated in excited vibrational levels which are identified by fluorescent H 2 lines in the spectrum excited by solar O vi λ1031.9 and solar Lyα. The amount of H 2 produced by H 2 CO dissociation is comparable to the amount produced by photodissociation of H 2 O. Electron impact excitation of CO, rather than resonance fluorescence, appears to be the primary source of the observed B 1 Σ + – X 1 Σ + (0,0) band at 1151 Å.