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

TL;DR

This study analyzes far ultraviolet spectra of four comets, revealing the presence of formaldehyde and carbon dioxide signatures, and distinguishes between thermal and non-thermal molecular populations caused by different dissociation processes.

Contribution

It identifies specific molecular dissociation pathways in comets, including formaldehyde and CO2 contributions, using spectroscopic signatures in the far ultraviolet range.

Findings

Detection of cold and hot CO components with distinct rotational temperatures

Identification of formaldehyde as a source of non-thermal CO populations

Electron impact dissociation as a dominant process near the comet nucleus

Abstract

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 - X) at 1088 A 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 55-75 K. We identify the "hot" component as coming from the dissociation of CO2 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 H2. This process also leaves the H2 preferentially populated in excited vibrational levels which are identified by fluorescent H2 lines in the spectrum excited by solar OVI 1031.9 and solar Lyman-alpha. The amount of H2 produced by H2CO dissociation is comparable to the amount produced by photodissociation of H2O. Electron impact excitation of CO, rather than resonance fluorescence, appears to be the primary source of the observed (B - X) (0,0) band at 1151 A.