The Fourth Positive System of Carbon Monoxide in theHubble Space TelescopeSpectra of Comets

Abstract

The rich A 1 Å À X 1 AE þ system of carbon monoxide accounts for most spectral features seen in long-slit HST STIS observations of comets 153P/ Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM 1 (LINEAR), as well as in the HST GHRS spectrum of comet C/1996 B2 ( Hyakutake). A detailed CO fluorescence model is developed to derive the CO abundances in these comets by simultaneously fitting all of the observed A À X bands. The model includes the latest values for the oscillator strengths and state parameters, and accounts for optical depth effects due to line overlap and self-absorption. The derived CO abundances relative to water in these comets span a wide range, from 0:44% AE 0:03% for C/2000 WM 1 (LINEAR), 7:2% AE 0:4% for 153P/Ikeya-Zhang, and 8:8% AE 0:8% for C/2001 Q4 (NEAT) to 20:9% AE 0:3% for C/1996 B2 (Hyakutake). The radial profiles of the CO column density are consistent with a predominantly native source for all the comets observed by STIS. A complete fitting of the CO spectral features using the fluorescence model leads to the first identification of a molecular hydrogen line pumped by solar H

FUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov–Gerasimenko

“…3 and 4. In addition to electron excited emission, CO Fourth Positive fluorescence is also present as indicated by the red synthetic spectrum based on the model of Lupu et al (2007), with rotational temperature (75 K) and outflow velocity (0.7 km s −1 ) typical of comets near 1 AU. As we did above, we can use the observed C I λ1657/Lyman-β ratio to determine the relative CO 2 abundance along the line-of-sight.…”

“…The green line is the same for CO 2 . The red line is a synthetic CO Fourth Positive fluorescence spectrum following Lupu et al (2007) for a CO column density of 1.6 × 10 14 cm −2 (April 26) and 1.0 × 10 14 cm −2 (May 14). Note the distorted Lyman-α line shape due to detector degradation effects described in Section 2.1. of ∼ 1.0 × 10 −9 photons s −1 molecule −1 at 1 AU.…”

The Photodissociation of Formaldehyde in Comets

“…The C I multiplets at 1561 and 1657 Å are well known features of cometary spectra (e.g., Lupu et al 2007), reflecting the abundance of carbon in the volatile constituents of a comet's nucleus. The principal series of C I leading up to the ionization limit at ∼1101 Å has many transitions in the FUSE spectral range and several are easily identified once all of the H 2 features are accounted for.…”

“…In Figure 5 we identify multiplets at 1129, 1140, and 1158 Å in comet C/2001 Q4 that are clearly separated from the nearby H 2 lines. We restrict our analysis to this comet in order to compare the derived carbon column density with that derived from the nearly simultaneous Hubble Space Telescope (HST) observations of the longer wavelength multiplets of Lupu et al (2007), although they are also present in the spectrum of C/2001 A2.…”

“…The modeled C I spectrum, for a mean C column density of 1.0 × 10 13 cm −2 derived by Feldman & Weaver (2011) from the HST/STIS spectra of Lupu et al (2007), is shown in Figure 5. The STIS spectral image shown by Lupu et al, obtained during the day following the FUSE observations, shows that the long wavelength carbon emissions are uniformly distributed along the 25″ length of the STIS slit, making direct comparison with the FUSE data (aperture size 30″ × 30″) possible.…”