Unidentified Ions in Comets

Optical spectra were taken of comets Halley, West and Brorsen-Metcalf in the wavelength region 3200-6400 Å. These spectra were offset ~10-5 — 10-6 km tailward from the nucleus so that the features detected were all ions with the exception of a very small residual C2 emission at 5165 Å. The full labelled tail spectrum of comet Halley is shown in Figure 1. While most of the features detected are attributable to CO+, H2O+, CO2+, CH+ and OH+, there are three moderately strong bands in the spectra of comets Halley and Brorsen-Metcalf which remain unidentified, These features were not detected in comet West. All three spectra in the 4800-5400 Å region are shown in Figure 2, in order of decreasing gas-to-dust ratio, Brorsen-Metcalf being the gassiest of the three. The central wavelengths of the unidentified features are 4930 Å, 5300 Å and 6000 Å. In an effort to identify the ions responsible for these features as well as to confirm previous identifications, laboratory spectra of ions were compared with the comet spectra.

H2O+ and CO+ are known to have extensive emission in the 4800-6200 Å legion. Therefore, the possibility that these unidentified features are due to H2O+ and CO+ was investigated. A search for the following bands was conducted: H2O+ (11-0), H2O+ (12-0), H2O+ (13-1), CO+ (1-1) and CO+ (0-0). CO+ (1-1) and CO+ (0-0) have been previously identified and are present in all the spectra examined here. No positive identifications of the H2O+ (12-0) and (13-1) bands could be made. However, the H2O+ (11-0) band was identified. Comparison of the H2O+ (11-0) synthetic spectrum with comet Brorsen-Metcalf data is shown in-Figure 3. The relative fluxes of the blended H20+ lines in the synthetic spectrum match those of the cometary data. The central peak at 5198 Å may be due to residual night sky [NI] contamination in this region of the data.

After eliminating H2O+ and CO+ as significant contributors to the stronger unidentified features, spectra of ions which have-not yet been identified in the optical region of comets were compared to the data. These ions are NH+, CS+ and C2+ (Figures 4, 5, 6). None of these ions appears to be a significant contributor to the optical spectra of these three comets.

In conclusion, H2O+ (11-0) has--been unambiguously identified in cometary optical spectra. However, no other molecular ions for which there are laboratory spectra can explain the unidentified features found in the ion tails- of comets Halley and Brorsen-Metcalf.