Rare Atoms, Molecules and Radicals in the Coma of P/Halley

We have searched for rare molecules and radicals in the coma of P/Halley using the ion data obtained by IMS-Giotto. Whereas our established methods were used in the ionosphere, a new model was developed for the interpretation of the ion data in the outer coma. Ne/H2O < 1.5 × 10-3was determined in the coma of the comet. Upper limits for the production of Na were derived from the very low abundance of Na+. Methyl cyanide and (probably) ethyl cyanide were identified with abundances of CH3CN/H2O = (1.4 ± .6) × 10-3and C2H5CN/H2O = (2.8 ± 1.6) × 10-4. These results and upper limits for other N-bearing species confirm that nitrogen is depleted in the Halley material. C4H was identified and a point source strength of C4H/H2O = (2.3 ± .8) × 10-3was derived. Our upper limit for C3H is lower than the abundance of C4H. This is in agreement with the enhanced abundances of CnH species with even numbers of C-atoms found in interstellar molecular clouds, suggesting that the C4H in Halley was synthesized under molecular cloud conditions. Thus, C4H and other organics with unpaired electrons may turn out to be indicators for a molecular cloud origin of cometary constituents.