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

摘要

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/H_2O < 1.5 X 10~(-3) was 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 CH_3CN/H_2O = (1.4 +- 6) X 10~(-3) and C_2H_5CN/H_2O = (2.8 +- 1.6) X 10~(-4). These results and upper limits for other N-bearing species confirm that nitrogen is depleted in the Halley material. C_4H was identified and a point source strength of C_4H/H_2O = (2.3 +- 8) X 10~(-3) was derived. Our upper limit for C_3H is lower than the abundance of C_4H. This is in agreement with the enhanced abundances of C_nH species with even numbers of C-atoms found in interstellar molecular clouds, suggesting that the C_4H in Halley was synthesized under molecular cloud conditions. Thus, C_4H and other organics with unpaired electrons may turn out to be indicators for a molecular cloud origin of cometary constituents.