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Y. Benilan (LISA, UMR 7583 du CNRS, Univ. Paris 7 et 12), H. Cottin (NASA Goddard Space Flight Center), N. Fray, M.-C. Gazeau, F. Raulin (LISA, UMR 7583 du CNRS, Univ. Paris 7 et 12)
Measurements performed with the NMS mass spectrometer on board Giotto, show that the formaldehyde density profile, as a function of the distance to nucleus of comet Halley, cannot be explained by a direct release of formaldehyde from the nucleus. Thus, there is an extended source of formaldehyde in comet Halley. Such an extended source of formaldehyde has also been detected in comets Hyakutake and Hale-Bopp. Polyoxymethylene (POM) has been often pushed forward to interpret H2CO extended source but until very recently important quantitative data were missing to allow a discerning analysis of this hypothesis.
Formaldehyde production rate from POM on solid grains is a function of several unknown parameters: the matrix in which POM is imbedded, the geometry of the grains, the chemical structure of the polymer, its photodegradation rate and its temperature. We have obtained experimental data concerning the photo and thermal degradation of POM and modeled the contribution of solid POM present on particles to the gaseous phase. First results were presented in (Cottin et al., 2001) for a single size population of grains, and temperature as free parameter. We have now extended our model to a more realistic grain population, based on actual measurements in Halley coma, for which velocity and temperature of each grain are a function of its size and composition.
We have calculated the best fits to Giotto measurements and derived the amount of POM required to account H2CO observation. The results show that the presence of solid POM on grains can provide a realistic explanation of the formaldehyde extended source observed in several comets. Even if the model still needs to be improved, for example to include different grains geometry, it can already be used to test other polymers like poly-HCN against known extended sources such as CN, C2 or C3, as soon as laboratory measurements are available.
Cottin, H., Gazeau, M.C., Bénilan, Y. & Raulin, F. 2001, ApJ, 556, 417