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N. Fray, Y. Benilan (LISA), N. Biver, D. Bockelee-Morvan (LESIA), H. Cottin (LISA), J. Crovisier (LESIA), M.-C. Gazeau (LISA)
The H2CO density profile measured inside the coma of comet 1P/Halley by Giotto spacecraft is not coherent with a production of H2CO from the nucleus sublimation alone. The H2CO is in part produced by an “extended source” and we have shown that the decomposition by heating and UV irradiation of polyoxymethylene [formaldehyde polymers: (-CH2-O-)n also called POM] present on cometary grains can produce gaseous H2CO needed to explain the density profile (Cottin et al., 2004).
In comet C/1995 O1 (Hale-Bopp), the production rates of H2CO display a much steeper evolution with heliocentric distance than others species of similar volatility, like HCN or H2S. This behavior seems also characteristic of molecules produced by “extended sources” (Biver et al., 2002). Moreover the “extended source” of H2CO in comet Hale-Bopp has been established by interferometric observations (Wink et al., 1999).
We have modeled H2CO production rates as a function of the heliocentric distance in comet Hale-Bopp considering three mechanisms of H2CO production: 1.) direct sublimation from the nucleus, 2.) thermal degradation of POM on grains and 3.) photolytic degradation of POM on grains. These production pathways exhibit very different heliocentric evolutions. We show that the slope of the H2CO production rates measured as a function of the heliocentric distance can be accurately reproduced if one considers the presence of a few percent of solid POM on grains. Two production zones can be distinguished: close to the perihelion, observed H2CO could be almost entirely produced by the thermal decomposition of POM, whereas for heliocentric distance greater than 2 AU, H2CO could be produced in majority by sublimation from the nucleus and photolytic degradation of POM.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.