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H. Cottin (Nasa/Goddard Space Flight Center), M.S. Lowenthal (University of Maryland), R.L. Hudson (Eckerd College, Florida), M.H. Moore (Nasa/Goddard Space Flight Center)
The assignment of the interstellar 4.62 µm (2165 cm-1) infrared feature has been controversial since the band's discovery in 1979 (Soifer et al. 1979). Nevertheless, recent laboratory works have shown that this infrared feature can be assigned to the cyanate ion, OCN- (Demyk et al. 1998, Hudson et al. 2001). If comets contain unaltered interstellar grains, OCN- could then be present on cometary grains and in nuclei. But to date cometary OCN- has not been detected, essentially because the organic component of the solid phase has not yet been investigated through remote sensing. However, by studying the cyanate ion's chemistry we can provide information about its potential presence in comets, and indirect ways to detect it.
In the laboratory, we have synthesized ices containing OCN-, by reaction of HNCO with NH3 and by proton irradiation of CO + NH3 ice mixtures at low temperature (about 10 K). We have studied the stability of OCN- vs photolysis, proton irradiation and temperature, in order to simulate the main processes encountered by grains in the ISM, during and after the formation of the Solar system, or when a comet approaches the Sun. When associated with NH4+ as a counter-ion (to form ammonium cyanate NH4OCN), OCN- is extremely stable to photolysis or radiolysis. But, on the other hand, its thermal decomposition becomes efficient near 200 K and has been quantitatively studied. Hence, if NH4OCN is present on cometary nuclei, its degradation products (suspected to be HNCO and NH3) might be produced when the temperature of the nucleus rises as it gets closer to the Sun. Moreover, NH4OCN could be ejected from the nucleus into the coma on warm grains and then dissociate to become extended sources of molecules such as HNCO and NH3. Those implications will be discussed in light of our experimental results.
This research is funded through NRA 344-33-01 and 344-02-57.
Demyk, K. et al. 1998, Astron. Astophys., 339, 553 Hudson, R.L.et al. 2001, ApJ, Volume 550, Issue 2, pp. 1140-1150, 550, 1140 Soifer, B.T. et al. 1979, ApJL, 232, L53
The author(s) of this abstract have provided an email address for comments about the abstract: hcottin@lepvax.gsfc.nasa.gov