Deuterium in Comet C/1995 O1 (Hale-Bopp)

R. Meier, T.C. Owen (IfA, UHawaii), H.E. Matthews (Joint Astronomy Centre), D.C. Jewitt (IfA, UHawaii), D. Bockelée-Morvan, N. Biver (Obs. de Paris-Meudon), M. Senay (UMass), J. Crovisier (Obs. de Paris-Meudon), D. Gautier (IAP, CNRS)

We report on sub-mm observations of HDO and DCN in comet C/1995 O1 (Hale-Bopp). Both deuterated species have been observed with the 15 m James Clerk Maxwell Telescope on Mauna Kea. On 1997 April 4.9 (UT) we detected the HDO ground-state transition tex2html_wrap_inline14 at 464.925 GHz. The main-beam corrected line area was tex2html_wrap_inline16  K km s tex2html_wrap_inline18 . A detailed model that includes all major excitation mechanisms reveals a D/H in H tex2html_wrap_inline20 O similar to what has been found in other potential Oort cloud comets 1P/Halley and C/1996 B2 (Hyakutake). Three weeks later we made a 10 tex2html_wrap_inline22 detection of the DCN 5-4 transition at 362.046 GHz, showing that deuterium is more abundant in cometary HCN than cometary H tex2html_wrap_inline20 O. This is the first detection of cometary deuterium in a species other than water. The fact that the D/H ratio varies significantly from species to species provides convincing evidence in favor of the interstellar origin of cometary ices and the concomitant absence of extensive radial mixing in the early solar nebula. Based on calculations of ion-molecule reactions in interstellar clouds, the observed D/H ratios suggest a kinetic formation temperature in the range of 25 to 45 K. This temperature is distinctly higher than a temperature of tex2html_wrap_inline28 10 K typically found in dark interstellar clouds.