Abstract :
[en] In the past few years, high-resolution (R ˜ 70,000) and high quality spectra of the CN B^2Sigma^+ - X^2Sigma^+ (0,0) band (at 388 nm) of several Oort Could comets have been collected by our team in different observatories and have allowed the first optical detections of [SUP]12[/SUP]C[SUP]15[/SUP]N. Observations with first class spectrographs on large 8m telescopes, such as UVES at the ESO VLT, have shown that such very efficient equipment are able to provide isotopic ratios for comets as faint as m_r ˜ 9 (heliocentric magnitude), opening up the possibility of carrying out a systematic analysis over a significant sample of comets. While the measured [SUP]12[/SUP]C/[SUP]13[/SUP]C isotope ratios in the different comets are in good agreement with the Solar system value (89), the derived [SUP]14[/SUP]N/[SUP]15[/SUP]N ratios are only about half the value in Earth's atmosphere (272), indicating an excess of [SUP]15[/SUP]N by a factor of about 2 with respect to the "cosmic" value. It is striking to note that the optical determinations of [SUP]14[/SUP]N/[SUP]15[/SUP]N in the different comets are consistent with each other (140 � 30), but not with the ratios derived for comet Hale-Bopp from sub-millimeter measurements on HCN, generally believed to be the main parent of CN. This discrepancy could indicate the existence of (an) other unknown parent(s) of CN, with an even higher [SUP]15[/SUP]N excess. Organic compounds like those found in interplanetary dust particles (IDPs) are good candidates. Further determinations from HCN are now badly needed in other comets. Obtaining the value of the N isotopic ratio in Jupiter-family short-period comets would be of great significance in view of their presumed different place of birth. Much is also expected in this respect, as in many others, from several space missions such as Deep Impact, Stardust, and Rosetta.
