Our aim is to determine the isotopic ratios ^ { 12 } C/ ^ { 13 } C and ^ { 14 } N/ ^ { 15 } N in a variety of comets and link these measurements to the formation and evolution of the solar system .
The ^ { 12 } C/ ^ { 13 } C and ^ { 14 } N/ ^ { 15 } N isotopic ratios are measured for the CN radical by means of high-resolution optical spectra of the R branch of the B-X ( 0,0 ) violet band .
23 comets from different dynamical classes have been observed , sometimes at various heliocentric and nucleocentric distances , in order to estimate possible variations of the isotopic ratios in parent molecules .
The ^ { 12 } C/ ^ { 13 } C and ^ { 14 } N/ ^ { 15 } N isotopic ratios in CN are remarkably constant ( average values of , respectively , 91.0 \pm 3.6 and 147.8 \pm 5.7 ) within our measurement errors , for all comets whatever their origin or heliocentric distance .
While the carbon isotopic ratio does agree with the terrestrial value ( 89 ) , the nitrogen ratio is a factor of two lower than the terrestrial value ( 272 ) , indicating a fractionation in the early solar system , or in the protosolar nebula , common to all the comets of our sample .
This points towards a common origin of the comets independently of their birthplaces , and a relationship between HCN and CN .