The CN isotopic ratios in comets

Abstract

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 ± 3.6 and 147.8 ± 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.

The CN/C15N isotopic ratio towards dark clouds

“…Isotopic ratios have proven extremely powerful in providing quantitative constraints on the composition of the protosolar disk from which the solar system formed (Chaussidon & Gounelle 2007;Jehin et al 2009). For carbon, the isotopic ratio 12 C/ 13 C measured in comets shows a remarkable homogeneity, with a mean value of 91 that agrees with the solar value of 90, and is also very close to the value of 70 determined in the local ISM (Milam et al 2005;Asplund et al 2009;Manfroid et al 2009). For oxygen, measurements in comets lead to 16 O/ 18 O ratios between 500 and 600, which again are in good agreement with the solar and local ISM value of 500 (Wilson & Rood 1994).…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…heavily enriched in 15 N) that obtained for HCN (e.g. Arpigny et al 2003;Manfroid et al 2009;Hutsemékers et al 2005;Bockelée-Morvan et al 2008). This differential between radio and visible measurements extended to Hale-Bopp (Arpigny et al 2003), which had already been shown to have a consistent isotopic ratio for carbon, as discussed above.…”

“…Of these, the CN and HCN measurements are the most extensive (e.g. Manfroid et al 2009), while C 2 measurements have the longest history (e.g. Danks et al 1974;Vanysek 1973;Owen 1973;Lambert and Danks 1983), extending from comet Ikeya (1963a) (Stawikowski and Greenstein 1964) through comet C/2002 T7 (LINEAR) (Rousselot et al 2012).…”

“…Danks et al 1974;Vanysek 1973;Owen 1973;Lambert and Danks 1983), extending from comet Ikeya (1963a) (Stawikowski and Greenstein 1964) through comet C/2002 T7 (LINEAR) (Rousselot et al 2012). The most recent compilations of data (Jehin et al 2009;Manfroid et al 2009 and references therein) include measurements of 23 comets including 8 JFCs and 15 OCCs at heliocentric distances from 1-3 AU and covering multiple compositional classes (e.g. A 'Hearn et al 1995;Fink 2009).…”

“…We know a little bit more about the isotopic composition of comets, thanks to spectroscopic measurements of the D/H and N isotopes (Bockelée-Morvan et al 2008;Ceccarelli et al 2014;Hartogh et al 2011a;Manfroid et al 2009;Rousselot et al 2014;Shinnaka et al 2014;Altwegg et al 2015). As described in Sect.…”