High nitrogen and carbon isotopic ratios in the interstellar comet 3I/ATLAS

Abstract

Interstellar objects provide a unique opportunity to further our understanding of the planetary formation process by studying in detail material formed around another star. Their ices contain precious clues about the environment and conditions prevailing in their home system. As fractionation processes can be sensitive to the temperature and radiation environment, isotopic ratios are powerful tracers of the origin and evolution of different species. While isotopic ratios have been measured in solar system comets, previously detected interstellar objects have been too faint to measure isotopic ratios. Here we report the measurement of two ratios in 3I/ATLAS from observations of the CN molecule: $^{12}$C/$^{13}$C and $^{14}$N/$^{15}$N. We report $^{12}$C/$^{13}$C=$147^{+87}_{-40}$ and $^{14}$N/$^{15}$N=$343^{+454}_{-124}$. The $^{14}$N/$^{15}$N is higher than the value of $\sim$~150 usually measured for solar system comets, close to the values measured in the interstellar medium, pre-stellar phases or the outside of protoplanetary discs. The $^{12}$C/$^{13}$C is marginally higher than the values usually measured for solar system comets and in the interstellar medium. These measurements could indicate an origin of 3I in the outer disc around an older low-metallicity star.

Figures (3)

• Figure 1: Co-added observed spectra of 3I with best-fit model Observed and modelled $^{13}$CN-centred (left) and C$^{15}$N-centred (right) line profiles. The shaded region represents the uncertainty of the profile resulting from the combination of the different lines, estimated from the MCMC fitting.
• Figure 2: Comparison of $^{14}$N/$^{15}$N in different types of objects$^{14}$N/$^{15}$N isotopic ratios in solar system comets measured from HCN, CN, and NH$_2$ are at the bottom, with different molecules represented by different symbols (values and references are listed in Table \ref{['tab:n_ratios']}). 3I is the red circle, with a $^{14}$N/$^{15}$N ratio measured in CN. Values for protoplanetary discs, pre-stellar cores, and protostars follow the same convention. Values from protoplanetary discs (PPD) come from Hily-Blant2019Rampinelli2025. Prestellar $^{14}$N/$^{15}$N values come from Jensen2024 for HCN and from Nomura2023 for the other molecules.
• Figure 3: Comparison of $^{12}$C/$^{13}$C in different types of objects$^{12}$C/$^{13}$C isotopic ratios in solar system comets were measured from various molecules, with different molecules represented by different symbols. Plotted values are the weighted average and uncertainty of all published value listed in Table \ref{['tab:c_ratios']}; in some cases multiple measurements were included for a single comet. The 67P point represents the weighted average of nine measurements of eight molecules obtained in situ during the Rosetta mission. 3I is the red circle. Values for protoplanetary discs Zhang2017Hily-Blant2019Yoshida2022Rampinelli2025, pre-stellar cores Nomura2023, galactic Milam2005, and ISM Wilson1999 follow the same color convention and are identified in the legend.