Unusual Isotopic Abundances in a Fully Convective Stellar Binary

Low-mass M dwarfs represent the most common outcome of star formation, but their complex emergent spectra hinder detailed studies of their composition and initial formation. The measurement of isotopic ratios is a key tool that has been used to unlock the formation of our solar system, the Sun, and the nuclear processes within more massive stars. We observed GJ 745AB, two M dwarfs orbiting in a wide binary, with the NASA Infrared Telescope Facility/iSHELL spectrograph. Our spectroscopy of CO in these stars at the 4.7 μm fundamental and 2.3 μm first-overtone rovibrational bandheads reveals ${}^{12}{{\rm{C}}}^{16}{\rm{O}}$, ${}^{13}{{\rm{C}}}^{16}{\rm{O}}$, and ${}^{12}{{\rm{C}}}^{18}{\rm{O}}$ in their photospheres. Because the stars are fully convective, the atomic constituents of these isotopologues should be uniformly mixed throughout the stars' interiors. We find that in these M dwarfs, both ${}^{12}{\rm{C}}$/${}^{13}{\rm{C}}$ and ${}^{16}{\rm{O}}$/${}^{18}{\rm{O}}$ greatly exceed the Solar values. These measurements cannot be explained solely by models of Galactic chemical evolution, but require that the stars formed from an interstellar medium significantly enriched by material ejected from an exploding core-collapse supernova. These isotopic measurements complement the elemental abundances provided by large-scale spectroscopic surveys, and open a new window onto studies of Galactic evolution, stellar populations, and individual systems.