Unusual Isotopic Abundances in a Fully Convective Stellar Binary

I. J. M. Crossfield1, J. D. Lothringer2, B. Flores1,3, E. A. C. Mills4, R. Freedman5,6, J. Valverde1,7,8, B. Miles9, X. Guo1, and A. Skemer9
Astrophysical Journal Letters 871, L3 Link to Article [DOI: 10.3847/2041-8213/aaf9b6]
1Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA
2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
3Department of Physics and Astronomy, California State University Northridge, Northridge, CA, USA
4Department of Physics, Brandeis University, Waltham, MA, USA
5NASA Ames Research Center, Moffett Field, CA, USA
6SETI Institute, Mountain View, CA, USA
7Department of Physics, University of California, Santa Cruz, Santa Cruz, CA, USA
8Chabot-Las Positas Community College, Dublin, CA, USA
9Department of Astronomy, University of California, Santa Cruz, Santa Cruz, CA, USA

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.

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