  We present the first isotopic abundances of both $^{13}$CO and C$^{18}$O in
solar twin stars and test the results against several galactic chemical
evolution (GCE) models with different nucleosynthesis prescriptions. First, we
compare M-band spectra from IRTF/iSHELL to synthetic spectra generated from
custom solar atmosphere models using the PHOENIX atmosphere code. Next, we
compare our calculated abundances to GCE models that consider isotopic yields
from massive stars, asymptotic giant branch (AGB) stars and fast-rotating
stars. The $^{12}$C/$^{13}$C ratios determined for this sample of solar twins
are consistent with predictions from the selected GCE models; however, the
$^{16}$O/$^{18}$O ratios tentatively contradict these predictions. This project
constitutes the first in a stellar chemical abundance series seeking to: (1)
support the James Webb Space Telescope (JWST) as it characterizes exoplanet
atmospheres, interiors, and biosignatures by providing host star abundances (2)
identify how unexplored stellar abundances reveal the process of galactic
chemical evolution and correlate with star formation, interior, age,
metallicity, and activity; and (3) provide improved stellar ages using stellar
abundance measurements. By measuring elemental and isotopic abundances in a
variety of stars, we not only supply refined host star parameters, but also
provide the necessary foundations for complementary exoplanet characterization
studies and ultimately contribute to the exploration of galactic, stellar, and
planetary origins and evolution.
