  It has been suggested that a class of chemically peculiar metal-poor stars
called iron-rich metal-poor (IRMP) stars formed from molecular cores with metal
contents dominated by thermonuclear supernova nucleosynthesis. If this
interpretation is accurate, then IRMP stars should be more common in
environments where thermonuclear supernovae were important contributors to
chemical evolution. Conversely, IRMP stars should be less common in
environments where thermonuclear supernovae were not important contributors to
chemical evolution. At constant $[\text{Fe/H}] \lesssim -1$, the Milky Way's
satellite classical dwarf spheroidal (dSph) galaxies and the Magellanic Clouds
have lower $[\text{$\alpha$/Fe}]$ than the Milky Way field and globular cluster
populations. This difference is thought to demonstrate the importance of
thermonuclear supernova nucleosynthesis for the chemical evolution of the Milky
Way's satellite classical dSph galaxies and the Magellanic Clouds. We use data
from the Sloan Digital Sky Survey (SDSS) Apache Point Observatory Galactic
Evolution Experiment (APOGEE) and Gaia to infer the occurrence of IRMP stars in
the Milky Way's satellite classical dSph galaxies $\eta_{\text{dSph}}$ and the
Magellanic Clouds $\eta_{\text{Mag}}$ as well as in the Milky Way field
$\eta_{\text{MWF}}$ and globular cluster populations $\eta_{\text{MWGC}}$. In
order of decreasing occurrence, we find
$\eta_{\text{dSph}}=0.07_{-0.02}^{+0.02}$,
$\eta_{\text{Mag}}=0.037_{-0.006}^{+0.007}$,
$\eta_{\text{MWF}}=0.0013_{-0.0005}^{+0.0006}$, and a 1-$\sigma$ upper limit
$\eta_{\text{MWGC}}<0.00057$. These occurrences support the inference that IRMP
stars formed in environments dominated by thermonuclear supernova
nucleosynthesis and that the time lag between the formation of the first and
second stellar generations in globular clusters was longer than the
thermonuclear supernova delay time.
