Oxygen Isotopes Reveal Low-mass Star Dominance in the Small Magellanic Cloud

Oxygen isotope abundances and their ratios are fingerprints of stellar evolution, and therefore provide a powerful tool in tracing the enrichment history of galaxies. However, their behavior in low-metallicity dwarf galaxies remains largely unexplored. The Small Magellanic Cloud (SMC), a nearby analog of young high-redshift galaxies, offers an ideal laboratory to investigate this regime. Using the Atacama Compact Array, we observed the J = 2 → 1 transitions of 12 CO, 13 CO, C 18 O, and C 17 O from the massive star-forming region LIRS 36 (aka N12A), achieving the first detection of C 17 O in the SMC. This detection enables the first direct measurement of the 18 O/ 17 O abundance ratio of 0.87 ± 0.26 in this galaxy, which is substantially lower than all values in the literature, including molecular clouds in the Milky Way and other galaxies. Such a low ratio of 18 O/ 17 O, together with a high 13 CO/C 18 O ratio, indicates chemical enrichment dominated by low-mass stars, consistent with the observed paucity of high-mass stars in the SMC. We suggest that the SMC is governed by a top-light integrated galaxy-wide initial mass function, predicted by the SMC’s persistently low star formation activities.