Oxygen isotopes reveal low-mass star dominance in the Small Magellanic Cloud

Abstract

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 $J=2\to 1$ 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$\pm$0.26 in this galaxy, 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.

12 pages, 3 figures, 3 tables, accepted for publication in ApJ