Very Massive Stars and High N/O: A Tale of the Nitrogen-enriched Super Star Cluster in the Sunburst Arc

Abstract The lensed Sunburst Arc ( z = 2.369) hosts a young (∼2–4 Myr), massive ( M ⋆ ∼ 10 7 M ⊙ ), compact ( R eff ∼ 8 pc) Lyman-continuum leaking super star cluster, which powers a compact (<10 pc), high-pressure nebula at subsolar metallicity ∼0.2 Z ⊙ and with an anomalously elevated nitrogen-to-oxygen ratio log ( N / O ) ∼ − 0.2 . We present semianalytic models and 3D magnetohydrodynamic simulations in an attempt to reproduce this system. The results indicate that the progenitor giant molecular cloud (GMC) may have M cloud ≳ 3 × 10 7 M ⊙ and R cloud ∼ 70 pc, corresponding to a surface density ∼10 3 –10 4 M ⊙ pc −2 . Incorporating feedback from individual very massive stars (VMSs; ≥100 M ⊙ ) sampled from the Kroupa initial mass function, we find that their winds rapidly enrich ∼10 4 M ⊙ of nearby gas with nitrogen (∼1 dex) and helium (∼0.1–0.2 dex). In the first 1–3 Myr, some cold gas falls to the system center where a central cluster builds up from subcluster mergers. There, the gas is photoionized, pressurized, and chemically enriched by the newly formed VMSs, before being radiatively expelled in the next ∼1 Myr. We find that both VMS feedback and a high-surface-density progenitor GMC are necessary to reproduce the observed nebular properties, such as high N/O, high pressure, and stellar proximity. Low metallicity ( Z ≤ 0.004) may be essential to avoid overproduction of carbon from WC stars. Such enrichment processes localized to compact starbursts may have caused strong nitrogen emission from dense ionized gas as observed in high-redshift galaxies such as GN-z11 and GS_3073.