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

TL;DR

The study tackles the rapid nitrogen enrichment observed in the Sunburst Arc LyC cluster by combining a semi-analytic wind-enrichment toy model with 3D magnetohydrodynamic simulations that explicitly include Very Massive Stars (VMSs) and PARSEC-based chemical yields. The results show that reproducing the high N/O and the high-pressure, compact nebular environment requires a leaky giant molecular cloud hosting VMSs (with $m\gtrsim100\,M_\odot$) in a low-metallicity regime of $Z\sim0.002$--$0.004$, a high star-formation efficiency, and significant gas outflows; under these conditions, VMS winds enrich approximately $10^4\,M_\odot$ of nearby gas by about $1$ dex in N and $0.1$--$0.2$ dex in He within the first $1$--$3\, m Myr$, prior to radiative expulsion. The simulations imply a central cluster forms from sub-cluster mergers, is then photoionized and chemically enriched by the newly formed VMSs, and is subsequently cleared by winds on a timescale of about $1\, m Myr$. This VMS-driven, localized enrichment mechanism offers a plausible explanation for nitrogen-enriched nebulae in high-redshift galaxies and provides a framework to interpret similar abundance anomalies in systems like GN-z11 and GS_3073, highlighting the role of compact, high-surface-density starbursts in shaping early chemical evolution.

Abstract

The lensed Sunburst Arc ($z = 2.369$) hosts a young ($\sim2$--$4\,\rm Myr$), massive ($M_\star \sim 10^7\,M_\odot$), compact ($R_{\rm eff} \sim 8\,\rm pc$) Lyman-continuum (LyC) leaking super star cluster, which powers a compact ($< 10\,\rm pc$), high-pressure nebula at sub-solar metallicity $\sim0.2\,Z_\odot$ and with an anomalously elevated nitrogen-to-oxygen ratio $\log({\rm N/O}) \sim -0.2$. We present semi-analytic models and 3D magnetohydrodynamic simulations with radiative feedback in an attempt to reproduce this system. The results indicate that the progenitor giant molecular cloud (GMC) may have $M_{\rm cloud} \gtrsim 3 \times 10^7\,M_\odot$ and $R_{\rm cloud} \sim 70\,\rm pc$, corresponding to a surface density $\sim10^3$--$10^4\,M_\odot\,{\rm pc}^{-2}$. Incorporating feedback from individual Very Massive Stars (VMSs; $\ge 100\,M_\odot$) sampled from the Kroupa initial mass function, we find that their winds rapidly enrich $\sim 10^4\,M_\odot$ of nearby gas with nitrogen ($\sim 1\,$dex) and helium ($\sim 0.1$--$0.2\,$dex). In the first $1$--$3\,$Myr, some cold gas falls to the system center where a central cluster builds up from sub-cluster mergers. There, the gas is photoionized, pressurized, and chemically enriched by the newly formed VMSs, before being radiatively expelled in the next $\sim1\,\rm Myr$. 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 ($Z \le 0.004$) may be essential to avoid overproduction of carbon from WC stars. Such enrichment processes localized to compact starburst events may have caused strong nitrogen emission from dense ionized gas as observed in high-redshift galaxies such as GN-z11 and GS_3073.

Figures (5)

• Figure 1: Mass evolution of star formation and feedback in the toy model (§ \ref{['sec:toy_model']}), including the total mass of stars $M_\star$ and the mass of the pristine gas $M_{\rm pristine}$ (without or with feedback-driven outflow). Moreover, we show the cumulated mass of wind ejecta $M_{\rm ejecta}$ from stellar populations of different initial metallicity $Z$ and IMF cutoff $m_{\rm max}$.
• Figure 2: textcolorblackFitting relations of log(N/O) (Eq. \ref{['equ:no_fitting']}; top) and log(C/O) (Eq. \ref{['equ:co_fitting']}; bottom), which match the solar metallicity used in FIRE WiersmaSchayeSmith_2009MNRAS.393...99WHopkinsWetzelWheeler_2023MNRAS.519.3154H. We also present data points for log(N/O) MollaVilchezDiaz_2007astro.ph..1691M and log(C/O) BergSkillmanHenry_2016ApJ...827..126B, where the low-redshift LyC leaking galaxies (red dots) are compiled from IzotovSchaererWorseck_2023MNRAS.522.1228I.
• Figure 3: Evolution of the relative nitrogen (top) and carbon (bottom) of the interstellar medium in the toy model (§ \ref{['sec:toy_model']}). In each panel, we show 6 tracks with various initial metallicities $Z$, and the rough estimated range of the Sunburst Arc cluster for reference PascaleDaiMcKee_2023ApJ...957...77P. From left to right, in each column is the model with different assumptions (closed box or leaky box, with or without VMSs of $> 100\,M_\odot$). In particular, we also show the abundance ratios in the stellar wind (i.e., without mixing with the interstellar medium; dashed lines).
• Figure 4: Mass evolution of VMSs compiled from the PARSEC results CostaShepherdBressan_2025AA...694A.193C. Here we present the evolution tracks (solid) for stars with ZAMS mass between $100\,M_\odot$ and $300\,M_\odot$ and $Z=0.004, 0.014$. We also show fittings of these evolution tracks before (following Eq. \ref{['equ:mass_loss_fitting']}; dashed) and after (linear relation; dotted) the "kink."
• Figure 5: The SNe chemical yield of massive stars and VMSs compiled from the PARSEC data set, for stars with initial metallicity $Z=0.004$. Top panel - the SNe yield as a function of the zero-age main-sequence (ZAMS) stellar mass for selected elements (C, N, O, Mg, Fe); note that the fate of a star of a given metallicity is dependent on its ZAMS mass (color shaded regions), including core-collapse SN (CCSN), failed SN (FSN), pulsational pair-instability SN (PPISN), pair-instability SN (PISN), and direct-collapse black hole (DCBH). Bottom panel - the SNe yield as a function of stellar age.