Inflated Supermassive Stars as Little Red Dots and Progenitors of Supermassive Black Holes

Wenbin Lu

First listed 2026-09-22 | Last updated 2026-09-18

Abstract

JWST has uncovered an abundant population of compact, red sources at high redshifts, termed little red dots (LRDs). We study whether they could be explained by thermally relaxed, hydrogen-burning, metal-enriched supermassive stars (SMSs). We construct 1D models in hydrostatic and thermal equilibrium, including a radiation-pressure-dominated, CNO-burning core and an outer envelope with metallicity-dependent opacity, non-adiabatic convection, and an effective treatment of super-Eddington surface layers. The luminosity is close to the Thomson-scattering Eddington limit. The Fe-opacity bump drives the formation of a strongly inflated, low-mass envelope, while hydrogen recombination terminates the envelope at a bound photosphere. For M=$10^4$ to $10^6$ Msun and metal mass fractions $Z=10^{-4}$ to $10^{-2}$, we find that the most metal-rich models (Z ~ $10^{-2}$) develop highly inflated envelopes and reach Teff ~ 7000 K. We suggest that such enriched SMSs could instead be assembled through runaway stellar collisions in compact star clusters. The GR instability sets a maximum mass that depends on the core rotation rate. The nonrotating and rotating models have maximum masses $3\times10^5$ to $3\times 10^6$ Msun, and maximum luminosity of the order $10^{44}$ erg/s, comparable to the observed bright-end cutoff of the LRD luminosity function. Core hydrogen depletion can drive initially stable SMSs across the instability threshold after a lifetime of order 1 Myr. If each LRD leaves a black hole retaining most of its mass, the observed LRD abundance implies a present-day remnant density of order $10^{-2}\rm cMpc^{-3}$, consistent with the local abundance of supermassive black holes. Cool SMSs naturally produce weak X-ray and weak high-ionization lines. Non-LTE effects may significantly modify the Balmer features and the continuum opacity, and may lower Teff below our LTE value.

Short digest

Lu develops hydrostatic, thermally relaxed models of metal-enriched, hydrogen-burning supermassive stars as a stellar interpretation for JWST little red dots. In the most metal-rich cases, an iron-opacity bump inflates a low-mass envelope to cool photospheric temperatures near 7000 K, producing compact, luminous, nearly Eddington sources in the LRD-like regime while naturally remaining weak in X-rays and high-ionization lines. General-relativistic instability limits their masses to roughly 3×10^5–3×10^6 Msun depending on core rotation, yielding a luminosity ceiling near 10^44 erg s^-1 that matches the observed bright-end LRD cutoff; hydrogen depletion can then trigger collapse after about 1 Myr, leaving massive black-hole seeds. The key caveat is atmospheric physics: non-LTE Lyα trapping and hydrogen-level populations could substantially alter Balmer features, continuum opacity, and the predicted effective temperature.

Key figures to inspect

• Figure 7. This HR diagram provides the clearest population-level link to LRD phenomenology: increasing metallicity moves the models onto a cool, inflated branch, with Z≈10^-2 solutions reaching the LRD-like temperature range while retaining near-Eddington luminosities.
• Figure 9. This figure shows how general-relativistic instability sets the model’s maximum mass and how rotation materially shifts that threshold, underpinning the proposed 3×10^5–3×10^6 Msun progenitor mass range and luminosity cutoff.
• Figure 11. This remnant-abundance calculation connects the short SMS phase to the present-day supermassive-black-hole population, showing that an order-1 Myr lifetime can yield a descendant density comparable to local SMBH estimates after allowing for mergers.
• Figure 1. The representative stellar cross section makes the physical mechanism concrete: a CNO-burning core radiates near the Thomson Eddington limit, while the Fe-opacity bump drives the extended convective envelope that creates the cool photosphere.