Stellar Evolution with Radiative Feedback in AGN Disks

Abstract

Stars embedded in the inner pc region of an active galactic nucleus (AGN) experience extreme accretion conditions that significantly alter their evolution. We present one-dimensional MESA simulations of stars growing and decaying within AGN disks, implementing radiative-feedback-regulated accretion which limits stellar growth near the Eddington luminosity, as well as wind-driven mass loss. Unlike stand-alone stars in the field, these embedded stars follow unique evolutionary tracks with well-determined mass evolution and chemical yields. We distinguish two regimes: ``immortal" stars that indefinitely remain on the main sequence due to efficient hydrogen mixing; and ``metamorphic" stars that evolves off the main sequence, ultimately enriching the disk with heavy elements upon hydrogen and helium exhaustion in their cores. Results indicate that embedded stars in AGN disks can attain large masses, but gas retention and limited mixing likely render the ``immortal" track unsustainable. We show radiative feedback plays a critical role in preventing runaway growth, since it regulates the inflow to at most of order-unity the Eddington-limited mass-loss rate. Embedded metamorphic stars significantly enrich AGN disks with helium and $α$-elements, potentially explaining the observed high metallicity in broad-line regions (BLR) without excessive helium enrichment. This study underscores the critical interplay between stellar feedback and accretion physics in shaping the stellar populations and chemical evolution within AGN disks.

Figures (5)

• Figure 1: Schematic of a star embedded in an AGN disk. Disk gas feeds the star at a Bondi–Hoyle rate but radiative feedback limits accretion once luminosity nears a fraction of $L_{\rm Edd}$, with excess energy driving winds. With extra mixing (upper branch), the star remains "immortal" on the main sequence; without it (lower branch), the star becomes metamorphic and returns metal-rich winds to the disk.
• Figure 2: Stellar mass (top) and radius (bottom) evolution. Panels (a,c) vary feedback parameter $\lambda_0$; panels (b,d) vary disk density. Solid lines show standard mixing; triangles denote extra mixing (immortal track). Feedback limits runaway growth, larger $\lambda_0$ allows higher masses, while ambient density has little effect.
• Figure 3: Evolution of net mass–exchange rate $\dot M_{\rm net}$, i.e. the accretion rate deducts the absolute value of the wind–loss rate is shown in the upper panel. The Eddington ratio $L_\star/L_{\mathrm{Edd, \star}}$ is shown in the lower panel for the fiducial model with $\rho_{\mathrm{c}}=10^{-16}\, \mathrm{g\,cm^{-3}}$, $Y_{\rm d} = 0.25$, and various values of $\lambda_{0}$. Opaque curves indicate main–sequence and partially transparent curves represent post–main–sequence phases after core hydrogen exhaustion. The dashed line represents the immortal–star model with $\lambda_0 = 0.5$ and extra mixing.
• Figure 4: Stellar evolutionary tracks for varying disk helium mass fractions $Y_{\rm d}=0.3$–$0.7$, with fixed $\rho_{\rm AGN} = 10^{-16} \mathrm{g\,cm^{-3}}$ and $\lambda_0 = 0.75$. Panels show the evolution of (a) stellar mass $M_\star$, (b) helium mass fraction $Y_\star$, (c) luminosity $\log L_\star$, and (d) the $Y_\star$–$M_\star$ relation during main–sequence evolution. As $Y_{\rm d}$ increases, higher mean molecular reduces the mass–to–light ratio $\Upsilon$, causing stars to reach the feedback limit with lower final masses. Panel (d) shows the best–fit power–law relation (grey dashed) between $Y_\star$ and $M_\star$ during equilibrium growth, as predicted by Eq. (\ref{['eq:y_time']}). Because models start from 10$M_\odot$, the ramp–up time is shortened.
• Figure 5: Metamorphic star's cumulative C, N, and O yields (wind minus gain) with $D_{\mathrm{mix, rad}}=10^7\,\mathrm{cm^2\,s^{-1}}$ (red) vs $D_{\mathrm{mix, rad}}=10^5\,\mathrm{cm^2\,s^{-1}}$ (green) during pre and main–sequence phases 1+2 (top left), He-burning phase 3 (top right), and carbon-burning phase 4 (middle left). Pre-collapse mass of various elements and compositional stratification inside a metamorphic star are shown in the middle-right and lower-left panels respectively. Immortal star's steady He and N yield versus C and O drain, accumulated over $\approx$5 Myrs, are shown in the bottom-right panel. All stellar models use $Y_{\rm d}=0.25$, $\lambda_0=0.75$, and $\rho_{\rm c}=10^{-16}\,\mathrm{g\,cm^{-3}}$.