A quasi-star is born: formation and evolution of accreting quasi-stars as a pathway to Little Red Dots at non-zero metallicity
The recently discovered Little Red Dots identified by the James Webb Space Telescope are compact high-redshift sources whose properties have motivated models involving black holes embedded within optically thick gaseous envelopes. We investigate their rest-frame optical emission by modeling quasi-stars, i.e. stellar envelopes powered by accretion onto a central black hole, formed from rapidly accreting proto-stars that reach the supermassive star regime ($>10^4$~\Msun) before undergoing general relativistic instability. We compute stellar evolution models with mass gain rates of 0.01, 0.1, and 1~\Msun/yr and metallicities $Z=0$-$0.01$. For accretion rates $\ge0.1$~\Msun/yr, stars remain nearly fully convective with $T_\mathrm{eff}\sim4000$-$9000$~K. General relativistic instability occurs at $M_\star\sim3.5\times10^4$~\Msun\ ($6.8\times10^4$~\Msun) for $\dot{m}=0.1$~\Msun/yr (1~\Msun/yr), at $L\sim10^9$~\Lsun. Assuming the black hole supports the envelope until complete accretion ($M_{\rm BH,max}/M_{\rm QS}=1$), quasi-stars reach maximum lifetimes of $10^7$-$10^8$~yr, $\sim100$-$1000$ times longer than their progenitors. Their formation and evolution are nearly independent of metallicity. Matching our models to Little Red Dots at $z<4.5$ ($L_\mathrm{bol}\sim10^{9.5}$-$10^{11.5}$~\Lsun) implies quasi-star masses of $10^{4.5}$-$10^{6.5}$~\Msun, while the minimum observed luminosity requires progenitor accretion rates $\gtrsim0.1$~\Msun/yr. Our models support quasi-stars as the origin of Little Red Dot optical emission and constrain their masses, lifetimes, progenitor environments, and luminosities. Our models offer a framework supporting quasi-stars as the source of Little Red Dot optical emission, and provide insights into their lifetimes, composition, progenitor's environment as well on their minimum and maximum observed luminosities.