arXiv subjects

J. Roman-Garza

Publications and source records attributed to J. Roman-Garza.

3 recordsLinked to original sources

arXiv2026

Ballistic mixing: stellar mergers drive internal mixing in extremely massive stars, with implications for proto-globular cluster pollution

Extremely massive stars, with masses of $10^3$-$10^4$ M$_\odot$, have been proposed as potential sources of the chemical enrichment associated with multiple populations in globular clusters. Early on the main sequence, these stars reach central temperatures of 65 to 75 MK, enabling nucleosynthesis through the CNO, NeNa, and MgAl cycles, producing abundance patterns similar to those observed in the so-called cluster's second population. Stellar mergers are among the leading formation channels for extremely massive stars. In this work, we follow the formation of a $\sim6700$ M$_\odot$ star via 253 stellar mergers, drawn from a direct $N$-body star cluster simulation with properties similar to the densest star clusters observed at redshift $z\sim10$. We evolve each star as a 1D stellar model and construct the merger products from mixing and shock heating prescriptions calibrated to 3D simulations of parabolic collisions. We find that entropy-based mixing efficiently transports nuclear-processed material from the convective core to the envelope, while merger-driven heating promotes chemical homogenization. As a result, the stellar surface becomes enriched with H-burning products, producing abundance patterns consistent with those observed in globular cluster multiple populations and supporting the role of merger-formed extremely massive stars as potential sources of early chemical enrichment in proto-globular cluster-like conditions.

astro-ph.GA↗

arXiv2026

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.

astro-ph.SR↗

arXiv2026

Massive stellar cannibals: How stellar mergers drive mass-loss in extremely massive stars

It has been theorized that the formation of extremely massive and supermassive stars ($>10^3\ {\rm M}_\odot$) could plausibly be the outcome of stellar mergers in low metallicity ($Z<10^{-1}$~Z$_\odot$) and dense ($\gtrsim10^3\ {\rm M}_\odot\ {\rm pc}^{-3}$) stellar environments. These objects remain relevant as they can serve as the progenitors of intermediate-mass black holes and they are also formidable chemical polluter candidates, as evidenced by the peculiar abundances seen across cosmic history. This work investigates merger-induced mass loss in extremely massive stars within a hydrodynamic framework and provides a prescription derived from the simulations to estimate both the mass loss and the outcome of the interaction. We adapted the 1D hydrodynamic, stellar structure, and evolution code MESA to simulate stellar inspirals. In our simulations, we considered stars of $>1000\,\rm M_{\odot}$ with inspiraling companions of $<100$ M$_\odot$; hence, with mass ratios of $<0.1$. As the inspiral progresses, the orbital energy of the system is lost through the hydrodynamic and gravitational drag forces. This energy gets deposited as thermal energy in the extremely massive star's envelope. We find that the total ejected mass is $\sim$10-30$\%$ of the system's mass. Our results point out that most of the energy deposited by the inspiral is used to eject mass. These findings demonstrate that merger-induced mass loss is non-negligible for the considered configurations. Thus, it is an important process to account for when investigating the formation of extremely massive stars and predicting their possible role throughout cosmic history.

astro-ph.SR↗