arXiv subjects

Amedeo Romagnolo

Publications and source records attributed to Amedeo Romagnolo.

13 recordsLinked to original sources

arXiv2026

The Stellar Winds Atlas I: Current uncertainties in mass-loss rates

Stellar winds are a major source of uncertainty in understanding the life and deaths of massive stars. Across studies in the field, prescriptions for stellar winds differ substantially in both their physical assumptions and implementation, making them a dominant contributor to model-to-model variation. In this work, we present a systematic analysis of the physical assumptions underlying commonly adopted wind prescriptions for optically thin and optically thick winds of hot stars, as well as the winds of cool supergiants. Our analysis reveals substantial discrepancies across all regimes: predicted mass-loss rates for optically thin winds differ by more than an order of magnitude, while rates for cool supergiants vary by several orders of magnitude, with even wider uncertainties arising in extrapolation regimes beyond the Humphreys-Davidson limit. These disparities introduce significant ambiguity into the predicted formation of Wolf-Rayet (WR) stars, a problem further compounded by the inconsistent application of transition criteria. A central issue is the "cool Wolf-Rayet problem", a temperature regime where the classical electron-scattering Eddington factor ($Γ_{\rm e}$) loses physical consistency. Because this factor is widely used to determine WR mass-loss rates, its failure forces models to rely on uncertain extrapolations and ad-hoc corrections. We conclude that the dominant stellar wind uncertainties arise from a mismatch between the physical assumptions in stellar wind models and the structure of the stars to which they are applied. Our framework clarifies the origins of current theoretical discrepancies and identifies the key physical bottlenecks that must be addressed to improve mass-loss modeling for massive stars.

astro-ph.SR↗

arXiv2026

A binary black hole merger rate comparison within the same metallicity - star formation rate framework

Recent studies have suggested that binary population synthesis models, when coupled with observationally based, metallicity-dependent star formation rate density, overpredict the observed local binary black hole (BBH) merger rate density. The significance of this tension might vary depending on the specific code and parameters adopted. Thus, a more extensive exploration of the parameter space is required. In this work, we perform such an extended analysis by considering BBH merger efficiencies coming from multiple population synthesis codes across a wide range of physical assumptions and parameter's choices. We adopt an observationally motivated metallicity distribution, exploring several variations to encompass observational uncertainties. We find that the tension persists: in almost all our metallicity variations, 14 out of 18, none of the models considered predicts a local BBH merger rate within or below the observed $90\%$ credible interval. Even in the four most favorable metallicity variations, only $\lesssim 10\%$ of the models are consistent with the observational constraints. We show that such a discrepancy originates from the low-metallicity tail contributed by low-mass galaxies and starbursts, as well as from the use of iron abundance rather than oxygen abundance in deriving the metallicity distribution. Even literature models that predict moderate BBH merger rates shift toward higher merger rates when combined with observationally motivated metallicity distributions. Although not comprehensive of all the literature models, our analysis suggests that models featuring stronger natal kicks and/or non-standard treatments of mass-transfer and common-envelope physics provide the most promising avenue for alleviating the tension with the observed local BBH merger rate.

astro-ph.HE↗

arXiv2026

The Stellar Winds Atlas II: Black Hole Formation at Solar Metallicity

Stellar winds are a primary source of uncertainty in predicting the masses of black holes (BHs) from massive stars. At solar metallicity, theoretical models lead to widely divergent results due to differing wind prescriptions. A key obstacle remains the lack of systematic investigations across a common parameter space. To address this, we construct a ``Wind Atlas'' using detailed 1D MESA stellar evolution models and population synthesis techniques to estimate the Galactic population of solar metallicity BH progenitors. We systematically investigate 14 distinct wind models, ranging from the most traditional and widespread prescriptions to the most recent. By evaluating stellar evolution across this extensive grid, we show that the final BH mass is dictated by a fundamental bifurcation: whether a star collapses as a cool supergiant or is first stripped of its envelope to become a Wolf-Rayet (WR) star. If a star enters the WR stage, its strong thick winds dominate, making the final mass sensitive to the WR wind prescription while largely erasing the memory of its prior mass-loss history. Conversely, stars that face core collapse as supergiants form significantly more massive BHs, producing a mass peak around an initial mass of 40 $M_\odot$. Rather than simply reproducing these divergent outcomes, our comprehensive evaluation demonstrates that this bifurcation is universally controlled by the highly uncertain mass loss during the cool supergiant phase. This framework strongly constrains the problem of BH mass prediction by identifying two key bottlenecks for future studies: envelope stripping efficiency and WR mass-loss rates. Our atlas provides a clear baseline for interpreting current theoretical discrepancies and testing wind models against observational constraints, such as the Galactic WR/OB population ratio.

astro-ph.SR↗

arXiv2026

The role of accretion efficiency, natal kicks, and angular momentum transport in the formation of the Gaia black holes

Gaia has the potential to deliver several tens of new dormant black holes (BHs) with low-mass stellar companions (hereafter, Gaia BHs) in the upcoming fourth data release. Three Gaia BHs are already known, but their formation pathways remain uncertain. Here, we perform a large parametric study to explore the formation of Gaia BHs from isolated binary systems with the population-synthesis code SEVN and compare our models with the properties of the three already reported Gaia BHs. Specifically, we explore the impact of accretion efficiency, mass transfer stability, natal kicks, angular momentum transport, and core-collapse supernova prescriptions. We find that models in which stable mass transfer is highly non-conservative and angular momentum is lost as a wind from the donor surface (Jeans mode) maximize the probability of forming dormant systems that match the properties of the observed Gaia BHs in terms of both orbital period and eccentricity, because such assumptions prevent the initial orbit from shrinking too much when the BH progenitor fills its Roche lobe. If we allow for common-envelope evolution, we find that models with common-envelope ejection efficiency $α < 1$ predict dormant systems with orbital periods that are too short compared to the observed Gaia BHs. The eccentricity of the observed Gaia BHs, when combined with information about orbital period and BH mass, favors relatively large natal kicks, similar to those inferred from Galactic neutron stars. Finally, models in which BH natal kicks are low - e.g. because they are modulated by fallback - result in the formation of a large population of dormant BHs with long orbital periods ($P_{\rm orb}>10^4$ days), which will be tested soon by the fourth Gaia data release.

astro-ph.HE↗

arXiv2026

How Common Are Common Envelopes? Quantifying Their Role in Forming Gravitational-Wave Sources

A central goal of gravitational-wave astronomy is to use merging binary black hole (BBH), black hole-neutron star (BHNS), and binary neutron star (BNS) systems as fossils to reconstruct the formation and evolution of massive stars across cosmic time. In practice, this inference relies on population-synthesis models that map massive stellar binaries to merging compact objects. However, these models disagree on the dominant orbital-hardening mechanisms within isolated binary evolution, particularly on whether common-envelope (CE) evolution is required. To address this, we compile and systematically compare formation-channel predictions from more than 200 isolated-binary population-synthesis simulations, organized within a unified hierarchical taxonomy. We find that BBH and BHNS formation pathways span nearly the full allowed range from CE-dominated to without-CE-dominated evolution (0-100%), while often predicting similar merger rates, revealing a fundamental degeneracy: merger-rate measurements alone do not uniquely constrain the underlying evolutionary pathways. In contrast, BNS formation proceeds almost exclusively through channels involving at least one CE phase (>90-100%), suggesting CE evolution plays a qualitatively different role in BNS than in BBH and BHNS formation. The relative contributions of with-CE and without-CE pathways are governed primarily by assumptions controlling mass-transfer stability, angular-momentum loss, CE efficiency, and supernova physics, which often act non-linearly and in correlated fashion, such that trends from one-at-a-time parameter variations do not generalize across simulation frameworks. Robust interpretation of gravitational-wave populations will therefore require transparent formation-channel definitions, reproducible analysis pipelines, systematic cross-code comparisons, and observational constraints that extend beyond merger rates alone.

astro-ph.HE↗

arXiv2026

Can current models predict the local black hole merger rate?

After four observational runs, the Ligo-Virgo-Kagra collaboration estimated a local binary black hole (BBH) merger rate density of $R_{0,\textrm{LVK}}\simeq 14-26\,\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$ within the 90% credible interval. Some previous studies already pointed out that, when a realistic evolution of the metallicity-dependent cosmic star formation rate density (SFRD) is adopted, theoretical models predict a local BBH merger rate density that exceeds the observed value by at least a factor of $\sim 10$ (Sgalletta et al. 2025). In this paper, we confirm and strengthen this claim by constructing an empirical model for the SFRD and metallicity evolution that includes a correction accounting for iron abundance. The adopted metallicity relation is flexible, enabling us to bracket the wide range of observational uncertainties. We show that, even under the most conservative assumptions regarding both the SFRD and the metallicity relation, the local BBH merger rate density is overestimated by a factor $> 10$. Attempts to reconcile the predicted and observed merger rates by modifying only the metallicity-dependent SFRD would require unrealistically high metallicities ($Z>Z_\odot$) even in low-mass galaxies at high redshift. This finding indicates that revisions to the treatment of stellar and binary evolution are necessary to achieve consistency between theoretical predictions and observations. We suggest that even a modest steepening of the delay-time distribution could help alleviate this tension.

astro-ph.HE↗

arXiv2026

Massquerade: Impacts of Mass Ratio Reversals on Binary Black Hole Merger Rates and Mass Distributions

We investigate the role of mass ratio reversal (MRR), in which the initially less massive star in a binary forms the more massive compact object, in shaping the astrophysical binary black hole (BBH) merger rate and mass distribution inferred by LIGO-Virgo-KAGRA, comparing simulation outcomes from population synthesis frameworks COMPAS and SEVN. We find that the observational imprint of MRR differs qualitatively between the two models. In COMPAS, MRR systems dominate the merger rate density at high primary masses ( $\gtrsim$ 12 M$_\odot$), high secondary masses ( $\gtrsim$ 20 M$_\odot$), and high mass ratios ($q>0.6$), whereas in SEVN, MRR systems remain subdominant across the BBH mass distribution. This implies that the initially less massive star can massquerade as the observed primary black hole, such that the primary-mass distribution is not a direct tracer of the initially more massive stars, but instead a superposition of physically distinct evolutionary populations. We identify in the simulations three distinct evolutionary pathways leading to MRR systems: core-growth, in which stable mass transfer increases the helium-core mass of the secondary; PPISN-shrinking, where pulsational pair-instability episodes reduce the primary remnant mass; and asymmetric-CCSN, where differential supernova mass loss drives the reversal. When weighted by the local BBH merger-rate density, the core-growth channel dominates almost exclusively. MRR systems predominantly originate from massive ($\gtrsim$ 50 M$_\odot$), low-metallicity progenitors, with most of the systems forming below 0.1 $Z_\odot$. Our results demonstrate that MRR is a physically distinct and potentially observable feature of isolated binary evolution. Accounting for MRR will be important for robustly connecting future gravitational-wave observations to the physics of massive binary evolution and compact-object formation.

astro-ph.HE↗

arXiv2026

From cosmological simulations to binary black hole mergers: The impact of using analytical star formation history models on gravitational-wave source populations

Observations of binary black hole (BBH) mergers provide a unique window into the lives of massive stars across cosmic time. Connecting redshift-dependent merger properties to massive star progenitors requires accurate models of cosmic star formation and chemical enrichment histories. Analytical fits for the metallicity-specific cosmic star formation rate density S(Z, z) are commonly used as proxies for the complex underlying star formation history, yet they remain unconstrained. Using the IllustrisTNG cosmological simulations, we evaluate the accuracy of these analytical S(Z, z) prescriptions and assess how simulation resolution and volume affect the inferred S(Z, z). By coupling the simulated and analytical S(Z, z) to the population synthesis code COMPAS, we investigate the resulting BBH merger rates and mass distributions. We find that analytical S(Z, z) prescriptions can overestimate BBH merger rates at high redshift ($z \gtrsim 6$) by up to a factor of $10$-$10^4$, depending on cosmological simulation resolution, and can introduce spurious features in the BBH mass distribution. For example, they can produce an artificial feature near $8\,M_\odot$ in the primary mass distribution at $z \lesssim 2$, which is absent when using the full simulation-based S(Z, z), while simultaneously suppressing high-mass features. These discrepancies arise because simple analytical models fail to capture a high-metallicity bump and a more flattened low-metallicity tail in the simulated S(Z, z) metallicity distribution. Our results highlight the importance of accurate star formation histories for modeling BBH populations, demonstrate the limitation of widely used analytical S(Z, z) fits, and underscore the need for careful integration of cosmological simulations, analytical fits, and population synthesis when interpreting gravitational-wave observations.

astro-ph.HE↗

arXiv2025

StarEstate: A Python Package for Galactic Population Synthesis

I present StarEstate, an open-source Python package for producing rapid, statistically robust galactic population synthesis models. By utilizing optimized pre-calculated inverse-cumulative distribution function samplers, the tool generates synthetic populations from pre-generated grids of stellar tracks orders of magnitude faster than traditional numerical integration methods. A key morphological feature is the probabilistic assignment of stars to spiral arms based on age-dependent dynamical temperature, reproducing the observation that young tracers tightly confine to arms while older populations disperse. The software combines statistical generation with stellar physics by mapping synthetic populations to MESA or rapid SSE/BSE evolutionary tracks. Users can inspect specific evolutionary stages through automatic hierarchical classification, distinguishing evolutionary phases and spectral classes like Wolf-Rayet, O-type, or red supergiant stars across different metallicity environments. StarEstate's features allow the user to predict spatial distributions of diverse stellar objects, providing a flexible resource for interpreting galactic surveys.

astro-ph.IM↗

arXiv2025

Evolution of stars with 60 and 200 Msun: predictions for WNh stars in the Milky Way

We study in detail the evolution of two massive stars at solar metallicity ($Z=0.014$), by calculating their final masses, radial expansion, and chemical enrichment. We run evolutionary models for initial masses 60 and 200 $M_\odot$, using MESA and the Geneva-evolution-code (GENEC). For the mass loss, we adopt the self-consistent m-CAK prescription for the optically thin winds of OB-type stars, a semi-empirical formula for H-rich optically thick wind of WNh stars, and a hydrodynamically consistent formula for the H-poor thick wind of classical Wolf-Rayet stars. The transition from thin to thick winds is set at $Γ_\text{e}=0.5$. For the 60 $M_\odot$ case, the GENEC model predicts a more efficient rotational mixing and more chemically homogeneous evolution, whereas the MESA model predicts a large radial expansion reaching the LBV phase. For the 200 $M_\odot$ case, differences between both evolution codes are less relevant because their evolution is dominated by wind mass loss with a weaker dependence on internal mixing. The switch of the mass-loss prescription based on the Eddington factor instead of the removal of outer layers, implies the existence of WNh stars with a large mass fraction of hydrogen at the surface ($X_\text{surf}\ge0.3$) formed from initial masses of $\gtrsim60$ $M_\odot$. These stars are constrained in a $T_\text{eff}$ range of the HRD which corresponds to the MS band, in agreement with the observations of Galactic WNh stars at $Z=0.014$. While our models employ a fixed $Γ_\text{e,trans}$ threshold for the switch to thick winds, rather than a continuous thin-to-thick wind model, the good reproduction of observations during the MS supports the robustness of the wind model upgrades, allowing its application to studies of late-stage stellar evolution before core collapse.

astro-ph.SR↗

arXiv2024

Development of convective envelopes in massive stars: Implications for gravitational wave sources

The structure of stellar envelopes strongly influences the course and outcome of binary mass transfer, in particular of common envelope (CE) evolution. Convective envelopes can most easily be ejected during CE events, leading to short-period binaries and potentially gravitational wave (GW) sources. Conversely, radiative envelope are thought to lead to CE mergers and Thorne-Zytkow objects (TZOs) or quasi-stars (QS). Rapid binary models based on Hurley et al. (2000) often assume that any CE event with a Hertzsprung gap donor results in a CE merger, in tension with literature. We improve this with a more self-consistent criterion based on the presence of a convective envelope. Using 1D stellar models (MESA), we systematically investigate the development of convective envelopes in massive stars. We provide fitting formulae for rapid binary codes and implement them into the StarTrack population synthesis code to refine the CE treatment and examine the impact on GW sources, TZOs, and QSs. We show that convective envelopes in massive stars are highly sensitive to the treatment of superadiabacity and the mixing length. Our revised CE model significantly reduces (factor 20) the predicted merger rate of binary black hole (BH-BH) mergers with total masses between roughly 20 and 50 Msun. This leads to a bimodal mass distribution with a strong metallicity dependence. We also predict that the current TZO/QS formation rate in the Galaxy (up to roughly 10-4 yr-1), combined with their predicted lifetimes, makes their detection unlikely. Our study strongly suggests that the role of CE evolution in the formation of BH-BH mergers has been considerably overestimated for BH-BH mergers with Mtot > 20 Msun. We highlight that any prediction from the CE channel for massive BH-BH mergers (>50 Msun) heavily hinges on our limited understanding of stellar structure and mass loss close to the Eddington limit.

astro-ph.SR↗

arXiv2024

Large Interferometer For Exoplanets (LIFE). XIV. Finding terrestrial protoplanets in the galactic neighborhood

The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization to distances from the solar system far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based mid-infrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. Here, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres, in young stellar associations in particular. Using the LIFE mission instrument simulator (LIFEsim) we assess how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplanets. We focus on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature using a blackbody assumption, and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Our simulations suggest that LIFE will be able to detect (S/N $\geq$ 7) hot protoplanets in young stellar associations up to distances of $\approx$100 pc from the solar system for reasonable integration times (up to $\sim$hours). Detection of an Earth-sized protoplanet orbiting a solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions $<$6 $μ$m is important for decreasing the detection threshold and discriminating emission temperatures.

astro-ph.EP↗

arXiv2024

On the maximum black hole mass at solar metallicity

In high metallicity environments the mass that black holes (BHs) can reach just after core-collapse widely depends on how much mass their progenitor stars lose via winds. On one hand new theoretical and observational insights suggest that early-stage winds should be weaker than what many canonical models prescribe. On the other hand the proximity to the Eddington limit should affect the formation of optically thick envelopes already during the earliest stages of stars with initial masses $M_{\rm ZAMS}\gtrsim 100$ M$_\odot$, hence resulting in higher mass-loss rates during the main sequence. We use the evolutionary codes MESA and Genec to calculate a suite of tracks for massive stars at solar metallicity Z$_\odot=0.014$ which incorporate these changes in our wind mass loss prescription. In our calculations we employ moderate rotation, high overshooting and magnetic angular momentum transport. We find a maximum BH mass $M_{\rm BH, max}=28.3$ M$_\odot$ at Z$_\odot$. The most massive BHs are predicted to form from stars with $M_{\rm ZAMS}\gtrsim 250$ M$_\odot$, with the BH mass directly proportional to its progenitor's $M_{\rm ZAMS}$. We also find in our models that at Z$_\odot$ almost any BH progenitor naturally evolves into a Wolf-Rayet star due to the combined effect of internal mixing and wind mass loss. These results are considerably different from most recent studies regarding the final mass of stars before their collapse into BHs. While we acknowledge the inherent uncertainties in stellar evolution modelling, our study underscores the importance of employing the most up-to-date physics in BH mass predictions.

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