arXiv subjects

Filippo Simonato

Publications and source records attributed to Filippo Simonato.

4 recordsLinked to original sources

arXiv2026

The STARfall pathway: forging intermediate-mass black holes through stellar collisions in dense star clusters

Intermediate-mass black holes (IMBHs, $10^2 < m_{\rm BH}/\mathrm{M}_{\odot} < 10^5$) are elusive objects bridging the gap between stellar-mass and supermassive black holes, whose origin remains uncertain. They may form from the collapse of very massive stars (VMSs, $m_*\geq150\,\mathrm{M}_{\odot}$) assembled through repeated stellar collisions in compact and fast evolving star clusters. We investigate the formation of VMSs and IMBH seeds via repeated mergers and collisions as a function of cluster initial conditions, assessing the implications for globular clusters and dense high-redshift system. We develop STARfall, a semi-analytic tool that follows the evolution of a massive stellar binary via binary-single interactions, mergers, collisions, and stellar evolution, while consistently accounting for the host cluster evolution. Our results show that clusters with central densities above $10^6\,\mathrm{M}_{\odot}\,\mathrm{pc}^{-3}$ can undergo a catastrophic sequence of stellar collisions, or "starastrophe", within $3$ Myr. Through this process, VMSs rapidly grow in mass and collapse into IMBHs with masses of up to $10^3-10^4\,\mathrm{M}_{\odot}$, despite the effects of intense stellar winds. Applying our models to observed systems, we find that compact clusters in the Cosmic Gems Arc and Milky Way globular clusters could have hosted VMSs that collapsed into IMBHs in this mass range. Despite their longer core-collapse timescales, the inferred central densities of Little Red Dots may also support IMBH formation in the same mass interval. Our models show that VMS assembly requires two essential ingredients: short core-collapse times and large central densities. Compared to observed systems, our models suggest that clusters in the Cosmic Gems Arcs and several Galactic globular clusters could have nurtured VMSs that directly collapsed to IMBHs with masses $10^3-10^4\,\text{M}_{\odot}$.

astro-ph.GA↗

arXiv2026

Isolated or Dynamical? Tracing Black Hole Binary Formation through the Population of Gravitational-Wave Sources

The population of binary black hole (BBH) mergers observed by the LIGO-Virgo-KAGRA (LVK) collaboration offers a window into the cosmic evolution of compact binaries and their formation. We employ the semi-analytic population-synthesis code B-POP to model BBHs assembled through isolated binary evolution and dynamical interactions in young, globular, and nuclear star clusters. Our framework incorporates star formation history, metallicity evolution, and single and binary stellar evolution to quantify their impact on the observable properties of the BBH population and on the relative contribution of distinct formation channels. Our models are characterized by a merger rate, $\mathcal{R} = 17.5-24.1\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$, broadly consistent with LVK constraints. Moreover, the predicted distributions of primary mass, mass ratio, and effective inspiral spin parameter are compatible with those inferred from current LVK observations. Our primary-mass distribution is dominated by isolated binaries at $m_1 < 20$ M$_\odot$, while dynamically assembled first- and higher-generation mergers dominate at larger masses. As a consequence, the sub-population of mergers with $m_1 > 45$ M$_\odot$ exhibits a nearly flat mass-ratio distribution and distinctive spin properties. We leverage our models to explore how: (i) the fraction of stars in isolated binaries and the fraction of stellar mass bound in clusters regulate the merger rate; (ii) common-envelope physics shapes the primary-mass distribution and its redshift evolution; (iii) the inclusion of stellar-collision products enhances the formation of higher-generation mergers; and (iv) the natal spin distribution influences the effective spin. Using our models to assess possible origins of selected GW events, we illustrate how the complexity of the underlying astrophysical processes can hinder the possibility to draw definitive conclusions.

astro-ph.GA↗

arXiv2025

Optically thick winds of very massive stars suppress intermediate-mass black hole formation

Intermediate-mass black holes (IMBHs) are the link between stellar-mass and supermassive black holes. Gravitational waves have started unveiling a population of IMBHs in the $\sim 100-300 \, \mathrm{M_{\odot}}$ range. Here, we investigate the formation of IMBHs from non-rotating very massive stars (VMSs, $>100\,{} \mathrm{M_{\odot}}$). We calculate new VMS models that account for the transition from optically thin to optically thick winds, and study how this enhanced mass loss affects IMBH formation and the black hole mass function at intermediate and high metallicity ($Z=10^{-4}-0.02$). We show that optically thick winds suppress the formation of IMBHs from direct VMS collapse at metallicities $Z>0.001$, one order of magnitude lower than predicted by previous models. Our models indicate that the stellar progenitors of GW231123 must have had a metallicity $Z<0.002$, if the primary black hole formed via direct VMS collapse.

astro-ph.HE↗

arXiv2025

Impact of stellar winds on the pair-instability supernova rate

Very massive stars (VMSs, $M_{\star}$ $\geq$ 100 M$_{\odot}$) play a crucial role in several astrophysical processes. At low metallicity, they might collapse directly into black holes, or end their lives as pair-instability supernovae. Recent observational results set an upper limit of $0.7\,{}\mathrm{ yr}^{-1} \,{}\mathrm{ Gpc}^{-3}$ on the rate density of pair-instability supernovae in the nearby Universe. However, most theoretical models predict rates exceeding this limit. Here, we compute new VMS tracks with the MESA code, and use them to analyze the evolution of the (pulsational) pair-instability supernova rate density across cosmic time. We show that stellar wind models accounting for the transition between optically thin and thick winds yield a pair-instability supernova rate $\mathcal{R}_{\mathrm{PISN}}\sim{}0.1$ Gpc$^{-3}$ yr$^{-1}$ at redshift $z\sim{}0$, about two orders of magnitude lower than our previous models. We find that the main contribution to the pair-instability supernova rate comes from stars with metallicity $Z\sim{}0.001-0.002$. Stars with higher metallicities cannot enter the pair-instability supernova regime, even if their zero-age main sequence mass is up to 500 M$_\odot$. The main reason is that VMSs enter the regime for optically thick winds during the main sequence at metallicity as low as $Z\sim{4}\times{}10^{-4}$. This enhances the mass loss rate, quenching the growth of the He core and thus preventing the onset of pair-instability in later evolutionary stages. This result highlights the critical role of mass loss in shaping the final fate of very massive stars and the rate of pair-instability supernovae.

astro-ph.HE↗