arXiv subjects

M. Spera

Publications and source records attributed to M. Spera.

16 recordsLinked to original sources

arXiv2025

The initial mass-remnant mass relation for core collapse supernovae

The first direct detection of gravitational waves in 2015 marked the beginning of a new era for the study of compact objects. Upcoming detectors, such as the Einstein Telescope, are expected to add thousands of binary coalescences to the list. However, from a theoretical perspective, our understanding of compact objects is hindered by many uncertainties, and a comprehensive study of the nature of stellar remnants from core-collapse supernovae is still lacking. In this work, we investigate the properties of stellar remnants using a homogeneous grid of rotating and non-rotating massive stars at various metallicities from Limongi and Chieffi 2018. We simulate the supernova explosion of the evolved progenitors using the HYdrodynamic Ppm Explosion with Radiation diffusION (HYPERION) code (Limongi and Chieffi 2020), assuming a thermal bomb model calibrated to match the main properties of SN1987A. We find that the heaviest black hole that can form depends on the initial stellar rotation, metallicity, and the assumed criterion for the onset of pulsational pair-instability supernovae. Non-rotating progenitors at $\big[\rm Fe/H \big]=-3$ can form black holes up to $\sim 87 M_\odot$, falling within the theorized pair-instability mass gap. Conversely, enhanced wind mass loss prevents the formation of BHs more massive than $\sim 41.6 M_\odot$ from rotating progenitors. We use our results to study the black hole mass distribution from a population of $10^6$ isolated massive stars following a Kroupa initial mass function. Finally, we provide fitting formulas to compute the mass of compact remnants as a function of stellar progenitor properties. Our up-to-date prescriptions can be easily implemented in rapid population synthesis codes.

astro-ph.HE↗

arXiv2023

Nebular emission from young stellar populations including binary stars

We investigate the nebular emission produced by young stellar populations using the new GALSEVN model based on the combination of the SEVN population-synthesis code including binary-star processes and the GALAXEV code for the spectral evolution of stellar populations. Photoionization calculations performed with the CLOUDY code confirm that accounting for binary-star processes strongly influences the predicted emission-line properties of young galaxies. In particular, we find that our model naturally reproduces the strong HeII/Hb ratios commonly observed at high Hb equivalent widths in metal-poor, actively star-forming galaxies, which have proven challenging to reproduce using previous models. Including bursty star formation histories broadens the agreement with observations, while the most extreme HeII equivalent widths can be reproduced by models dominated by massive stars. GALSEVN also enables us to compute, for the first time in a way physically consistent with stellar emission, the emission from accretion discs of X-ray binaries (XRBs) and radiative shocks driven by stellar winds and supernova explosions. We find that these contributions are unlikely to prominently affect the predicted HeII/Hb ratio, and that previous claims of a significant contribution by XRBs to the luminosities of high-ionization lines are based on models predicting improbably high ratios of X-ray luminosity to star formation rate, inconsistent with the observed average luminosity function of XRBs in nearby galaxies. The results presented here provide a solid basis for a more comprehensive investigation of the physical properties of observed galaxies with GALSEVN using Bayesian inference.

astro-ph.GA↗

arXiv2022

The Black Hole Mass Function Across Cosmic Times II. Heavy Seeds and (Super)Massive Black Holes

This is the second paper in a series aimed at modeling the black hole (BH) mass function, from the stellar to the (super)massive regime. In the present work we focus on (super)massive BHs and provide an ab-initio computation of their mass function across cosmic times. We consider two main mechanisms to grow the central BH, that are expected to cooperate in the high-redshift star-forming progenitors of local massive galaxies. The first is the gaseous dynamical friction process, that can cause the migration toward the nuclear regions of stellar-mass BHs originated during the intense bursts of star formation in the gas-rich host progenitor galaxy, and the buildup of a central heavy BH seed $M_\bullet\sim 10^{3-5}\, M_\odot$ within short timescales $\lesssim$ some $10^7$ yr. The second mechanism is the standard Eddington-type gas disk accretion onto the heavy BH seed, through which the central BH can become (super)massive $M_\bullet\sim 10^{6-10}\, M_\odot$ within the typical star-formation duration $\lesssim 1$ Gyr of the host. We validate our semi-empirical approach by reproducing the observed redshift-dependent bolometric AGN luminosity functions and Eddington ratio distributions, and the relationship between the star-formation and the bolometric luminosity of the accreting central BH. We then derive the relic (super)massive BH mass function at different redshifts via a generalized continuity equation approach, and compare it with present observational estimates. Finally, we reconstruct the overall BH mass function from the stellar to the (super)massive regime, over more than ten orders of magnitudes in BH mass.

astro-ph.GA↗

arXiv2022

The impact of very massive stars on the chemical evolution of extremely metal-poor galaxies

In recent observations of extremely metal-poor low-mass starburst galaxies, almost solar Fe/O ratios are reported, despite N/O ratios consistent with the low metallicity. We investigate if the peculiar Fe/O ratios can be a distinctive signature of an early enrichment produced by very massive objects dying as Pair-Instability Supernovae (PISN). We run chemical evolution models with yields that account for the contribution by PISN. We use both the recent non-rotating stellar yields from Goswami et al. 2021, and new yields from rotating very massive stars calculated on purpose in this work. We also search for the best initial mass function (IMF) that is able to reproduce the observations. We can reproduce the observations by adopting a bi-modal IMF and by including an initial burst of rotating very massive stars. Only with a burst of very massive stars can we reproduce the almost solar Fe/O ratios at the estimated young ages. We also confirm that rotation is absolutely needed to concomitantly reproduce the observed N/O ratios. These results stress the importance of very massive stars in galactic chemical evolution studies and strongly support a top-heavy initial mass function in the very early evolutionary stages of metal poor starburst galaxies.

astro-ph.GA↗

arXiv2021

On the effects of the Initial Mass Function on Galactic chemical enrichment

There is mounting evidence that the stellar initial mass function (IMF) could extend much beyond the canonical Mi ~100, Msun limit, but the impact of such hypothesis on the chemical enrichment of galaxies still remains to be clarified. We aim to address this question by analysing the observed abundances of thin- and thick-disc stars in the Milky Way with chemical evolution models that account for the contribution of very massive stars dying as pair-instability supernovae. We built new sets of chemical yields from massive and very massive stars up to Mi ~ 350 Msun, by combining the wind ejecta extracted from our hydrostatic stellar evolution models with explosion ejecta from the literature. Using a simple chemical evolution code we analyse the effects of adopting different yield tables by comparing predictions against observations of stars in the solar vicinity. After several tests, we focus on the [O/Fe] ratio which best separates the chemical patterns of the two Milky Way components. We find that with a standard IMF, truncated at Mi ~ 100 Msun, we can reproduce various observational constraints for thin-disc stars, but the same IMF fails to account for the [O/Fe] ratios of thick-disc stars. The best results are obtained by extending the IMF up to Mi = 350 Msun and including the chemical ejecta of very massive stars, in the form of winds and pair-instability supernova explosions.Our study indicates that PISN played a significant role in shaping the chemical evolution of the Milky Way thick disc. By including their chemical yields it is easier to reproduce not only the level of the alpha-enhancement but also the observed slope of thick-disc stars in the [O/Fe] vs [Fe/H] diagram. The bottom line is that the contribution of very massive stars to the chemical enrichment of galaxies is potentially quite important and should not be neglected in chemical evolution models.

astro-ph.GA↗

arXiv2019

Evolution of dwarf galaxies hosting GW150914-like events

Here we present a detailed analysis of the properties and evolution of different dwarf galaxies, candidate to host the coalescence of black hole binary systems (BHB) generating GW150914-like events. By adopting a novel theoretical framework coupling the binary population synthesis code \texttt{SeBa} with the Galaxy formation model \texttt{GAMESH}, we can investigate the detailed evolution of these objects in a well resolved cosmological volume of 4~cMpc, having a Milky Way-like (MW) galaxy forming at its center. We identify three classes of interesting candidate galaxies: MW progenitors, dwarf satellites and dwarf galaxies evolving in isolation. We find that: (i) despite differences in individual histories and specific environments the candidates reduce to only nine representative galaxies; (ii) among them, $\sim44\%$ merges into the MW halo progenitors by the redshift of the expected signal, while the remaining dwarfs are found as isolated or as satellites of the MW and their evolution is strongly shaped by both peculiar dynamical history and environmental feedback; (iii) a stringent condition for the environments where GW150914-like binaries can form comes from a combination of the accretion history of their DM halos and the radiative feedback in the high redshift universe; (iv) by comparing with the observed catalogues from DGS and ALLSMOG surveys we find two observed dwarfs respecting the properties predicted by our model. We finally note how the present analysis opens the possibility to build future strategies for host galaxy identification.

astro-ph.GA↗

arXiv2017

Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of gamma-rays, a gravitational wave signal, and a transient optical/near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named "macronovae" or "kilonovae", are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short gamma-ray burst at z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational wave source GW 170817 and gamma-ray burst GRB 170817A associated with a galaxy at a distance of 40 Mpc from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03-0.05 solar masses of material, including high-opacity lanthanides.

astro-ph.HE↗

arXiv2017

GRAWITA: VLT Survey Telescope observations of the gravitational wave sources GW150914 and GW151226

We report the results of deep optical follow-up surveys of the first two gravitational-wave sources, GW150914 and GW151226, done by the GRAvitational Wave Inaf TeAm Collaboration (GRAWITA). The VLT Survey Telescope (VST) responded promptly to the gravitational-wave alerts sent by the LIGO and Virgo Collaborations, monitoring a region of $90$ deg$^2$ and $72$ deg$^2$ for GW150914 and GW151226, respectively, and repeated the observations over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the $r-$band. The paper describes the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Several transients have been discovered but no candidates are recognized to be related to the gravitational-wave events. Interestingly, among many contaminant supernovae, we find a possible correlation between the supernova VSTJ57.77559-59.13990 and GRB150827A detected by {\it Fermi}-GBM. The detection efficiency of VST observations for different types of electromagnetic counterparts of gravitational-wave events are evaluated for the present and future follow-up surveys.

astro-ph.HE↗

arXiv2017

Supernova Kicks and Dynamics of Compact Remnants in the Galactic Centre

The Galactic Centre (GC) is a unique place to study the extreme dynamical processes occurring near a super-massive black hole (SMBH). Here we investigate the role of supernova (SN) explosions occurring in massive binary systems lying in a disc-like structure within the innermost parsec. We use a regularized algorithm to simulate 30,000 isolated three-body systems composed of a stellar binary orbiting the SMBH. We start the integration when the primary member undergoes a SN explosion, and we analyze the impact of SN kicks on the orbits of stars and compact remnants. We find that SN explosions scatter the lighter stars in the pair on completely different orbits, with higher eccentricity and inclination. In contrast, stellar-mass black holes (BHs) and massive stars retain memory of the orbit of their progenitor star. Our results suggest that SN kicks are not sufficient to eject BHs from the GC. We thus predict that all BHs that form in situ in the central parsec of our Galaxy remain in the GC, building up a cluster of dark remnants. In addition, the change of NS orbits induced by SNe may partially account for the observed dearth of NSs in the GC. About 40 per cent of remnants stay bound to the stellar companion after the kick; we expect up to 70 per cent of them might become X-ray binaries through Roche-lobe filling. Finally, the eccentricity of some light stars becomes > 0.7 as an effect of the SN kick, producing orbits similar to those of the G1 and G2 dusty objects.

astro-ph.GA↗

arXiv2016

Stripe and short range order in the charge density wave of 1T-Cu$_x$TiSe$_2$

We study the impact of Cu intercalation on the charge density wave (CDW) in 1T-Cu$_{\text{x}}$TiSe$_{\text{2}}$ by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x$<$0.02) and a breaking up of the commensurate order into 2$\times$2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase-shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material.

cond-mat.mtrl-sci↗

arXiv2016

Brownian motion of massive black hole binaries and the final parsec problem

Massive black hole binaries (BHBs) are expected to be one of the most powerful sources of gravitational waves (GWs) in the frequency range of the pulsar timing array and of forthcoming space-borne detectors. They are believed to form in the final stages of galaxy mergers, and then harden by slingshot ejections of passing stars. However, evolution via the slingshot mechanism may be ineffective if the reservoir of interacting stars is not readily replenished, and the binary shrinking may come to a halt at roughly a parsec separation. Recent simulations suggest that the departure from spherical symmetry, naturally produced in merger remnants, leads to efficient loss cone refilling, preventing the binary from stalling. However, current N-body simulations able to accurately follow the evolution of BHBs are limited to very modest particle numbers. Brownian motion may artificially enhance the loss cone refilling rate in low-N simulations, where the binary encounters a larger population of stars due its random motion. Here we study the significance of Brownian motion of BHBs in merger remnants in the context of the final parsec problem. We simulate mergers with various particle numbers (from 8k to 1M) and with several density profiles. Moreover, we compare simulations where the BHB is fixed at the centre of the merger remnant with simulations where the BHB is free to random walk. We find that Brownian motion does not significantly affect the evolution of BHBs in simulations with particle numbers in excess of one million, and that the hardening measured in merger simulations is due to collisionless loss cone refilling.

astro-ph.GA↗

arXiv2013

The Dense Stellar Systems Around Galactic Massive Black Holes

The central regions of galaxies show the presence of massive black holes and/or dense stellar systems. The question about their modes of formation is still under debate. A likely explanation of the formation of the central dense stellar systems in both spiral and elliptical galaxies is based on the orbital decay of massive globular clusters in the central region of galaxies due to kinetic energy dissipation by dynamical friction. Their merging leads to the formation of a nuclear star cluster, like that of the Milky Way, where a massive black hole (Sgr A*) is also present. Actually, high precision N-body simulations (Antonini, Capuzzo-Dolcetta et al. 2012, ApJ, 750, 111) show a good fit to the observational characteristics of the Milky Way nuclear cluster, giving further reliability to the cited `migratory' model for the formation of compact systems in the inner galaxy regions.

astro-ph.CO↗

arXiv2012

A fully parallel, high precision, N-body code running on hybrid computing platforms

We present a new implementation of the numerical integration of the classical, gravitational, N-body problem based on a high order Hermite's integration scheme with block time steps, with a direct evaluation of the particle-particle forces. The main innovation of this code (called HiGPUs) is its full parallelization, exploiting both OpenMP and MPI in the use of the multicore Central Processing Units as well as either Compute Unified Device Architecture (CUDA) or OpenCL for the hosted Graphic Processing Units. We tested both performance and accuracy of the code using up to 256 GPUs in the supercomputer IBM iDataPlex DX360M3 Linux Infiniband Cluster provided by the italian supercomputing consortium CINECA, for values of N up to 8 millions. We were able to follow the evolution of a system of 8 million bodies for few crossing times, task previously unreached by direct summation codes. The code is freely available to the scientific community.

astro-ph.IM↗

arXiv2011

High Precision Simulations of the Evolution of a Super Star Cluster Around a Massive Black Hole

We present preliminary results of the application of a new sophisticated code which allows high precision integration of orbits of stars belonging to a dense stellar system moving in the vicinity of a massive black hole. This mimics the situation observed in the center of many galaxies, where a nuclear star cluster contains a massive black hole which, in the past, was, likely, an active engine of violent emission of radiation. The main scope of our work is the investigation of the relaxation of the super star cluster on a sufficiently long time, together with the investigation of its feedback with the massive black hole.

astro-ph.GA↗

arXiv2011

High performance astrophysics computing

The application of high end computing to astrophysical problems, mainly in the galactic environment, is under development since many years at the Dep. of Physics of Sapienza Univ. of Roma. The main scientific topic is the physics of self gravitating systems, whose specific subtopics are: i) celestial mechanics and interplanetary probe transfers in the solar system; ii) dynamics of globular clusters and of globular cluster systems in their parent galaxies; iii) nuclear clusters formation and evolution; iv) massive black hole formation and evolution; v) young star cluster early evolution. In this poster we describe the software and hardware computational resources available in our group and how we are developing both software and hardware to reach the scientific aims above itemized.

astro-ph.IM↗

arXiv2000

String Limit of Vortex Current Algebra

The Poisson structure generating the hamiltonian dynamics of string vortices is reconstructed within the current algebra picture as a limiting case of the standard brackets associated to fluids with a smooth vorticity field. The approach implemented bypasses the use of Dirac's procedure. The fine structure of the dynamical algebra is derived for planar fluids by implementing an appropriate spatial fragmentation of the vorticity field, and the limit to the point vortex gas is effected. The physical interpretation of the resulting local currents is provided. Nontrivial differences characterizing the canonical quantization of point vortices and the current algebra quantization are also illustrated.

cond-mat.mes-hall↗