314 research outputs found
Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster
We investigate the formation of circumnuclear gas structures from the tidal
disruption of molecular clouds in galactic nuclei, by means of smoothed
particle hydrodynamics simulations. We model galactic nuclei as composed of a
supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider
different mass ratios between the two components. We find that the relative
masses of the SMBH and the NSC have a deep impact on the morphology of the
circumnuclear gas. Extended disks form only inside the sphere of influence of
the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere
of influence, where the gravity is dominated by the NSC. This result is in
agreement with the properties of the Milky Way's circumnuclear ring, which
orbits outside the SMBH sphere of influence. Our results indicate that compact
circumnuclear rings can naturally form outside the SMBH sphere of influence.Comment: Accepted for publication in ApJ. 12 pages, 6 figures, 3 tables.
Comments welcom
Weighing the IMBH candidate CO-0.40-0.22* in the Galactic Centre
The high velocity gradient observed in the compact cloud CO-0.40-0.22, at a
projected distance of 60 pc from the centre of the Milky Way, has led its
discoverers to identify the closeby mm continuum emitter, CO-0.40-0.22*, with
an intermediate mass black hole (IMBH) candidate. We describe the interaction
between CO-0.40-0.22 and the IMBH, by means of a simple analytical model and of
hydrodynamical simulations. Through such calculation, we obtain a lower limit
to the mass of CO-0.40-0.22* of few . This result
tends to exclude the formation of such massive black hole in the proximity of
the Galactic Centre. On the other hand, CO-0.40-0.22* might have been brought
to such distances in cosmological timescales, if it was born in a dark matter
halo or globular cluster around the Milky Way.Comment: 9 pages, 4 figures. To be published on MNRA
Formation of black holes in the pair-instability mass gap: evolution of a post-collision star
The detection of GW190521 by the LIGO-Virgo collaboration revealed the
existence of black holes (BHs) in the pair-instability (PI) mass gap. Here, we
investigate the formation of BHs in the PI mass gap via star -- star collisions
in young stellar clusters. To avoid PI, the stellar-collision product must have
a relatively small core and a massive envelope. We generate our initial
conditions from the outputs of a hydro-dynamical simulation of the collision
between a core helium burning star ( M) and a main-sequence
star ( M). The hydro-dynamical simulation allows us to take
into account the mass lost during the collision ( M) and to
build the chemical composition profile of the post-collision star. We then
evolve the collision product with the stellar evolution codes PARSEC and MESA.
We find that the post-collision star evolves through all the stellar burning
phases until core collapse, avoiding PI. At the onset of core collapse, the
post-collision product is a blue super-giant star. We estimate a total mass
loss of about 1 M during the post-collision evolution, due to stellar
winds and shocks induced by neutrino emission in a failed supernova. The final
BH mass is M. Therefore, we confirm that the collision
scenario is a suitable formation channel to populate the PI mass gap.Comment: 9 pages, 6 figures, comments welcome
Measuring the spectral index of turbulent gas with deep learning from projected density maps
Turbulence plays a key role in star formation in molecular clouds, affecting
star cluster primordial properties. As modelling present-day objects hinges on
our understanding of their initial conditions, better constraints on turbulence
can result in windfalls in Galactic archaeology, star cluster dynamics and star
formation. Observationally, constraining the spectral index of turbulent gas
usually involves computing spectra from velocity maps. Here we suggest that
information on the spectral index might be directly inferred from column
density maps (possibly obtained by dust emission/absorption) through deep
learning. We generate mock density maps from a large set of adaptive mesh
refinement turbulent gas simulations using the hydro-simulation code RAMSES. We
train a convolutional neural network (CNN) on the resulting images to predict
the turbulence index, optimize hyper-parameters in validation and test on a
holdout set. Our adopted CNN model achieves a mean squared error of 0.024 in
its predictions on our holdout set, over underlying spectral indexes ranging
from 3 to 4.5. We also perform robustness tests by applying our model to
altered holdout set images, and to images obtained by running simulations at
different resolutions. This preliminary result on simulated density maps
encourages further developments on real data, where observational biases and
other issues need to be taken into account.Comment: 7 pages, 7 figures, 1 tabl
Merging black hole binaries with the SEVN code
Studying the formation and evolution of black hole binaries (BHBs) is essential for the interpretation of current and forthcoming gravitational wave (GW) detections. We investigate the statistics of BHBs that form from isolated binaries, by means of a new version of the
SEVN population-synthesis code. SEVN integrates stellar evolution by interpolation over a grid of stellar evolution tracks. We upgraded SEVN to include binary stellar evolution processes and we used it to evolve a sample of 1.5 x 10(8) binary systems, with metallicity in the range [10(-4); 4 x 10(-2)]. From our simulations, we find that the mass distribution of black holes (BHs) in double compact-object binaries is remarkably similar to the one obtained considering only single stellar evolution. The maximum BH mass we obtain is similar to 30, 45, and 55 M-circle dot at metallicity Z = 2 x 10(-2), 6 x 10(-3), and 10(-4), respectively. A few massive single BHs may also form (less than or similar to 0.1 per cent of the total number of BHs), with mass up to similar to 65, 90, and 145 M-circle dot at Z = 2 x 10(-2), 6 x 10(-3),
and 10(-4), respectively. These BHs fall in the mass gap predicted from pair-instability supernovae. We also show that the most massive BHBs are unlikely to merge within a Hubble time. In our simulations, merging BHs like GW151226 and GW170608, form at all metallicities, the high-mass systems (like GW150914, GW170814, and GW170104) originate from metal-poor (Z less than or similar to 6 x 10(-3)) progenitors, whereas GW170729-like systems are hard to form, even at Z = 10(-4). The BHB merger rate in the local Universe obtained from our simulations is similar to 90Gpc(-3)yr(-1), consistent with the rate inferred from LIGO-Virgo data
SNAP House. Modulo abitativo temporaneo per i rifugiati in Europa
The refugees\u80crisis results one of the most relevant social and medical issues in the international panorama. The Aim of the research is to develop a design project for a refugees\u80 temporary housing module, able to solving specific social, typological and functional needs in the context of anthropogenic emergencies in Europe. Starting from the study of the refugee crisis phenomenon, through a careful analysis of case studies and by identifying the main requirements of temporary dwellings for housing emergencies, the SNAP House project presents innovative solutions related to modularity, flexibility, adaptability to the different users needs and it is able to ensure hygiene of spaces and indoor well-being
Massive binary black holes from Population II and III stars
Population III stars, born from the primordial gas in the Universe, lose a
negligible fraction of their mass via stellar winds and possibly follow a
top-heavy mass function. Hence, they have often been regarded as the ideal
progenitors of massive black holes (BHs), even above the pair instability mass
gap. Here, we evolve a large set of Population III binary stars (metallicity
) with our population-synthesis code SEVN, and compare them with
Population II binary stars (). In our models, the lower edge of the
pair-instability mass gap corresponds to a BH mass of
() M for single Population III (II) stars. Overall, we
find only mild differences between the properties of binary BHs (BBHs) born
from Population III and II stars, especially if we adopt the same initial mass
function and initial orbital properties. Most BBH mergers born from Population
III and II stars have primary BH mass below the pair-instability gap, and the
maximum secondary BH mass is M. Only up to %
(%) BBH mergers from Population III (II) progenitors have
primary mass above the gap. Unlike metal-rich binary stars, the main formation
channel of BBH mergers from Population III and II stars involves only stable
mass transfer episodes in our fiducial model.Comment: 15 pages, 17 figures, comments are welcom
Compact object mergers: exploring uncertainties from stellar and binary evolution with SEVN
Population-synthesis codes are an unique tool to explore the parameter space
of massive binary star evolution and binary compact object (BCO) formation.
Most population-synthesis codes are based on the same stellar evolution model,
limiting our ability to explore the main uncertainties. Our code SEVN overcomes
this issue by interpolating the main stellar properties from a set of
pre-computed evolutionary tracks. With SEVN, we evolved
binaries in the metallicity range , exploring a number
of models for electron-capture, core-collapse and pair-instability supernovae,
different assumptions for common envelope, stability of mass transfer,
quasi-homogeneous evolution and stellar tides. We find that stellar evolution
has a dramatic impact on the formation of single and binary compact objects.
Just by slightly changing the overshooting parameter () and the pair-instability model, the maximum mass of a black hole
can vary from to . Furthermore,
the formation channels of BCOs and the merger efficiency we obtain with SEVN
show significant differences with respect to the results of other
population-synthesis codes, even when the same binary-evolution parameters are
used. For example, the main traditional formation channel of BCOs is strongly
suppressed in our models: at high metallicity () only % of
the merging binary black holes and binary neutron stars form via this channel,
while other authors found fractions %. The local BCO merger rate density
of our fiducial models is consistent with the most recent estimates by the
LIGO--Virgo--KAGRA collaboration.Comment: Submitted to MNRAS, comments welcome! The SEVN code is available at
https://gitlab.com/sevncodes/sevn.git. All the data underlying this article
are available in Zenodo at the link https://doi.org/10.5281/zenodo.7260771.
All the Jupyter notebooks used to produce the plots in the paper are
available in the gitlab repository https://gitlab.com/iogiul/iorio22_plot.gi
The Influence of Dense Gas Rings on the Dynamics of a Stellar Disk in the Galactic Center
The Galactic center hosts several hundred early-type stars, about 20% of which lie in the so-called clockwise disk, while the remaining 80% do not belong to any disks. The circumnuclear ring (CNR), a ring of molecular gas that orbits the supermassive black hole (SMBH) with a radius of similar to 1.5 pc, has been claimed to induce precession and Kozai-Lidov oscillations onto the orbits of stars in the innermost parsec. We investigate the perturbations exerted by a gas ring on a nearly Keplerian stellar disk orbiting an SMBH by means of combined direct N-body and smoothed particle hydrodynamics simulations. We simulate the formation of gas rings through the infall and disruption of a molecular gas cloud, adopting different inclinations between the infalling gas cloud and the stellar disk. We find that a CNR-like ring is not efficient in affecting the stellar disk on a timescale of 3 Myr. In contrast, a gas ring in the innermost 0.5 pc induces precession of the longitude of the ascending node Omega, which significantly affects the stellar disk inclination. Furthermore, the combined effect of two-body relaxation and Omega-precession drives the stellar disk dismembering, displacing the stars from the disk. The impact of precession on the star orbits is stronger when the stellar disk and the inner gas ring are nearly coplanar. We speculate that the warm gas in the inner cavity might have played a major role in the evolution of the clockwise disk
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