133 research outputs found
The Dragon-II simulations -- II. Formation mechanisms, mass, and spin of intermediate-mass black holes in star clusters with up to 1 million stars
The processes that govern the formation of intermediate-mass black holes
(IMBHs) in dense stellar clusters are still unclear. Here, we discuss the role
of stellar mergers, star-BH interactions and accretion, as well as BH binary
(BBH) mergers in seeding and growing IMBHs in the \textsc{Dragon-II} simulation
database, a suite of 19 direct -body models representing dense clusters with
up to stars. \textsc{Dragon-II} IMBHs have typical masses of M and relatively large spins . We find a link between the IMBH formation mechanism and the cluster
structure. In clusters denser than M pc, the
collapse of massive star collision products represents the dominant IMBH
formation process, leading to the formation of heavy IMBHs ( M), possibly slowly rotating, that form over times Myr and
grow further via stellar accretion and mergers in just Myr. BBH mergers
are the dominant IMBH formation channel in less dense clusters, for which we
find that the looser the cluster, the longer the formation time ( Myr)
and the larger the IMBH mass, although remaining within M. Strong
dynamical scatterings and relativistic recoil efficiently eject all IMBHs in
\textsc{Dragon-II} clusters, suggesting that IMBHs in this type of cluster are
unlikely to grow beyond a few M.Comment: 15 pages, 6 figures, 2 tables, 1 appendix. Comments welcome.
Submitted to MNRA
The Dragon-II simulations -- III. Compact binary mergers in clusters with up to 1 million stars: mass, spin, eccentricity, merger rate and pair instability supernovae rate
Compact binary mergers forming in star clusters may exhibit distinctive
features that can be used to identify them among observed gravitational-wave
(GW) sources. Such features likely depend on the host cluster structure and the
physics of massive star evolution. Here, we dissect the population of compact
binary mergers in the \textsc{Dragon-II} simulation database, a suite of 19
direct -body models representing dense star clusters with up to stars
and of stars in primordial binaries. We find a substantial population
of black hole binary (BBH) mergers, some of them involving an intermediate-mass
BH (IMBH), and a handful mergers involving a stellar BH and either a neutron
star (NS) or a white dwarf (WD). Primordial binary mergers, of the
whole population, dominate ejected mergers. Dynamical mergers, instead,
dominate the population of in-cluster mergers and are systematically heavier
than primordial ones. Around of \textsc{Dragon-II} mergers are eccentric
in the LISA band and in the LIGO band. We infer a mean cosmic merger rate
of yr Gpc for BBHs, NS-BH, and WD-BH
binary mergers, respectively, and discuss the prospects for multimessenger
detection of WD-BH binaries with LISA. We model the rate of pair-instability
supernovae (PISNe) in star clusters and find that surveys with a limiting
magnitude can detect yr PISNe. Comparing
these estimates with future observations could help to pin down the impact of
massive star evolution on the mass spectrum of compact stellar objects in star
clusters.Comment: 22 pages, 14 figures, 3 tables. Comments welcome. Submitted to MNRA
The Dragon-II simulations -- I. Evolution of single and binary compact objects in star clusters with up to 1 million stars
We present the first results of the \textsc{Dragon-II} simulations, a suite
of 19 -body simulations of star clusters with up to stars, with up to
of them initially paired in binaries. In this work, we describe the main
evolution of the clusters and their compact objects (COs). All
\textsc{Dragon-II} clusters form in their centre a black hole (BH) subsystem
with a density times larger than the stellar density, with the cluster
core containing of the whole BH population. In all models, the BH
average mass steeply decreases as a consequence of BH burning, reaching values
M within relaxation times.
Generally, our clusters retain only BHs lighter than M over
relaxation times. Looser clusters retain a higher binary fraction, because in
such environments binaries are less likely disrupted by dynamical encounters.
We find that BH-main sequence star binaries have properties similar to recently
observed systems. Double CO binaries (DCOBs) ejected from the cluster exhibit
larger mass ratios and heavier primary masses than ejected binaries hosting a
single CO (SCOBs). Ejected SCOBs have BH masses M,
definitely lower than those in DCOBs ( M).Comment: 22 pages, 21 figures, 4 tables. Comments welcome. Submitted to MNRA
Superdeformed rotational bands in the Mercury region; A Cranked Skyrme-Hartree-Fock-Bogoliubov study
A study of rotational properties of the ground superdeformed bands in \Hg{0},
\Hg{2}, \Hg{4}, and \Pb{4} is presented. We use the cranked
Hartree-Fock-Bogoliubov method with the {\skm} parametrization of the Skyrme
force in the particle-hole channel and a seniority interaction in the pairing
channel. An approximate particle number projection is performed by means of the
Lipkin-Nogami prescription. We analyze the proton and neutron quasiparticle
routhians in connection with the present information on about thirty presently
observed superdeformed bands in nuclei close neighbours of \Hg{2}.Comment: 26 LaTeX pages, 14 uuencoded postscript figures included, Preprint
IPN-TH 93-6
Foundations of self-consistent particle-rotor models and of self-consistent cranking models
The Kerman-Klein formulation of the equations of motion for a nuclear shell
model and its associated variational principle are reviewed briefly. It is then
applied to the derivation of the self-consistent particle-rotor model and of
the self-consistent cranking model, for both axially symmetric and triaxial
nuclei. Two derivations of the particle-rotor model are given. One of these is
of a form that lends itself to an expansion of the result in powers of the
ratio of single-particle angular momentum to collective angular momentum, that
is essentual to reach the cranking limit. The derivation also requires a
distinct, angular-momentum violating, step. The structure of the result implies
the possibility of tilted-axis cranking for the axial case and full
three-dimensional cranking for the triaxial one. The final equations remain
number conserving. In an appendix, the Kerman-Klein method is developed in more
detail, and the outlines of several algorithms for obtaining solutions of the
associated non-linear formalism are suggested.Comment: 29 page
Solution of the Skyrme-Hartree-Fock-Bogolyubov equations in the Cartesian deformed harmonic-oscillator basis. (VI) HFODD (v2.38j): a new version of the program
We describe the new version (v2.38j) of the code HFODD which solves the
nuclear Skyrme-Hartree-Fock or Skyrme-Hartree-Fock-Bogolyubov problem by using
the Cartesian deformed harmonic-oscillator basis. In the new version, we have
implemented: (i) projection on good angular momentum (for the Hartree-Fock
states), (ii) calculation of the GCM kernels, (iii) calculation of matrix
elements of the Yukawa interaction, (iv) the BCS solutions for state-dependent
pairing gaps, (v) the HFB solutions for broken simplex symmetry, (vi)
calculation of Bohr deformation parameters, (vii) constraints on the Schiff
moments and scalar multipole moments, (viii) the D2h transformations and
rotations of wave functions, (ix) quasiparticle blocking for the HFB solutions
in odd and odd-odd nuclei, (x) the Broyden method to accelerate the
convergence, (xi) the Lipkin-Nogami method to treat pairing correlations, (xii)
the exact Coulomb exchange term, (xiii) several utility options, and we have
corrected two insignificant errors.Comment: 45 LaTeX pages, 4 figures, submitted to Computer Physics
Communication
The nuclear energy density functional formalism
The present document focuses on the theoretical foundations of the nuclear
energy density functional (EDF) method. As such, it does not aim at reviewing
the status of the field, at covering all possible ramifications of the approach
or at presenting recent achievements and applications. The objective is to
provide a modern account of the nuclear EDF formalism that is at variance with
traditional presentations that rely, at one point or another, on a {\it
Hamiltonian-based} picture. The latter is not general enough to encompass what
the nuclear EDF method represents as of today. Specifically, the traditional
Hamiltonian-based picture does not allow one to grasp the difficulties
associated with the fact that currently available parametrizations of the
energy kernel at play in the method do not derive from a genuine
Hamilton operator, would the latter be effective. The method is formulated from
the outset through the most general multi-reference, i.e. beyond mean-field,
implementation such that the single-reference, i.e. "mean-field", derives as a
particular case. As such, a key point of the presentation provided here is to
demonstrate that the multi-reference EDF method can indeed be formulated in a
{\it mathematically} meaningful fashion even if does {\it not} derive
from a genuine Hamilton operator. In particular, the restoration of symmetries
can be entirely formulated without making {\it any} reference to a projected
state, i.e. within a genuine EDF framework. However, and as is illustrated in
the present document, a mathematically meaningful formulation does not
guarantee that the formalism is sound from a {\it physical} standpoint. The
price at which the latter can be enforced as well in the future is eventually
alluded to.Comment: 64 pages, 8 figures, submitted to Euroschool Lecture Notes in Physics
Vol.IV, Christoph Scheidenberger and Marek Pfutzner editor
A re-interpretation of the concept of mass and of the relativistic mass-energy relation
For over a century the definitions of mass and derivations of its relation
with energy continue to be elaborated, demonstrating that the concept of mass
is still not satisfactorily understood. The aim of this study is to show that,
starting from the properties of Minkowski spacetime and from the principle of
least action, energy expresses the property of inertia of a body. This implies
that inertial mass can only be the object of a definition - the so called
mass-energy relation - aimed at measuring energy in different units, more
suitable to describe the huge amount of it enclosed in what we call the
"rest-energy" of a body. Likewise, the concept of gravitational mass becomes
unnecessary, being replaceable by energy, thus making the weak equivalence
principle intrinsically verified. In dealing with mass, a new unit of
measurement is foretold for it, which relies on the de Broglie frequency of
atoms, the value of which can today be measured with an accuracy of a few parts
in 10^9
PHANGS-JWST First Results: A combined HST and JWST analysis of the nuclear star cluster in NGC 628
We combine archival HST and new JWST imaging data, covering the ultraviolet
to mid-infrared regime, to morphologically analyze the nuclear star cluster
(NSC) of NGC 628, a grand-design spiral galaxy. The cluster is located in a 200
pc x 400 pc cavity, lacking both dust and gas. We find roughly constant values
for the effective radius (r_eff ~ 5 pc) and ellipticity ({\epsilon} ~ 0.05),
while the S\'ersic index (n) and position angle (PA) drop from n ~ 3 to ~ 2 and
PA ~ 130{\deg} to 90{\deg}, respectively. In the mid-infrared, r_eff ~ 12pc,
{\epsilon} ~ 0.4, and n ~ 1-1.5, with the same PA ~ 90{\deg}. The NSC has a
stellar mass of log10 (M_nsc / M_Sun) = 7.06 +- 0.31, as derived through B-V,
confirmed when using multi-wavelength data, and in agreement with the
literature value. Fitting the spectral energy distribution, excluding the
mid-infrared data, yields a main stellar population's age of (8 +- 3) Gyr with
a metallicity of Z = 0.012 +- 0.006. There is no indication of any significant
star formation over the last few Gyr. Whether gas and dust were dynamically
kept out or evacuated from the central cavity remains unclear. The best-fit
suggests an excess of flux in the mid-infrared bands, with further indications
that the center of the mid-infrared structure is displaced with respect to the
optical center of the NSC. We discuss five potential scenarios, none of them
fully explaining both the observed photometry and structure.Comment: 26 pages, 10 figures, 6 tables. Accepted for publication by ApJ
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