98,394 research outputs found
Supernova Type Ia progenitors from merging double white dwarfs: Using a new population synthesis model
The study of Type Ia supernovae (SNIa) has lead to greatly improved insights
into many fields in astrophysics, however a theoretical explanation of the
origin of these events is still lacking. We investigate the potential
contribution to the SNIa rate from the population of merging double
carbon-oxygen white dwarfs. We aim to develope a model that fits the observed
SNIa progenitors as well as the observed close double white dwarf population.
We differentiate between two scenarios for the common envelope (CE) evolution;
the alpha-formalism based on the energy equation and the gamma-formalism that
is based on the angular momentum equation. In one model we apply the
alpha-formalism always. In the second model the gamma-formalism is applied,
unless the binary contains a compact object or the CE is triggered by a tidal
instability for which the alpha-formalism is used. The binary population
synthesis code SeBa was used to evolve binary systems from the zero-age main
sequence to the formation of double white dwarfs and subsequent mergers. SeBa
has been thoroughly updated since the last publication of the content of the
code. The limited sample of observed double white dwarfs is better represented
by the simulated population using the gamma-formalism than the alpha-formalism.
For both CE formalisms, we find that although the morphology of the simulated
delay time distribution matches that of the observations within the errors, the
normalisation and time-integrated rate per stellar mass are a factor 7-12 lower
than observed. Furthermore, the characteristics of the simulated populations of
merging double carbon-oxygen white dwarfs are discussed and put in the context
of alternative SNIa models for merging double white dwarfs.Comment: 16 pages (including 4 pages appendix), 15 figure
Quasars at z=6: the survival of the fittest
The Sloan Digital Sky survey detected luminous quasars at very high redshift,
z>6. Follow-up observations indicated that at least some of these quasars are
powered by supermassive black holes (SMBHs) with masses in excess of billion
solar masses. SMBHs, therefore, seem to have already existed when the Universe
was less than 1 Gyr old, and the bulk of galaxy formation still has to take
place. We investigate in this paper to which extent accretion and dynamical
processes influence the early growth of SMBHs. We assess the impact of (i)
black hole mergers, (ii) the influence of the merging efficiency and (iii) the
negative contribution due to dynamical effects which can kick black holes out
of their host halos (gravitational recoil). We find that if accretion is always
limited by the Eddington rate via a thin disc, the maximum radiative efficiency
allowed to reproduce the LF at z=6 is of order 12%, when the adverse effect of
the gravitational recoil is taken into consideration. Dynamical effects cannot
be neglected in studies of high-redshift SMBHs. If black holes can accrete at
super-critical rate during an early phase, reproducing the observed SMBH mass
values is not an issue, even in the case that the recoil velocity is in the
upper limits range, as the mass ratios of merging binaries are skewed towards
low values, where the gravitational recoil effect is very mild. We propose that
SMBH growth at early times is very selective, and efficient only for black
holes hosted in high density peak halos.Comment: Accepted for publication in the ApJ. 9 pages, 6 b/w figure
A dual modelling of evolving political opinion networks
We present the result of a dual modeling of opinion network. The model
complements the agent-based opinion models by attaching to the social agent
(voters) network a political opinion (party) network having its own intrinsic
mechanisms of evolution. These two sub-networks form a global network which can
be either isolated from or dependent on the external influence. Basically, the
evolution of the agent network includes link adding and deleting, the opinion
changes influenced by social validation, the political climate, the
attractivity of the parties and the interaction between them. The opinion
network is initially composed of numerous nodes representing opinions or
parties which are located on a one dimensional axis according to their
political positions. The mechanism of evolution includes union, splitting,
change of position and of attractivity, taken into account the pairwise node
interaction decaying with node distance in power law. The global evolution ends
in a stable distribution of the social agents over a quasi-stable and
fluctuating stationary number of remaining parties. Empirical study on the
lifetime distribution of numerous parties and vote results is carried out to
verify numerical results
Relativistic neutron star merger simulations with non-zero temperature equations of state I. Variation of binary parameters and equation of state
An extended set of binary neutron star (NS) merger simulations is performed
with an approximative conformally flat treatment of general relativity to
systematically investigate the influence of the nuclear equation of state
(EoS), the neutron star masses, and the NS spin states prior to merging. We
employ the two non-zero temperature EoSs of Shen et al. (1998a,b) and Lattimer
& Swesty (1991). In addition, we use the cold EoS of Akmal et al. (1998) with a
simple ideal-gas-like extension according to Shibata & Taniguchi (2006), and an
ideal-gas EoS with parameters fitted to the supernuclear part of the Shen-EoS.
We estimate the mass sitting in a dilute high-angular momentum ``torus'' around
the future black hole (BH). The dynamics and outcome of the models is found to
depend strongly on the EoS and on the binary parameters. Larger torus masses
are found for asymmetric systems (up to ~0.3 M_sun for a mass ratio of 0.55),
for large initial NSs, and for a NS spin state which corresponds to a larger
total angular momentum. We find that the postmerger remnant collapses either
immediately or after a short time when employing the soft EoS of Lattimer&
Swesty, whereas no sign of post-merging collapse is found within tens of
dynamical timescales for all other EoSs used. The typical temperatures in the
torus are found to be about 3-10 MeV depending on the strength of the shear
motion at the collision interface between the NSs and thus depending on the
initial NS spins. About 10^{-3}-10^{-2} M_sun of NS matter become
gravitationally unbound during or right after the merging process. This matter
consists of a hot/high-entropy component from the collision interface and (only
in case of asymmetric systems) of a cool/low-entropy component from the spiral
arm tips. (abridged)Comment: 20 pages, 15 figures, accepted for publication in A&A, included
changes based on referee comment
Bounds on Expected Black Hole Spins in Inspiraling Binaries
As a first step towards understanding the angular momentum evolution history
of black holes in merging black-hole/neutron-star binaries, we perform
population synthesis calculations to track the distribution of accretion
histories of compact objects in such binaries. We find that there are three
distinct processes which can possibly contribute to the black-hole spin
magnitude: a birth spin for the black hole, imparted at either (i) the collapse
of a massive progenitor star to a black hole or (ii) the accretion-induced
collapse of a neutron star to a black hole; and (iii) an accretion spin-up when
the already formed black hole [via (i) or (ii)] goes through an accretion
episode (through an accretion disk or a common-envelope phase). Our results
show that, with regard to accretion-induced spinup in merging BH-NS binaries
[method (iii) above], only
{\em accretion episodes associated with common-envelope phases and
hypercritical accretion rates} occur in the formation history of merging black
hole/neutron star binaries. Lacking unambiguous experimental information about
BH birth spins [i.e., regarding the results of processes (i) and (ii)], we
choose two fiducial values for the BH birth angular momentum parameter a=J/M^2,
consistent with observations of (i) NS birth spins (a roughly 0) and (ii) X-ray
binaries (a=0.5). Using these two fiducial values and a conservative upper
bound on the specific angular momentum of accreted matter, we discuss the
expected range of black hole spins in the binaries of interest. We conclude
with comments on the significance of these results for ground-based
gravitational-wave searches of inspiral signals from black hole binaries.Comment: Submitted to ApJ. (v1) Uses emulateapj.cls. 5 figures. (v2):
corrected reference list and uses smaller figures (v3): Includes changes in
response to referee comments, including new discussion of XRBs. Figures
merged, so only 3 figures (v4) Minor typo correction, plus updated abstract
posted onlin
Identification of Group Changes in Blogosphere
The paper addresses a problem of change identification in social group
evolution. A new SGCI method for discovering of stable groups was proposed and
compared with existing GED method. The experimental studies on a Polish
blogosphere service revealed that both methods are able to identify similar
evolution events even though both use different concepts. Some differences were
demonstrated as wellComment: The 2012 IEEE/ACM International Conference on Advances in Social
Networks Analysis and Mining, IEEE Computer Society, 2012, pp. 1233-123
Coalescing Binary Neutron Stars
Coalescing compact binaries with neutron star or black hole components
provide the most promising sources of gravitational radiation for detection by
the LIGO/VIRGO/GEO/TAMA laser interferometers now under construction. This fact
has motivated several different theoretical studies of the inspiral and
hydrodynamic merging of compact binaries. Analytic analyses of the inspiral
waveforms have been performed in the Post-Newtonian approximation. Analytic and
numerical treatments of the coalescence waveforms from binary neutron stars
have been performed using Newtonian hydrodynamics and the quadrupole radiation
approximation. Numerical simulations of coalescing black hole and neutron star
binaries are also underway in full general relativity. Recent results from each
of these approaches will be described and their virtues and limitations
summarized.Comment: Invited Topical Review paper to appear in Classical and Quantum
Gravity, 35 pages, including 5 figure
Coalitional Games in MISO Interference Channels: Epsilon-Core and Coalition Structure Stable Set
The multiple-input single-output interference channel is considered. Each
transmitter is assumed to know the channels between itself and all receivers
perfectly and the receivers are assumed to treat interference as additive
noise. In this setting, noncooperative transmission does not take into account
the interference generated at other receivers which generally leads to
inefficient performance of the links. To improve this situation, we study
cooperation between the links using coalitional games. The players (links) in a
coalition either perform zero forcing transmission or Wiener filter precoding
to each other. The -core is a solution concept for coalitional games
which takes into account the overhead required in coalition deviation. We
provide necessary and sufficient conditions for the strong and weak
-core of our coalitional game not to be empty with zero forcing
transmission. Since, the -core only considers the possibility of
joint cooperation of all links, we study coalitional games in partition form in
which several distinct coalitions can form. We propose a polynomial time
distributed coalition formation algorithm based on coalition merging and prove
that its solution lies in the coalition structure stable set of our coalition
formation game. Simulation results reveal the cooperation gains for different
coalition formation complexities and deviation overhead models.Comment: to appear in IEEE Transactions on Signal Processing, 14 pages, 14
figures, 3 table
r-process enrichment of ultra-faint dwarf galaxies by fast merging double neutron stars
The recent aLIGO/aVirgo discovery of gravitational waves from the neutron
star merger (NSM) GW170817 and the follow up kilonova observations have shown
that NSMs produce copious amount of r-process material. However, it is
difficult to reconcile the large natal kicks and long average merging times of
Double Neutron Stars (DNSs), with the levels of r-process enrichment seen in
ultra faint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming
that such dwarf systems have lost a significant fraction of their stellar mass
through tidal stripping, we conclude that contrary to most current models, it
is the DNSs with rather large natal kicks but very short merging timescales
that can enrich UFD-type galaxies. These binaries are either on highly
eccentric orbits, or form with very short separations due to an additional
mass-transfer between the first-born neutron star and a naked helium star,
progenitor of the second-born neutron star. These DNSs are born with a
frequency that agrees with the statistics of the r-process UFDs, and merge well
within the virial radius of their host halos, therefore contributing
significantly to their r-process enrichment.Comment: Accepted for publication in Ap
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