31,848 research outputs found
Watch This Space: Observing Merging White Dwarfs
The Laser Interferometer Space Antenna (LISA) will open the low-frequency
(0.1-100 mHz) part of the gravitational wave spectrum to direct observation. Of
order 3600 galactic close binary white dwarfs will be individually resolvable
in its all-sky spectrum, of which a dozen systems are expected to be on the
verge of merger, showing the effects of strong tidal heating and/or early onset
of tidal mass transfer. Optical study of these systems would provide important
insights into tidal dissipation mechanisms, and internal heating in merging
white dwarfs that sets ignition conditions for potential type Ia supernovae.
Theoretical modeling and instrumentation programs are needed now to enable a
campaign for optical identifications to exploit this opportunity.Comment: 6 pages. To be published in Binary-Star Evolution: Mass Loss,
Accretion and Mergers, ed. V. Kalogera and M. van der Sluys (AIP Conf. Ser.
Star Clusters and Super Massive Black Holes: High Velocity Stars Production
One possible origin of high velocity stars in the Galaxy is that they are the
product of the interaction of binary systems and supermassive black holes. We
investigate a new production channel of high velocity stars as due to the close
interaction between a star cluster and supermassive black holes in galactic
centres. The high velocity acquired by some stars of the cluster comes from
combined effect of extraction of their gravitational binding energy and from
the slingshot due to the interaction with the black holes. Stars could reach a
velocity sufficient to travel in the halo and even overcome the galactic
potential well, while some of them are just stripped from the cluster and start
orbiting around the galactic centre.Comment: 2 pages, 1 figure. Presented at the MODEST 16/Cosmic Lab conference
in Bologna, Italy, April 18-22 2016. To be pusblshed in Mem. S.A.It.
Conference Serie
Combined effects of tidal and rotational distortions on the equilibrium configuration of low-mass, pre-main sequence stars
In close binary systems, rotation and tidal forces of the component stars
deform each other and destroy their spherical symmetry. We present new models
for low-mass, pre-main sequence stars that include the combined distortion
effects of tidal and rotational forces on the equilibrium configuration of
stars. We investigate the effects of interaction between tides and rotation on
the stellar structure and evolution. The Kippenhahn & Thomas (1970)
approximation, along with the Clairaut-Legendre expansion for the gravitational
potential of a self-gravitating body, is used to take the distortion effects
into account. We obtained values of internal structure constants for low-mass,
pre-main sequence stars from stellar evolutionary models that consider the
combined effects of rotation and tidal forces due to a companion star. We also
derived a new expression for the rotational inertia of a tidally and
rotationally distorted star. Our distorted models were successfully used to
analyze the eclipsing binary system EK Cep, reproducing the stellar radii,
effective temperature ratio, lithium depletion, rotational velocities, and the
apsidal motion rate in the age interval of 15.5-16.7 Myr. In the low-mass
range, the assumption that harmonics greater than j=2 can be neglected seems
not to be fully justified, although it is widely used when analyzing the
apsidal motion of binary systems. The non-standard evolutionary tracks are
cooler than the standard ones, mainly for low-mass stars. Distorted models
predict more mass-concentrated stars at the zero-age main-sequence than
standard models
Orbital evolution of LIGO/Virgo binaries in stellar clusters driven by cluster tides, stellar encounters and general relativity
Origin of LIGO/Virgo gravitational wave events may involve production of
binaries with relativistic components in dense stellar systems - globular or
nuclear star clusters - and their subsequent evolution towards merger. Orbital
parameters of these binaries (the inner orbit) and their motion inside the
cluster (the outer orbit) evolve due to both external agents - random
encounters with cluster stars and cluster tides due to the smooth cluster
potential - and the internal ones - various sources of dissipation and
precession within the binary. We present a numerical framework - Binary
Evolution in Stellar Clusters (BESC) - that follows the evolution of the binary
inner and outer orbits accounting for all these effects simultaneously,
enabling efficient Monte Carlo studies. The secular effect of cluster tides is
computed in the singly-averaged approximation, without averaging over the outer
binary orbit. As to stellar encounters, we include the effects of both close
and distant flybys on the inner and outer orbits of the binary, respectively.
In particular, this allows us to explicitly account for the dynamical friction
sinking the binary towards the cluster centre. Also, given our focus on the
LIGO/Virgo sources, we include the general relativistic precession (which
suppresses cluster tides at high eccentricities) and the gravitational wave
emission (shrinking the binary orbit). We use BESC to illustrate a number of
characteristic binary evolutionary outcomes and discuss relative contributions
of different physical processes. BESC can also be used to study other objects
in clusters, e.g. blue stragglers, hot Jupiters, X-ray binaries, etc.Comment: 20 pages, 16 figures, submitted to MNRA
The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas Cloud
Using high-resolution SPH numerical simulations, we investigate the effects
of gas on the inspiral and merger of a massive black hole binary. This study is
motivated by both observational and theoretical work that indicate the presence
of large amounts of gas in the central regions of merging galaxies. N-body
simulations have shown that the coalescence of a massive black hole binary
eventually stalls in a stellar background. However, our simulations suggest
that the massive black hole binary will finally merge if it is embedded in a
gaseous background. Here we present results in which the gas is assumed to be
initially spherical with a relatively smooth distribution. In the early
evolution of the binary, the separation dimishes due to the gravitational drag
exerted by the background gas. In the later stages, when the binary dominates
the gravitational potential in its vicinity, the medium responds by forming an
ellipsoidal density enhancement whose axis lags behind the binary axis, and
this offset produces a torque on the binary that causes continuing loss of
angular momentum and is able to reduce the binary separation to distances where
gravitational radiation is efficient. Assuming typical parameters from
observations of Ultra Luminous Infrared Galaxies, we predict that a black hole
binary will merge within yrs; therefore these results imply that in a
merger of gas-rich galaxies, any massive central black holes will coalescence
soon after the galaxies merge. Our work thus supports scenarios of massive
black hole evolution and growth where hierarchical merging plays an important
role. The final coalescence of the black holes leads to gravitational radiation
emission that would be detectable up to high redshift by LISA. We show that
similar physical effects are important for the formation of close binary stars.Comment: 38 pages, 14 figures, submitted to Ap
Gravitational wave signals from long lasting binary-single black hole encounters
In the dense regions of star clusters, close encounters with black holes
(BHs) can occur giving rise to a new class of gravitational-wave (GW) signals.
Binary-single encounters between three BHs are expected to dominate the rate of
signals from unbound systems in the frequency band of terrestrial GW detectors.
The encounter can describe a quasi-hyperbolic trajectory, which was the focus
of a recent study. In some cases, the encounter can take a more complex form
including one or two BH mergers as a result of the encounter, repeating cycles
of close proximity between the BHs, and the exchange of a BH that is part of
the binary. The variety of types of encounters leads to a variety of GW signals
emerging from these encounters. Using the ARWV N-body code, we performed 42
numerical simulations, to explore various outcomes of binary-single
interaction, and we characterize the diverse GW signatures produced during
these encounters. Additionally, we evaluated the detectability of these GW
signals by injecting them into the simulated noise of the Einstein Telescope
and exploring different methods to detect the signals. Our findings shed light
on the complexities of these interactions and their potential implications for
GW astronomy.Comment: 20 pages, 9 figure
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
Using binary stars to bound the mass of the graviton
Interacting white dwarf binary star systems, including helium cataclysmic
variable (HeCV) systems, are expected to be strong sources of gravitational
radiation, and should be detectable by proposed space-based laser
interferometer gravitational wave observatories such as LISA. Several HeCV star
systems are presently known and can be studied optically, which will allow
electromagnetic and gravitational wave observations to be correlated.
Comparisons of the phases of a gravitational wave signal and the orbital light
curve from an interacting binary white dwarf star system can be used to bound
the mass of the graviton. Observations of typical HeCV systems by LISA could
potentially yield an upper bound on the inverse mass of the graviton as strong
as km (
eV), more than two orders of magnitude better than present solar system derived
bounds.Comment: 21 pages plus 4 figures; ReVTe
Numerical Models of Binary Neutron Star System Mergers. I.: Numerical Methods and Equilibrium Data for Newtonian Models
The numerical modeling of binary neutron star mergers has become a subject of
much interest in recent years. While a full and accurate model of this
phenomenon would require the evolution of the equations of relativistic
hydrodynamics along with the Einstein field equations, a qualitative study of
the early stages on inspiral can be accomplished by either Newtonian or
post-Newtonian models, which are more tractable. In this paper we offer a
comparison of results from both rotating and non-rotating (inertial) frame
Newtonian calculations. We find that the rotating frame calculations offer
significantly improved accuracy as compared with the inertial frame models.
Furthermore, we show that inertial frame models exhibit significant and
erroneous angular momentum loss during the simulations that leads to an
unphysical inspiral of the two neutron stars. We also examine the dependence of
the models on initial conditions by considering initial configurations that
consist of spherical neutron stars as well as stars that are in equilibrium and
which are tidally distorted. We compare our models those of Rasio & Shapiro
(1992,1994a) and New & Tohline (1997). Finally, we investigate the use of the
isolated star approximation for the construction of initial data.Comment: 32 pages, 19 gif figures, manuscript with postscript figures
available at http://www.astro.sunysb.edu/dswesty/docs/nspap1.p
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