771 research outputs found
Gravitational waves from compact binaries
In this review, I give a summary of the history of our understanding of
gravitational waves and how compact binaries were used to transform their
status from mathematical artefact to physical reality. I also describe the
types of compact (stellar) binaries that LISA will observe as soon as it is
switched on. Finally, the status and near future of LIGO, Virgo and GEO are
discussed, as well as the expected detection rates for the Advanced detectors,
and the accuracies with which binary parameters can be determined when BH/NS
inspirals are detected.Comment: 15 pages, 3 figures, 2 tables. To be published in "Evolution of
compact binaries", editors: Linda Schmidtobreick, Matthias Schreiber and
Claus Tapper
The exclusive (e,ep) reaction at high missing momenta
The reduced (e,ep) cross section is calculated for kinematics that probe
high missing momenta. The final-state interaction is handled within a
non-relativistic many-body framework. One- and two-body nuclear currents are
included. Electron distortion effects are treated in an exact distorted wave
calculation. It is shown that at high missing momenta the calculated (e,ep)
cross sections exhibit a pronounced sensitivity to ground-state correlations of
the RPA type and two-body currents. The role of these mechanisms is found to be
relatively small at low missing momenta.Comment: 15 pages in REVtex with embedded psfigure
Masses and envelope binding energies of primary stars at the onset of a common envelope
We present basic properties of primary stars that initiate a common envelope
(CE) in a binary, while on the giant branch. We use the population-synthesis
code described in Politano et al. (2010) and follow the evolution of a
population of binary stars up to the point where the primary fills its Roche
lobe and initiates a CE. We then collect the properties of each system, in
particular the donor mass and the binding energy of the donor's envelope, which
are important for the treatment of a CE. We find that for most CEs, the donor
mass is sufficiently low to define the core-envelope boundary reasonably well.
We compute the envelope-structure parameter {\lambda_\mathrm{env}} from the
binding energy and compare its distribution to typical assumptions that are
made in population-synthesis codes. We conclude that {\lambda_\mathrm{env}}
varies appreciably and that the assumption of a constant value for this
parameter results in typical errors of 20--50%. In addition, such an assumption
may well result in the implicit assumption of unintended and/or unphysical
values for the CE parameter {\alpha_\mathrm{CE}}. Finally, we discuss accurate
existing analytic fits for the envelope binding energy, which make these
oversimplified assumptions for {\lambda_\mathrm{env}}, and the use of
{\lambda_\mathrm{env}} in general, unnecessary.Comment: 6 pages, 3 figures, 1 table; to be published in the proceedings of
the conference "Binary Star Evolution", in Mykonos, Greece, held in June
22-25, 201
Analytical expressions for the envelope binding energy of giants as a function of basic stellar parameters
The common-envelope (CE) phase is an important stage in the evolution of
binary stellar populations. The most common way to compute the change in
orbital period during a CE is to relate the binding energy of the envelope of
the Roche-lobe filling giant to the change in orbital energy. Especially in
population-synthesis codes, where the evolution of millions of stars must be
computed and detailed evolutionary models are too expensive computationally,
simple approximations are made for the envelope binding energy. In this study,
we present accurate analytic prescriptions based on detailed stellar-evolution
models that provide the envelope binding energy for giants with metallicities
between Z = 10-4 and Z = 0.03 and masses between 0.8 Msun and 100 Msun, as a
function of the metallicity, mass, radius and evolutionary phase of the star.
Our results are also presented in the form of electronic data tables and
Fortran routines that use them. We find that the accuracy of our fits is better
than 15% for 90% of our model data points in all cases, and better than 10% for
90% of our data points in all cases except the asymptotic giant branches for
three of the six metallicities we consider. For very massive stars (M > 50
Msun), when stars lose more than ~20% of their initial mass due to stellar
winds, our fits do not describe the models as accurately. Our results are more
widely applicable - covering wider ranges of metallicity and mass - and are of
higher accuracy than those of previous studies
Move to improve:The effect of physical activity on cognitive control and antisocial behavior
The current thesis investigated the effect of physical activity on antisocial behavior and the possible role of cognitive control in multi-problem young adults. Earlier research uncovered a robust association between impaired cognitive control and antisocial behavior, implying individuals displaying antisocial behavior may benefit from interventions targeting neurocognitive functioning. Impaired cognitive control can severely impact one's life, as it is needed to adequately plan, regulate, and adapt goal-directed behavior. Understandably, deficiencies in these capabilities may sustain or promote antisocial behavior through the impaired ability to anticipate negative consequences, suppress unwanted impulses, and adjust behavior according to social expectations. Physical activity has been previously proposed as an effective intervention for decreasing antisocial behavior in children and adolescents (aged < 18) from the general population and in adult offenders or adults suffering from substance use disorders, yet existing research in young adults (especially clinical or at-risk young adults) is scarce and inconclusive. The observed reduction in antisocial behavior may be the result of an enhancement in neurocognitive functioning, as prior studies indicate robust positive effects on cognitive control following increased physical activity in healthy (but sedentary) youth aged <18 and the elderly. However, to date, the effect of physical activity on cognitive control has not been examined in young adults suffering from multiple problems including aggression, delinquent behavior, frequent substance use, and other externalizing behaviors despite their possible associated executive deficits. To deepen our understanding of the relation between physical activity, antisocial behavior, and cognitive control, we first studied the possible potential of neurobiological and (neuro)behavioral measures of cognitive control in the prediction of and association with real-world behavior, i.e., behavior following a multimodal day treatment and antisocial behavior. We included three indices of neurocognitive functions, i.e., error processing, response inhibition, and interference, using different measurement techniques including functional magnetic resonance imaging and electroencephalographic paradigms in male multi-problem young adults (aged 18 – 27). In the following chapter, we reviewed and quantified the overall effectiveness of previous physical activity interventions in reducing antisocial behavior in a meta-analytic review. Subsequently, in the next two chapters, we discussed the association between physical activity and the impact on behavioral measures of cognitive control using a randomized controlled intervention study. And lastly, we investigated other ways in which physical activity might contribute to the development of positive behavior and the decrease of negative behavior, according to the multi-problem young adults, using a thematic approach. Our findings suggest that overall, increased physical activity may be used to treat antisocial behavior in adults displaying a range of externalizing behaviors. Furthermore, regarding cognitive control, the current results first associated a neurocognitive biomarker (ACC activity during response inhibition) with better outcomes (higher odds of engaging in daytime activities) a year after a multimodal day treatment program in populations displaying antisocial behavior, linking cognitive control to real-world positive behavior. Secondly, our data suggest enhanced neurocognitive functioning following a physical activity program in young adults with impaired cognitive control, who are suffering from multiple problems, including several forms of antisocial behavior. Taken together with prior studies, the results of the current dissertation suggest that a: physical activity interventions and programs may possibly provide an easy-to-implement treatment of antisocial behavior and b: this positive behavioral effect may be partially explained by an exercise-induced enhancement of cognitive control, although future studies including both cognitive control and antisocial behavior are needed to verify this hypothesis
The Formation of Low-Mass Double White Dwarfs through an Initial Phase of Stable Non-Conservative Mass Transfer
Although many double white dwarfs (DWDs) have been observed, the evolutionary
channel by which they are formed from low-mass/long-period
red-giant-main-sequence (RG-MS) binaries remains uncertain. The canonical
explanations involve some variant of double common-envelope (CE) evolution,
however it has been found that such a mechanism cannot produce the observed
distribution. We present a model for the initial episode of mass transfer (MT)
in RG-MS binaries, and demonstrate that their evolution into double white
dwarfs need not arise through a double-CE process, as long as the initial
primary's core mass (Md,c) does not exceed 0.46M. Instead, the first
episode of dramatic mass loss may be stable, non-conservative MT. We find a
lower bound on the fraction of transferred mass that must be lost from the
system in order to provide for MT, and demonstrate the feasibility of this
channel in producing observed low-mass (with M < 0.46M) DWD
systems.Comment: 2 pages, 1 figure, Conference Proceedings for the International
Conference on Binaries, Mykonos, Greec
Why Halley did not discover proper motion and why Cassini did
In 1717 Halley compared contemporaneous measurements of the latitudes of four
stars with earlier measurements by ancient Greek astronomers and by Brahe, and
from the differences concluded that these four stars showed proper motion. An
analysis with modern methods shows that the data used by Halley do not contain
significant evidence for proper motion. What Halley found are the measurement
errors of Ptolemaios and Brahe. Halley further argued that the occultation of
Aldebaran by the Moon on 11 March 509 in Athens confirmed the change in
latitude of Aldebaran. In fact, however, the relevant observation was almost
certainly made in Alexandria where Aldebaran was not occulted. By carefully
considering measurement errors Jacques Cassini showed that Halley's results
from comparison with earlier astronomers were spurious, a conclusion partially
confirmed by various later authors. Cassini's careful study of the measurements
of the latitude of Arcturus provides the first significant evidence for proper
motion.Comment: 15 pages, 3 figures, accepted for publication in the Journal for the
History of Astronom
Population synthesis of classical low-mass X-ray binaries in the Galactic Bulge
Aims. We model the present-day population of 'classical' low-mass X-ray
binaries (LMXBs) with neutron star accretors, which have hydrogen-rich donor
stars. Their population is compared with that of hydrogen-deficient LMXBs,
known as ultracompact X-ray binaries (UCXBs). We model the observable LMXB
population and compare it to observations. Methods. We combine the binary
population synthesis code SeBa with detailed LMXB evolutionary tracks to model
the size and properties of the present-day LMXB population in the Galactic
Bulge. Whether sources are persistent or transient, and what their
instantaneous X-ray luminosities are, is predicted using the thermal-viscous
disk instability model. Results. We find a population of ~2.1 x 10^3 LMXBs with
neutron star accretors. Of these about 15 - 40 are expected to be persistent
(depending on model assumptions), with luminosities higher than 10^35 erg s^-1.
About 7 - 20 transient sources are expected to be in outburst at any given
time. Within a factor of two these numbers are consistent with the observed
population of bright LMXBs in the Bulge. This gives credence to our prediction
of the existence of a population of ~1.6 x 10^3 LMXBs with low donor masses
that have gone through the period minimum, and have present-day mass transfer
rates below 10^-11 Msun yr^-1. Conclusions. Even though the observed population
of hydrogen-rich LMXBs in the Bulge is larger than the observed population of
(hydrogen-deficient) UCXBs, the latter have a higher formation rate. While
UCXBs may dominate the total LMXB population at the present, the majority would
be very faint, or may have become detached and produced millisecond radio
pulsars. In that case UCXBs would contribute significantly more to the
formation of millisecond radio pulsars than hydrogen-rich LMXBs. [abridged]Comment: 8 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysics. v2: minor language correction
Reduced magnetic braking and the magnetic capture model for the formation of ultra-compact binaries
A binary in which a slightly evolved star starts mass transfer to a neutron
star can evolve towards ultra-short orbital periods under the influence of
magnetic braking. This is called magnetic capture. In a previous paper we
showed that ultra-short periods are only reached for an extremely small range
of initial binary parameters, in particular orbital period and donor mass. Our
conclusion was based on one specific choice for the law of magnetic braking,
and for the loss of mass and angular momentum during mass transfer. In this
paper we show that for less efficient magnetic braking it is impossible to
evolve to ultra-short periods, independent of the amount of mass and associated
angular momentum lost from the binary.Comment: 7 pages, 7 figures, accepted for publication in Astronomy and
Astrophysics. See http://www.astro.uu.nl/~sluys/PhD
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