2,050 research outputs found
Merging binary black holes formed through chemically homogeneous evolution in short-period stellar binaries
We explore a newly proposed channel to create binary black holes of stellar
origin. This scenario applies to massive, tight binaries where mixing induced
by rotation and tides transports the products of hydrogen burning throughout
the stellar envelopes. This slowly enriches the entire star with helium,
preventing the build-up of an internal chemical gradient. The stars remain
compact as they evolve nearly chemically homogeneously, eventually forming two
black holes, which, we estimate, typically merge 4--11 Gyr after formation.
Like other proposed channels, this evolutionary pathway suffers from
significant theoretical uncertainties, but could be constrained in the near
future by data from advanced ground-based gravitational-wave detectors. We
perform Monte Carlo simulations of the expected merger rate over cosmic time to
explore the implications and uncertainties. Our default model for this channel
yields a local binary black hole merger rate of about Gpc yr
at redshift , peaking at twice this rate at . This means that this
channel is competitive, in terms of expected rates, with the conventional
formation scenarios that involve a common-envelope phase during isolated binary
evolution or dynamical interaction in a dense cluster. The events from this
channel may be distinguished by the preference for nearly equal-mass components
and high masses, with typical total masses between 50 and 110
. Unlike the conventional isolated binary evolution scenario
that involves shrinkage of the orbit during a common-envelope phase, short time
delays are unlikely for this channel, implying that we do not expect mergers at
high redshift.Comment: Minor update to match the version published in MNRAS; 15 pages 10
figure
Merger rates of double neutron stars and stellar origin black holes: The Impact of Initial Conditions on Binary Evolution Predictions
The initial mass function (IMF), binary fraction and distributions of binary
parameters (mass ratios, separations and eccentricities) are indispensable
input for simulations of stellar populations. It is often claimed that these
are poorly constrained significantly affecting evolutionary predictions.
Recently, dedicated observing campaigns provided new constraints on the initial
conditions for massive stars. Findings include a larger close binary fraction
and a stronger preference for very tight systems. We investigate the impact on
the predicted merger rates of neutron stars and black holes.
Despite the changes with previous assumptions, we only find an increase of
less than a factor 2 (insignificant compared with evolutionary uncertainties of
typically a factor 10-100). We further show that the uncertainties in the new
initial binary properties do not significantly affect (within a factor of 2)
our predictions of double compact object merger rates. An exception is the
uncertainty in IMF (variations by a factor of 6 up and down). No significant
changes in the distributions of final component masses, mass ratios, chirp
masses and delay times are found.
We conclude that the predictions are, for practical purposes, robust against
uncertainties in the initial conditions concerning binary parameters with
exception of the IMF. This eliminates an important layer of the many uncertain
assumptions affecting the predictions of merger detection rates with the
gravitational wave detectors aLIGO/aVirgo.Comment: Accepted for publication in Ap
Has the Stability and Growth Pact Impeded Political Budget Cycles in the European Union?
This paper examines whether there is a political budget cycle (PBC) in countries in the euro area. Using a multivariate model for the period 1999-2004 and various election indicators we find strong evidence that the Stability and Growth Pact has not restricted fiscal policy makers in the euro area in pursuing expansionary policies before elections. In an election-year – but not in the year prior to the election – the budget deficit increases. This result is in line with third generation PBC models, which are based on moral hazard. We also find a significant but small partisan effect on fiscal policy outcomes.fiscal policy, political budget cycle, Stability and Growth Pact
Massive binaries and the enrichment of the interstellar medium in globular clusters
Abundance anomalies observed in globular cluster stars indicate pollution
with material processed by hydrogen burning. Two main sources have been
suggested: asymptotic giant branch stars and massive stars rotating near the
break-up limit. We discuss the potential of massive binaries as an interesting
alternative source of processed material.
We discuss observational evidence for mass shedding from interacting
binaries. In contrast to the fast, radiatively driven winds of massive stars,
this material is typically ejected with low velocity. We expect that it remains
inside the potential well of a globular cluster and becomes available for the
formation or pollution of a second generation of stars. We estimate that the
amount of processed low-velocity material that can be ejected by massive
binaries is larger than the contribution of two previously suggested sources
combined.Comment: 6 pages, 2 figures, to appear in the proceedings of IAU Symposium
266, "Star Clusters - Basic Galactic Building Blocks throughout Time and
Space", 10-14 August 2009, at the general assembly in Rio de Janeiro, Brazi
Extreme isolation of WN3/O3 stars and implications for their evolutionary origin as the elusive stripped binaries
Recent surveys of the Magellanic Clouds have revealed a subtype of Wolf-Rayet
(WR) star with peculiar properties. WN3/O3 spectra exhibit both WR-like
emission and O3 V-like absorption - but at lower luminosity than O3 V or WN
stars. We examine the projected spatial distribution of WN3/O3 stars in the LMC
as compared to O-type stars. Surprisingly, WN3/O3 stars are among the most
isolated of all classes of massive stars; they have a distribution similar to
red supergiants dominated by initial masses of 10-15 , and are far
more dispersed than classical WR stars or luminous blue variables (LBVs). Their
lack of association with clusters of O-type stars suggests strongly that WN3/O3
stars are not the descendants of single massive stars (30 or
above). Instead, they are likely products of interacting binaries at lower
initial mass (10-18 ). Comparison with binary models suggests a
probable origin with primaries in this mass range that were stripped of their H
envelopes through non-conservative mass transfer by a low-mass secondary. We
show that model spectra and positions on the Hertzsprung-Russell diagram for
binary stripped stars are consistent with WN3/O3 stars. Monitoring radial
velocities with high-resolution spectra can test for low-mass companions or
runaway velocities. With lower initial mass and environments that avoid very
massive stars, the WN3/O3 stars fit expectations for progenitors of Type Ib and
possibly Type Ibn supernovae.Comment: Accepted for publication in MNRA
Binaries are the best single stars
Stellar models of massive single stars are still plagued by major
uncertainties. Testing and calibrating against observations is essential for
their reliability. For this purpose one preferably uses observed stars that
have never experienced strong binary interaction, i.e. "true single stars".
However, the binary fraction among massive stars is high and identifying "true
single stars" is not straight forward. Binary interaction affects systems in
such a way that the initially less massive star becomes, or appears to be,
single. For example, mass transfer results in a widening of the orbit and a
decrease of the luminosity of the donor star, which makes it very hard to
detect. After a merger or disruption of the system by the supernova explosion,
no companion will be present.
The only unambiguous identification of "true single stars" is possible in
detached binaries, which contain two main-sequence stars. For these systems we
can exclude the occurrence of mass transfer since their birth. A further
advantage is that binaries can often provide us with direct measurements of the
fundamental stellar parameters. Therefore, we argue these binaries are worth
the effort needed to observe and analyze them. They may provide the most
stringent test cases for single stellar models.Comment: 5 pages, 1 figure, contribution to the proceedings of "The
multi-wavelength view of hot, massive stars", 39th Li`ege Int. Astroph.
Coll., 12-16 July 201
Can low metallicity binaries avoid merging?
Rapid mass transfer in a binary system can drive the accreting star out of
thermal equilibrium, causing it to expand. This can lead to a contact system,
strong mass loss from the system and possibly merging of the two stars. In low
metallicity stars the timescale for heat transport is shorter due to the lower
opacity. The accreting star can therefore restore thermal equilibrium more
quickly and possibly avoid contact.
We investigate the effect of accretion onto main sequence stars with
radiative envelopes with different metallicities. We find that a low
metallicity (Z<0.001), 4 solar mass star can endure a 10 to 30 times higher
accretion rate before it reaches a certain radius than a star at solar
metallicity. This could imply that up to two times fewer systems come into
contact during rapid mass transfer when we compare low metallicity. This factor
is uncertain due to the unknown distribution of binary parameters and the
dependence of the mass transfer timescale on metallicity. In a forthcoming
paper we will present analytic fits to models of accreting stars at various
metallicities intended for the use in population synthesis models.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 2 figure
Measuring Synchronicity and Co-movement of Business Cycles with an Application to the Euro Area
We develop multivariate measures of synchronicity and co-movement of business cycles. In addition to synchronicity, the co-movement measure takes differences between cycle amplitudes into account that have been overlooked in most previous studies. We apply the new measures to the euro area. Synchronicity and co-movement for the region as a whole do not exhibit a clear upward tendency. Although several countries saw the similarity of their business cycle vis-`a-vis the euro area reference cycle increase, national business cycles remain fairly diverse. Changes in business cycle amplitudes cause most of the observed change in cycle co-movement.business cycles, synchronisation, concordance, co-movement, cycle amplitudes, euro area
Binaries at Low Metallicity: ranges for case A, B and C mass transfer
The evolution of single stars at low metallicity has attracted a large
interest, while the effect of metallicity on binary evolution remains still
relatively unexplored. We study the effect of metallicity on the number of
binary systems that undergo different cases of mass transfer. We find that
binaries at low metallicity are more likely to start transferring mass after
the onset of central helium burning, often referred to as case C mass transfer.
In other words, the donor star in a metal poor binary is more likely to have
formed a massive CO core before the onset of mass transfer.
At solar metallicity the range of initial binary separations that result in
case C evolution is very small for massive stars, because they do not expand
much after the ignition of helium and because mass loss from the system by
stellar winds causes the orbit to widen, preventing the primary star to fill
its Roche lobe. This effect is likely to have important consequences for the
metallicity dependence of the formation rate of various objects through binary
evolution channels, such as long GRBs, double neutron stars and double white
dwarfs.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
LAWLESS WORLD: MAKING AND BREAKING GLOBAL RULES
Philippe Sands (Penguin Books, London 2006) Paperback, Pp 432, ISBN 9780141017990, £8.99This being the first book that I have ever read dedicated exclusively to international law, I was not certain what to expect. I did not feel disappointed or out of my depth with “Lawless World” however, as Professor Sands writes clearly and authoritatively on subject-matter which will be quite familiar to most readers: the Pinochet trial, the Kyoto Protocol, trade rules, foreign investment, Abu Ghraib and Guantanamo, the Iraq War and torture
- …