32 research outputs found
Formation of the first three gravitational-wave observations through isolated binary evolution
During its first 4 months of taking data, Advanced LIGO has detected
gravitational waves from two binary black hole mergers, GW150914 and GW151226,
along with the statistically less significant binary black hole merger
candidate LVT151012. We use our rapid binary population synthesis code COMPAS
to show that all three events can be explained by a single evolutionary channel
-- classical isolated binary evolution via mass transfer including a common
envelope phase. We show all three events could have formed in low-metallicity
environments (Z = 0.001) from progenitor binaries with typical total masses
, and , for
GW150914, GW151226, and LVT151012, respectively.Comment: Published in Nature Communication
Stellar triples with chemically homogeneously evolving inner binaries
Observations suggest that massive stellar triples are common. However, their
evolution is not yet fully understood. We investigate the evolution of
hierarchical triples in which the stars of the inner binary experience
chemically homogeneous evolution (CHE), particularly to understand the role of
the tertiary star in the formation of gravitational-wave (GW) sources. We use
the triple-star rapid population synthesis code TRES to determine the evolution
of these systems at two representative metallicities: and . About half of all triples harbouring a CHE inner binary (CHE triples)
experience tertiary mass transfer (TMT) episodes, an event which is rare for
classically evolving stars. In the majority of TMT episodes, the inner binary
consists of two main-sequence stars (58-60 per cent) or two black holes (BHs,
24-31 per cent). Additionally, we explore the role of von Zeipel-Lidov-Kozai
(ZLK) oscillations for CHE triples. ZLK oscillations can result in eccentric
stellar mergers or lead to the formation of eccentric compact binaries in
systems with initial outer pericenters smaller than 1200 .
Approximately 24-30 per cent of CHE triples form GW sources, and in 31 per cent
of these, the tertiary star plays a significant role and leads to
configurations that are not predicted for isolated binaries. We conclude that
the evolution of CHE binaries can be affected by a close tertiary companion,
resulting in astronomical transients such as BH-BH binaries that merge via GW
emission orders of magnitude faster than their isolated binary counterparts and
tertiary-driven massive stellar mergers.Comment: 27 pages, 17 figures, 3 tables. Submitted to MNRA
Stellar triples with chemically homogeneously evolving inner binaries
Observations suggest that massive stellar triples are common. However, their evolution is not yet fully understood. We investigatethe evolution of hierarchical triples in which the stars of the inner binary experience chemically homogeneous evolution (CHE),particularly to understand the role of the tertiary star in the formation of gravitational-wave (GW) sources. We use the triple-starrapid population synthesis code TRES to determine the evolution of these systems at two representative metallicities: Z = 0.005and Z = 0.0005. About half of all triples harbouring a CHE inner binary (CHE triples) experience tertiary mass transfer (TMT)episodes, an event which is rare for classically evolving stars. In the majority of TMT episodes, the inner binary consistsof two main-sequence stars (58–60 per cent) or two black holes (BHs, 24–31 per cent). Additionally, we explore the role ofvon Zeipel-Lidov-Kozai (ZLK) oscillations for CHE triples. ZLK oscillations can result in eccentric stellar mergers or lead tothe formation of eccentric compact binaries in systems with initial outer pericentre smaller than ∼ 1200 R. Approximately24–30 per cent of CHE triples form GW sources, and in 31 per cent of these, the tertiary star plays a significant role and leadsto configurations that are not predicted for isolated binaries. We conclude that the evolution of CHE binaries can be affected bya close tertiary companion, resulting in astronomical transients such as BH–BH binaries that merge via GW emission orders ofmagnitude faster than their isolated binary counterparts and tertiary-driven massive stellar merger
Constraints on Weak Supernova Kicks from Observed Pulsar Velocities
Observations of binary pulsars and pulsars in globular clusters suggest that
at least some pulsars must receive weak natal kicks at birth. If all pulsars
received strong natal kicks above \unit[50]{\kms}, those born in globular
clusters would predominantly escape, while wide binaries would be disrupted. On
the other hand, observations of transverse velocities of isolated radio pulsars
indicate that only have velocities below \unit[50]{\kms}. We explore
this apparent tension with rapid binary population synthesis modelling. We
propose a model in which supernovae with characteristically low natal kicks
(e.g., electron-capture supernovae) only occur if the progenitor star has been
stripped via binary interaction with a companion. We show that this model
naturally reproduces the observed pulsar speed distribution and without
reducing the predicted merging double neutron star yield. We estimate that the
zero-age main sequence mass range for non-interacting progenitors of
electron-capture supernovae should be no wider than .Comment: 7 pages including figures, tables, and references. 3 figures
including 5 pdfs, and 1 tabl
Accuracy of inference on the physics of binary evolution from gravitational-wave observations
The properties of the population of merging binary black holes encode some of
the uncertain physics of the evolution of massive stars in binaries. The binary
black hole merger rate and chirp mass distribution are being measured by
ground-based gravitational-wave detectors. We consider isolated binary
evolution and explore how accurately the physical model can be constrained with
such observations by applying the Fisher information matrix to the merging
black hole population simulated with the rapid binary population synthesis code
COMPAS. We investigate variations in four COMPAS parameters: common envelope
efficiency, kick velocity dispersion, and mass loss rates during the luminous
blue variable and Wolf--Rayet stellar evolutionary phases. We find that 1000
observations would constrain these model parameters to a fractional accuracy of
a few percent. Given the empirically determined binary black hole merger rate,
we can expect gravitational-wave observations alone to place strong constraints
on the physics of stellar and binary evolution within a few years.Comment: 12 pages, 9 figures; version accepted by Monthly Notices of the Royal
Astronomical Societ
STROOPWAFEL: Simulating rare outcomes from astrophysical populations, with application to gravitational-wave sources
Gravitational-wave observations of double compact object (DCO) mergers are
providing new insights into the physics of massive stars and the evolution of
binary systems. Making the most of expected near-future observations for
understanding stellar physics will rely on comparisons with binary population
synthesis models. However, the vast majority of simulated binaries never
produce DCOs, which makes calculating such populations computationally
inefficient. We present an importance sampling algorithm, STROOPWAFEL, that
improves the computational efficiency of population studies of rare events, by
focusing the simulation around regions of the initial parameter space found to
produce outputs of interest. We implement the algorithm in the binary
population synthesis code COMPAS, and compare the efficiency of our
implementation to the standard method of Monte Carlo sampling from the birth
probability distributions. STROOPWAFEL finds 25-200 times more DCO
mergers than the standard sampling method with the same simulation size, and so
speeds up simulations by up to two orders of magnitude. Finding more DCO
mergers automatically maps the parameter space with far higher resolution than
when using the traditional sampling. This increase in efficiency also leads to
a decrease of a factor 3-10 in statistical sampling uncertainty for the
predictions from the simulations. This is particularly notable for the
distribution functions of observable quantities such as the black hole and
neutron star chirp mass distribution, including in the tails of the
distribution functions where predictions using standard sampling can be
dominated by sampling noise.Comment: Accepted. Data and scripts to reproduce main results is publicly
available. The code for the STROOPWAFEL algorithm will be made publicly
available. Early inquiries can be addressed to the lead autho
Common-Envelope Episodes that lead to Double Neutron Star formation
Close double neutron stars have been observed as Galactic radio pulsars,
while their mergers have been detected as gamma-ray bursts and
gravitational-wave sources. They are believed to have experienced at least one
common-envelope episode during their evolution prior to double neutron star
formation. In the last decades there have been numerous efforts to understand
the details of the common-envelope phase, but its computational modelling
remains challenging. We present and discuss the properties of the donor and the
binary at the onset of the Roche-lobe overflow leading to these common-envelope
episodes as predicted by rapid binary population synthesis models. These
properties can be used as initial conditions for detailed simulations of the
common-envelope phase. There are three distinctive populations, classified by
the evolutionary stage of the donor at the moment of the onset of the
Roche-lobe overflow: giant donors with fully-convective envelopes, cool donors
with partially-convective envelopes, and hot donors with radiative envelopes.
We also estimate that, for standard assumptions, tides would not circularise a
large fraction of these systems by the onset of Roche-lobe overflow. This makes
the study and understanding of eccentric mass-transferring systems relevant for
double neutron star populations.Comment: 26 pages, 10 figures. Includes bug fix. Two new figures and an
appendix adde
Be X-ray binaries in the SMC as indicators of mass transfer efficiency
Be X-ray binaries (BeXRBs) consist of rapidly rotating Be stars with neutron
star companions accreting from the circumstellar emission disk. We compare the
observed population of BeXRBs in the Small Magellanic Cloud with simulated
populations of BeXRB-like systems produced with the COMPAS population synthesis
code. We focus on the apparently higher minimal mass of Be stars in BeXRBs than
in the Be population at large. Assuming that BeXRBs experienced only
dynamically stable mass transfer, their mass distribution suggests that at
least 30% of the mass donated by the progenitor of the neutron star is
typically accreted by the B-star companion. We expect these results to affect
predictions for the population of double compact object mergers. A convolution
of the simulated BeXRB population with the star formation history of the Small
Magellanic Cloud shows that the excess of BeXRBs is most likely explained by
this galaxy's burst of star formation around 20--40 Myr ago
Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave Observations I: Black Hole-Neutron Star Mergers
Mergers of black hole-neutron star (BHNS) binaries have now been observed by
GW detectors with the recent announcement of GW200105 and GW200115. Such
observations not only provide confirmation that these systems exist, but will
also give unique insights into the death of massive stars, the evolution of
binary systems and their possible association with gamma-ray bursts,
-process enrichment and kilonovae. Here we perform binary population
synthesis of isolated BHNS systems in order to present their merger rate and
characteristics for ground-based GW observatories. We present the results for
420 different model permutations that explore key uncertainties in our
assumptions about massive binary star evolution (e.g. mass transfer,
common-envelope evolution, supernovae), and the metallicity-specific star
formation rate density, and characterize their relative impacts on our
predictions. We find intrinsic local BHNS merger rates spanning
- for our
full range of assumptions. This encompasses the rate inferred from recent BHNS
GW detections, and would yield detection rates of - for a GW network consisting of LIGO, Virgo and
KAGRA at design sensitivity. We find that the binary evolution and
metallicity-specific star formation rate density each impact the predicted
merger rates by order . We also present predictions for the GW
detected BHNS merger properties and find that all 420 model variations predict
that of the BHNS mergers have BH masses ,
total masses , chirp masses ,
mass ratios or . Moreover, we find that massive NSs
are expected to be commonly detected in BHNS mergers in
almost all our model variations.Comment: 38 pages, 18 figures, accepted to MNRAS. The authors welcome
suggestions and feedback. All data and code to reproduce the results in this
paper are publicly availabl