Binary Population Synthesis Study Research Articles

From ZAMS to merger: Detailed binary evolution models of coalescing neutron star – black hole systems at solar metallicity

Neutron star – black hole (NSBH) merger events bring us new opportunities to constrain theories of stellar and binary evolution and understand the nature of compact objects. In this work, we investigated the formation of merging NSBH binaries at solar metallicity by performing a binary population synthesis study of merging NSBH binaries with the newly developed code POSYDON. The latter incorporates extensive grids of detailed single and binary evolution models, covering the entire evolution of a double compact object progenitor. We explored the evolution of NSBHs originating from different formation channels, which in some cases differ from earlier studies performed with rapid binary population synthesis codes. In this paper, we present the population properties of merging NSBH systems and their progenitors such as component masses, orbital features, and BH spins, and we detail our investigation of the model uncertainties in our treatment of common envelope (CE) evolution and the core-collapse process. We find that at solar metallicity, under the default model assumptions, most of the merging NSBHs have BH masses in the range of 3 − 11 M⊙ and chirp masses within 1.5 − 4 M⊙. Independently of our model variations, the BH always forms first with dimensionless spin parameter ≲0.2, which is correlated to the initial binary orbital period. Some BHs can subsequently spin up moderately (χBH ≲ 0.4) due to mass transfer, which we assume to be Eddington limited. Binaries that experience CE evolution rarely demonstrate large tilt angles. Conversely, approximately 40% of the binaries that undergo only stable mass transfer without CE evolution contain an anti-aligned BH. Finally, accounting for uncertainties in both the population modeling and the NS equation of state, we find that 0 − 18.6% of NSBH mergers may be accompanied by an electromagnetic counterpart.

POSYDON: A General-purpose Population Synthesis Code with Detailed Binary-evolution Simulations

Most massive stars are members of a binary or a higher-order stellar system, where the presence of a binary companion can decisively alter their evolution via binary interactions. Interacting binaries are also important astrophysical laboratories for the study of compact objects. Binary population synthesis studies have been used extensively over the last two decades to interpret observations of compact-object binaries and to decipher the physical processes that lead to their formation. Here, we present POSYDON, a novel, publicly available, binary population synthesis code that incorporates full stellar structure and binary-evolution modeling, using the MESA code, throughout the whole evolution of the binaries. The use of POSYDON enables the self-consistent treatment of physical processes in stellar and binary evolution, including: realistic mass-transfer calculations and assessment of stability, internal angular-momentum transport and tides, stellar core sizes, mass-transfer rates, and orbital periods. This paper describes the detailed methodology and implementation of POSYDON, including the assumed physics of stellar and binary evolution, the extensive grids of detailed single- and binary-star models, the postprocessing, classification, and interpolation methods we developed for use with the grids, and the treatment of evolutionary phases that are not based on precalculated grids. The first version of POSYDON targets binaries with massive primary stars (potential progenitors of neutron stars or black holes) at solar metallicity.

The fractions of post-binary-interaction stars and evolved blue straggler stars on the red giant branch of globular clusters

The red giant branch (RGB) of globular clusters (GCs) is home to some exotic stars, which may provide clues on the formation of multiple stellar populations in GCs. It is well known that binary interactions are responsible for many exotic stars. Thus, it is important to understand what fraction of stars on the RGB of GCs is the result of binary interactions. In this paper, we performed a binary population synthesis study to track the number of post-binary-interaction (post-BI) stars that appear on the RGB, with particular emphasis on the evolved blue straggler stars (E-BSSs). Assuming an initial binary fraction of nearly 50%, we find that about half of the objects on the RGB (called giants) underwent the binary interactions, and that E-BSSs account for around 10% of the giants in our standard simulation. We also compare the properties of post-BI giants that evolved from different channels. We find that the initial orbital period and mass ratio distributions significantly affect the fraction of post-BI giants. Our results imply that the non-standard stars from binary interactions provide a non-negligible contribution to the RGB stars in GCs, which should be considered in future investigations of the origin of multiple stellar populations.

He-shell flashes on the surface of oxygen-neon white dwarfs

Accretion induced collapse (AIC) may be responsible for the formation of some interesting neutron star binaries (e.g., millisecond pulsars, intermediate-mass binary pulsars, etc). It has been suggested that oxygen-neon white dwarfs (ONe WDs) can increase their mass to the Chandrasekhar limit by multiple He-shell flashes, leading to AIC events. However, the properties of He-shell flashes on the surface of ONe WDs are still not well understood. In this article, we aim to study He-shell flashes on the surface of ONe WDs in a systematic approach. We investigated the long-term evolution of ONe WDs accreting He-rich materialwith various constantmass-accretion rates by time-dependent calculationswith the stellar evolution codeModules for Experiments in Stellar Astrophysics (MESA), in which the initial ONeWD masses range from 1.1 to 1.35 M ⊙. We found that the mass-retention efficiency increases with the ONe WD mass and the mass-accretion rate, whereas both the nova cycle duration and the ignition mass decrease with the ONe WD mass and the mass-accretion rate. We also present the nuclear products in different accretion scenarios. The results presented in this article can be used in the future binary population synthesis studies of AIC events.

Towards a realistic explosion landscape for binary population synthesis

ABSTRACTA crucial ingredient in population synthesis studies involving massive stars is the determination of whether they explode or implode in the end. While the final fate of a massive star is sensitive to its core structure at the onset of collapse, the existing binary population synthesis studies do not reach core collapse. Instead, they employ simple prescriptions to infer their final fates without knowing the pre-supernova core structure. We explore a potential solution to this problem by treating the carbon-oxygen (CO) core independently from the rest of the star. Using the implicit hydrodynamics code $\mathrm{\tt {KEPLER}}$, we have computed an extensive grid of 3496 CO-core models from a diverse range of initial conditions, each evolved from carbon ignition until core collapse. The final core structure, and thus the explodability, varies non-monotonically and depends sensitively on both the mass and initial composition of the CO core. Although bare CO cores are not perfect substitutes for cores embedded in massive stars, our models compare well both with $\mathrm{\tt {MESA}}$ and full hydrogenic and helium star calculations. Our results can be used to infer the pre-supernova core structures from population synthesis estimates of CO-core properties, thus to determine the final outcomes based on the results of modern neutrino-driven explosion simulations. A sample application is presented for a population of Type-IIb supernova progenitors.

Constraining Galactic Structure with the LISA White Dwarf Foreground

Abstract White dwarfs (WDs) comprise 95% of all stellar remnants, and are thus an excellent tracer of old stellar populations in the Milky Way. Current and planned telescopes are not able to directly probe the WD population in its entirety due to its inherently low luminosity. However, the Galactic population of double white dwarf binaries (DWDs) gives rise to a millihertz gravitational-wave foreground detectable by the Laser Interferometer Space Antenna (LISA). Here we show how characterizing the angular power of the WD foreground will enable probes of the Galactic structure in a novel way to determine whether the Galactic WD population traces the spatial distribution of young, bright stars, or traces a vertically heated spatial distribution associated with the Galaxy’s oldest stellar populations. We do this using a binary population synthesis study that incorporates different Galactic spatial distributions for the DWD population. We find that the level of anisotropy in the WD foreground’s angular power spectrum is dependent on the vertical scale height of the population, but show that multipole coefficients from the spherical harmonic decomposition must be considered individually because of LISA’s angular resolution. Finally, we show that LISA can probe the vertical scale height of the Galactic WD population with an accuracy of 300 pc, using the hexadecapole moment of the WD foreground.

Observed binary populations reflect the Galactic history

Context. Wide hot subdwarf B (sdB) binaries with main-sequence companions are outcomes of stable mass transfer from evolved red giants. The orbits of these binaries show a strong correlation between their orbital periods and mass ratios. The origins of this correlation have, so far, been lacking a conclusive explanation. Aims. We aim to find a binary evolution model which can explain the observed correlation. Methods. Radii of evolved red giants, and hence the resulting orbital periods, strongly depend on their metallicity. We performed a small but statistically significant binary population synthesis study with the binary stellar evolution code MESA. We used a standard model for binary mass loss and a standard metallicity history of the Galaxy. The resulting sdB systems were selected based on the same criteria as was used in observations and then compared with the observed population. Results. We have achieved an excellent match to the observed period-mass ratio correlation without explicitly fine-tuning any parameters. Furthermore, our models produce a very good match to the observed period-metallicity correlation. We predict several new correlations, which link the observed sdB binaries to their progenitors, and a correlation between the orbital period, metallicity, and core mass for subdwarfs and young low-mass helium white dwarfs. We also predict that sdB binaries have distinct orbital properties depending on whether they formed in the Galactic bulge, thin or thick disc, or the halo. Conclusions. We demonstrate, for the first time, how the metallicity history of the Milky Way is imprinted in the properties of the observed post-mass transfer binaries. We show that Galactic chemical evolution is an important factor in binary population studies of interacting systems containing at least one evolved low-mass (Minit < 1.6 M⊙) component. Finally, we provide an observationally supported model of mass transfer from low-mass red giants onto main-sequence stars.

A Volume-limited Sample of Cataclysmic Variables from Gaia DR2: Space Density and Population Properties

ABSTRACT We present the first volume-limited sample of cataclysmic variables (CVs), selected using the accurate parallaxes provided by the second data release (DR2) of the European Space Agency Gaia space mission. The sample is composed of 42 CVs within 150 pc, including two new systems discovered using the Gaia data, and is $(77 \pm 10)$ per cent complete. We use this sample to study the intrinsic properties of the Galactic CV population. In particular, the CV space density we derive, $\rho =(4.8^{+0.6}_{-0.8}) \times 10^{-6}\, \mbox{$\mathrm{pc}^{-3}$}$, is lower than that predicted by most binary population synthesis studies. We also find a low fraction of period bounce CVs, seven per cent, and an average white dwarf mass of $\langle M_\mathrm{WD} \rangle = (0.83 \pm 0.17)\, \mathrm{M}_\odot$. Both findings confirm previous results, ruling out the presence of observational biases affecting these measurements, as has been suggested in the past. The observed fraction of period bounce CVs falls well below theoretical predictions, by at least a factor of five, and remains one of the open problems in the current understanding of CV evolution. Conversely, the average white dwarf mass supports the presence of additional mechanisms of angular momentum loss that have been accounted for in the latest evolutionary models. The fraction of magnetic CVs in the 150 pc sample is remarkably high at 36 per cent. This is in striking contrast with the absence of magnetic white dwarfs in the detached population of CV progenitors, and underlines that the evolution of magnetic systems has to be included in the next generation of population models.

Constraining Compact Object Formation with 2M0521

Abstract We show that the recently discovered binary 2M05215658+4359220 (2M0521), comprising a giant star (GS) orbiting a suspected black hole (BH) in an ∼80 day orbit, may be instrumental in shedding light on uncertain BH-formation physics and can be a test case for studying wind accretion models. Using binary population synthesis with a realistic prescription for the star formation history and metallicity evolution of the Milky Way, we analyze the evolution of binaries containing compact objects (COs) in orbit around GSs with properties that are similar to 2M0521. We find ∼102–103 CO–GS binaries in the Milky Way observable by Gaia and 0–12 BH–GS and 0–1 neutron star–GS binaries in the Milky Way with properties that are similar to 2M0521. We find that all CO–GSs with P orb < 5 yr, including 2M0521, go through a common envelope (CE), and hence form a class of higher-mass analogs to white dwarf post-CE binaries. We further show how the component masses of 2M0521-like binaries depend strongly on the supernova-engine model that we adopt. Thus, an improved measurement of the orbit of 2M0521, imminent with Gaia’s third data release, will strongly constrain its component masses and as a result inform supernova-engine models that are widely used in binary population synthesis studies. These results have widespread implications for the origins and properties of CO binaries, especially those detectable by the Laser Interferometer Gravitational-Wave Observatory and the Laser Interferometer Space Antenna. Finally, we show that the reported X-ray nondetection of 2M0521 is a challenge for wind accretion theory, making 2M0521-like CO–GS binaries a prime target for further study with accretion models.

Understanding the orbital periods of CEMP-s stars

The chemical enrichments detected in carbon- and s-element-enhanced metal-poor (CEMP-s) stars are believed to be the consequence of a past episode of mass transfer from a now extinct asymptotic-giant-branch primary star. This hypothesis is borne out by the evidence that most CEMP-s stars exhibit radial-velocity variations suggesting that they belong to binary systems in which the companion is not directly visible. We used the orbital-period distribution of an unbiased sample of observed CEMP-s stars to investigate the constraints it imposes on our models of binary evolution and on the properties of the metal-poor binary population in the Galactic halo. We generated synthetic populations of metal-poor binary stars using different assumptions about the initial period distribution and about the physics of the mass-transfer process, and we compared the predicted period distributions of our synthetic CEMP-s stars with the observed one. With a set of default assumptions often made in binary population-synthesis studies, the observed period distribution cannot be reproduced. The percentage of observed CEMP-s systems with periods shorter than about 2000 days is underestimated by almost a factor of three, and by about a factor of two between 3000 and 10 000 days. Conversely, about 40% of the simulated systems have periods longer than 104 days, which is approximately the longest measured period among CEMP-s stars. Variations in the assumed stability criterion for Roche-lobe overflow and the efficiency of wind mass transfer do not alter the period distribution enough to overcome this discrepancy. To reconcile the results of the models with the orbital properties of observed CEMP-s stars, one or both of the following conditions are necessary: (i) the specific angular momentum carried away by the material that escapes the binary system is approximately two to five times higher than currently predicted by analytical models and hydrodynamical simulations of wind mass transfer, and (ii) the initial period distribution of very metal-poor binary stars is significantly different from that observed in the solar vicinity and weighted towards periods shorter than about ten thousand days. Our simulations show that some, perhaps all, of the observed CEMP-s stars with apparently constant radial velocity could be undetected binaries with periods longer than 104 days, but the same simulations also predict that twenty to thirty percent of detectable binaries should have periods above this threshold, much more than are currently observed.

On the Role of Supernova Kicks in the Formation of Galactic Double Neutron Star Systems

Abstract In this work we focus on a group of Galactic double neutron star (DNS) systems with long orbital periods of ≳1 day and low eccentricities of ≲0.4. The feature of these orbital parameters is used to constrain the evolutionary processes of progenitor binaries and the supernova (SN) kicks of the second born NSs. Adopting that the mass transfer during primordial binary evolution is highly nonconservative (rotation-dependent), the formation of DNS systems involves a double helium star binary phase, the common envelope (CE) evolution initiates before the first NS formation. During the CE evolution the binary orbital energy is obviously larger when using a helium star rather than an NS to expel the donor envelope, this can help explain the formation of DNS systems with long periods. SN kicks at NS birth can lead to eccentric orbits and even the disruption of binary systems, and the low eccentricities require that the DNSs receive a small natal kick at the second collapse. Compared with the overall distribution of orbital parameters for observed DNS binaries, we propose that the second born NSs in most DNS systems are subject to small natal kicks with the Maxwellian dispersion velocity of less than 80 km s−1, which can provide some constraints on the SN explosion processes. The mass distribution of DNS binaries is also briefly discussed. We suggest that the rotation-dependent mass transfer mode and our results about SN kicks should be applied to massive binary evolution and population synthesis studies.

The cataclysmic variable QZ Lib: a period bouncer

While highly evolved cataclysmic variables (CVs) with brown dwarf donors, often called "period bouncers", are predicted to make up $\simeq40-70$ % of the Galactic CV population, only a handful of such systems are currently known. The identification and characterization of additional period bouncers is therefore important to probe this poorly understood phase of CV evolution. We investigate the evolution of the CV QZ Lib following its 2004 super-outburst using multi-epoch spectroscopy. From time-resolved spectroscopic observations we measure the orbital period of the system, $P_\mathrm{orb}= 0.06436(20)$ d, which, combined with the superhump period $P_\mathrm{SH}= 0.064602(24)$ d, yields the system mass ratio, $q = 0.040(9)$. From the analysis of the spectral energy distribution we determine the structure of the accretion disc and the white dwarf effective temperature, $T_\mathrm{eff} = 10\,500 \pm 1500\,\mathrm{K}$. We also derive an upper limit on the effective temperature of the secondary, $T_\mathrm{eff} \lesssim 1700\,\mathrm{K}$, corresponding to a brown dwarf of T spectral type. The low temperature of the white dwarf, the small mass ratio and the fact that the donor is not dominating the near-infrared emission are all clues of a post bounce system. Although it is possible that QZ Lib could have formed as a white dwarf plus a brown dwarf binary, binary population synthesis studies clearly suggest this scenario to be less likely than a period bouncer detection and we conclude that QZ Lib is a CV that has already evolved through the period minimum.

Constraining Roche-Lobe Overflow Models Using the Hot-Subdwarf Wide Binary Population

Abstract One of the important issues regarding the final evolution of stars is the impact of binarity. A rich zoo of peculiar, evolved objects are born from the interaction between the loosely bound envelope of a giant, and the gravitational pull of a companion. However, binary interactions are not understood from first principles, and the theoretical models are subject to many assumptions. It is currently agreed upon that hot subdwarf stars can only be formed through binary interaction, either through common envelope ejection or stable Roche-lobe overflow (RLOF) near the tip of the red giant branch (RGB). These systems are therefore an ideal testing ground for binary interaction models. With our long term study of wide hot subdwarf (sdB) binaries we aim to improve our current understanding of stable RLOF on the RGB by comparing the results of binary population synthesis studies with the observed population. In this article we describe the current model and possible improvements, and which observables can be used to test different parts of the interaction model.

Merging of a CO WD and a He-rich WD to produce a type Ia supernovae

Context: Although type Ia supernovae (SNe Ia) play a key role in astrophysics, the companions of the exploding carbon-oxygen white dwarfs (CO WDs) are still not completely identified. It has been suggested recently that a He-rich WD (a He WD or a hybrid HeCO WD) merges with a CO WD may produce an SN Ia. This theory was based on the double-detonation model, in which the shock compression in the CO core caused by the surface explosion of the He-rich shell might lead to the explosion of the whole CO WD. However, so far, very few binary population synthesis (BPS) studies have been made on the merger scenario of a CO WD and a He-rich WD in the context of SNe Ia. Aims: We aim to systematically study the Galactic birthrates and delay-time distributions of SNe Ia based on the merger scenario of a CO WD and a He-rich WD. Methods: We performed a series of Monte Carlo BPS simulations to investigate the properties of SNe Ia from the merging of a CO WD and a He-rich WD based on the Hurley rapid binary evolution code. We also considered the influence of different metallicities on the final results. Results: From our simulations, we found that no more than 15% of all SNe Ia stem from the merger scenario of a CO WD and a He-rich WD, and their delay times range from ~110 Myr to the Hubble time. This scenario mainly contributes to SN Ia explosions with intermediate and long delay times. The present work indicates that the merger scenario of a CO WD and a He-rich WD can roughly reproduce the birthrates of SN 1991bg-like events, and cover the range of their delay times. We also found that SN Ia birthrates from this scenario would be higher for the cases with low metallicities.

Mass retention efficiencies of He accretion onto carbon-oxygen white dwarfs and type Ia supernovae

Type Ia supernovae (SNe Ia) play a crucial role in studying cosmology and galactic chemical evolution. They are thought to be thermonuclear explosions of carbon-oxygen white dwarfs (CO WDs) when their masses reach the Chandrasekar mass limit in binaries. Previous studies have suggested that He novae may be progenitor candidates of SNe Ia. However, the mass retention efficiencies during He nova outbursts are still uncertain. In this article, we aim to study the mass retention efficiencies of He nova outbursts and to investigate whether SNe Ia can be produced through He nova outbursts. Using the stellar evolution code Modules for Experiments in Stellar Astrophysics, we simulated a series of multicycle He-layer flashes, in which the initial WD masses range from 0.7 to 1.35 Msun with various accretion rates. We obtained the mass retention efficiencies of He nova outbursts for various initial WD masses, which can be used in the binary population synthesis studies. In our simulations, He nova outbursts can increase the mass of the WD to the Chandrasekar mass limit and the explosive carbon burning can be triggered in the center of the WD; this suggests that He nova outbursts can produce SNe Ia. Meanwhile, the mass retention efficiencies in the present work are lower than those of previous studies, which leads to a lower birthrates of SNe Ia through the WD + He star channel. Furthermore, we obtained the elemental abundances distribution at the moment of explosive carbon burning, which can be used as the initial input parameters in studying explosion models of SNe Ia.

Mind Your Ps and Qs: The Interrelation between Period (P) and Mass-ratio (Q) Distributions of Binary Stars

Abstract We compile observations of early-type binaries identified via spectroscopy, eclipses, long-baseline interferometry, adaptive optics, common proper motion, etc. Each observational technique is sensitive to companions across a narrow parameter space of orbital periods P and mass ratios q = /M 1. After combining the samples from the various surveys and correcting for their respective selection effects, we find that the properties of companions to O-type and B-type main-sequence (MS) stars differ among three regimes. First, at short orbital periods P ≲ 20 days (separations a ≲ 0.4 au), the binaries have small eccentricities e ≲ 0.4, favor modest mass ratios , and exhibit a small excess of twins q > 0.95. Second, the companion frequency peaks at intermediate periods log P (days) ≈ 3.5 (a ≈ 10 au), where the binaries have mass ratios weighted toward small values q ≈ 0.2–0.3 and follow a Maxwellian “thermal” eccentricity distribution. Finally, companions with long orbital periods log P (days) ≈ 5.5–7.5 (a ≈ 200–5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q ≈ 0.1–1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% ± 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q > 0.1 and log P (days) < 8.0 per primary increases from 0.50 ± 0.04 for solar-type MS primaries to 2.1 ± 0.3 for O-type MS primaries. We fit joint probability density functions to the corrected distributions, which can be incorporated into binary population synthesis studies.

Neutron Star Kicks by the Gravitational Tug-boat Mechanism in Asymmetric Supernova Explosions: Progenitor and Explosion Dependence

Abstract Asymmetric mass ejection in the early phase of supernova (SN) explosions can impart a kick velocity to the new-born neutron star (NS). For neutrino-driven explosions the NS acceleration has been shown to be mainly caused by the gravitational attraction of the anisotropically expelled inner ejecta, while hydrodynamic forces contribute on a subdominant level, and asymmetric neutrino emission plays only a secondary role. Two- and three-dimensional hydrodynamic simulations have demonstrated that this gravitational tug-boat mechanism can explain the observed space velocities of young NSs up to more than 1000 km s−1. Here, we discuss how the NS kick depends on the energy, ejecta mass, and asymmetry of the SN explosion, and what role the compactness of the pre-collapse stellar core plays for the momentum transfer to the NS. We also provide simple analytic expressions for the NS velocity in terms of these quantities. Referring to results of hydrodynamic simulations in the literature, we argue why, within the discussed scenario of NS acceleration, electron-capture SNe, low-mass Fe-core SNe, and ultra-stripped SNe can be expected to have considerably lower intrinsic NS kicks than core-collapse SNe of massive stellar cores. Our basic arguments also remain valid if progenitor stars possess large-scale asymmetries in their convective silicon and oxygen burning layers. Possible scenarios for spin-kick alignment are sketched. Much of our discussion stays on a conceptual and qualitative level, and more work is necessary on the numerical modeling side to determine the dependences of involved parameters, whose prescriptions will be needed for recipes that can be used to better describe NS kicks in binary evolution and population synthesis studies.

Common envelope events with low-mass giants: understanding the energy budget

Common envelope events are important interactions between two binary stars that lead to the formation of close binary systems. We present here a systematic three-dimensional study in which we model common envelope events with low-mass giant donors. The results allow us to revise the energy formalism that is usually used to determine common envelope event outcomes. We show that the energy budget for this type of system should include the recombination energy, and that it also must take into account that a significant fraction of the released orbital energy is taken away by the ejecta. We provide three ways in which our results can be used by binary population synthesis studies: a relation that links the observed post-common envelope binary with the initial binary parameters, a fitting formula for the $\alpha_{\rm ce}\lambda$ parameter of the standard energy formalism, and a revised energy formalism that takes into account both the recombination energy and the energy that is taken away by the ejecta.

A binary population synthesis study on gravitational wave sources

AbstractGravitational waves (GW) are a natural consequence of Einstein's theory of gravity (general relativity), and minute distortions of space-time. Gravitational Wave Astronomy is an emerging branch of observational astronomy which aims to use GWs to collect observational data about objects such as neutron stars and black holes, about events such as supernovae and about the early universe shortly after the big bang.This field will evolve to become an established component of 21st century multi-messenger astronomy, and will stand shoulder-to-shoulder with gamma-ray, x-ray, optical, infrared and radio astronomers in exploring the cosmos. In this paper, we state a recent theoretical study on GW sources, and present the results of our studies on the field using a binary population synthesis (BPS) approach, which was designed to investigate the formation of many interesting binary-related objects, including close double white dwarfs, AM CVn stars, ultra-compact X-ray binaries(UCXBs), double neutron stars, double stellar black holes. Here we report how BPS can be used to determine the GW radiation from double compact objects.

NEUTRON STAR MERGERS AS THE ORIGIN OF r -PROCESS ELEMENTS IN THE GALACTIC HALO BASED ON THE SUB-HALO CLUSTERING SCENARIO

Binary mergers (NSMs) of double neutron star (and black hole-neutron star) systems are suggested to be major sites of r-process elements in the Galaxy by recent hydrodynamical and nucleosynthesis studies. It has been pointed out, however, that the estimated long lifetimes of neutron star binaries are in conflict with the presence of r-process-enhanced halo stars at metallicities as low as [Fe/H] ~ -3. To resolve this problem, we examine the role of NSMs in the early Galactic chemical evolution on the assumption that the Galactic halo was formed from merging sub-halos. We present simple models for the chemical evolution of sub-halos with total final stellar masses between 10^4 M_solar and 2 x 10^8 M_solar. Typical lifetimes of compact binaries are assumed to be 100 Myr (for 95% of their population) and 1 Myr (for 5%), according to recent binary population synthesis studies. The resulting metallcities of sub-halos and their ensemble are consistent with the observed mass-metallicity relation of dwarf galaxies in the Local Group, and the metallicity distribution of the Galactic halo, respectively. We find that the r-process abundance ratios [r/Fe] start increasing at [Fe/H] <= -3 if the star formation efficiencies are smaller for less massive sub-halos. In addition, the sub-solar [r/Fe] values (observed as [Ba/Fe] ~ -1.5 for [Fe/H] < -3) are explained by the contribution from the short-lived (~1 Myr) binaries. Our results indicate that NSMs may have a substantial contribution to the r-process element abundances throughout the Galactic history.