arXiv subjects

Krzysztof Belczynski

Publications and source records attributed to Krzysztof Belczynski.

At least 37 records · Page 2Linked to original sources

arXiv2022

A Nuclear Equation of State Inferred from Stellar r-process Abundances

Binary neutron star mergers (NSMs) have been confirmed as one source of the heaviest observable elements made by the rapid neutron-capture (r-) process. However, modeling NSM outflows -- from the total ejecta masses to their elemental yields -- depends on the unknown nuclear equation of state (EOS) that governs neutron-star structure. In this work, we derive a phenomenological EOS by assuming that NSMs are the dominant sources of the heavy element material in metal-poor stars with r-process abundance patterns. We start with a population synthesis model to obtain a population of merging neutron star binaries and calculate their EOS-dependent elemental yields. Under the assumption that these mergers were responsible for the majority of r-process elements in the metal-poor stars, we find parameters representing the EOS for which the theoretical NSM yields reproduce the derived abundances from observations of metal-poor stars. For our proof-of-concept assumptions, we find an EOS that is slightly softer than, but still in agreement with, current constraints, e.g., by the Neutron Star Interior Composition Explorer, with $R_{1.4}=12.25\pm 0.03$~km and $M_{\textrm TOV}$ of $2.17\pm 0.03$~M$_\odot$(statistical uncertainties, neglecting modeling systematics).

astro-ph.HE↗

arXiv2021

The implications of high BH spins on the origin of BH-BH mergers

The LIGO/Virgo collaboration has reported 50 black hole-black hole (BH-BH) mergers and 8 candidates recovered from digging deeper into the detectors noise. The majority of these mergers have low effective spins pointing toward low BH spins and efficient angular momentum transport in massive stars as proposed by several models (e.g., the Tayler-Spruit dynamo). However, out of these 58 mergers, 7 are consistent with having high effective spin parameter (chi_eff>0.3). Additionally, 2 events seem to have high effective spins sourced from the spin of the primary (more massive) BH. These particular observations could be used to discriminate between the isolated binary and dynamical formation channels. It might seem that high BH spins point to a dynamical origin if angular momentum transport in stars is efficient and forms low-spinning BHs. In such a case dynamical formation is required to produce second and third generations of BH-BH mergers with typically high-spinning BHs. Here we show, however, that isolated binary BH-BH formation also naturally reproduces such highly spinning BHs. Our models start with efficient angular momentum transport in massive stars that is needed to reproduce the majority of BH-BH mergers with low effective spins. Later, some of the binaries are subject to a tidal spin-up allowing the formation of a moderate fraction (~10%) of BH-BH mergers with high effective spins (chi_eff>=0.4-0.5). In addition, isolated binary evolution can produce a small fraction of BH-BH mergers with almost maximally spinning primary BHs. Therefore, the formation scenario of these atypical BH-BH mergers remains to be found.

astro-ph.HE↗

arXiv2021

Wind-powered ultraluminous X-ray sources

Although ultra-luminous X-ray sources (ULX) are important for astrophysics due to their extreme apparent super-Eddington luminosities, their nature is still poorly known. Theoretical and observational studies suggest that ULXs could be a diversified group of objects composed of low-mass X-ray binaries, high-mass X-ray binaries and marginally also systems containing intermediate-mass black holes, which is supported by their presence in a variety of environments. Observational data on the ULX donors could significantly boost our understanding of these systems, but only a few were detected. There are several candidates, mostly red supergiants (RSGs), but surveys are typically biased toward luminous near-infrared objects. Nevertheless, it is worth exploring if RSGs can be members of ULX binaries. In such systems matter accreted onto the compact body would have to be provided by the stellar wind of the companion, since a Roche-lobe overflow could be unstable for relevant mass-ratios. Here we present a comprehensive study of the evolution and population of wind-fed ULXs and provide a theoretical support for the link between RSGs and ULXs. Our estimated upper limit on contribution of wind-fed ULX to the overall ULX population is $\sim75$--$96\%$ for young ($<100$ Myr) star forming environments, $\sim 49$--$87\%$ for prolonged constant star formation (e.g., disk of Milky Way), and $\lesssim1\%$ for environments in which star formation ceased long time ($>2$ Gyr) ago. We show also that some wind-fed ULXs (up to $6\%$) may evolve into merging double compact objects (DCOs), but typical systems are not viable progenitors of such binaries because of their large separations. We demonstrate that, the exclusion of wind-fed ULXs from population studies of ULXs, might have lead to systematical errors in their conclusions.

astro-ph.HE↗

arXiv2021

The impact of common envelope development criteria on the formation of LIGO/Virgo sources

The treatment and criteria for development of unstable Roche lobe overflow (RLOF) that leads to the common envelope (CE) phase have hindered the evolutionary predictions for decades. In particular, the formation of black hole-black hole (BH-BH), black hole-neutron star (BH-NS), and neutron star-neutron star (NS-NS) merging binaries depends sensitively on the CE phase in classical isolated binary evolution model. All these mergers are now reported as LIGO/Virgo sources or source candidates. CE is even considered by some as a mandatory phase in the formation of BH-BH, BH-NS or NS-NS mergers in binary evolution. At the moment, there is no full first-principles model for development of CE. We employ the Startrack population synthesis code to test the current advancements in studies on stability of RLOF for massive donors to assess their effect on LIGO/Virgo source population. In particular, we allow for more restrictive CE development criteria for massive donors. We also test a modified condition for switching between different types of stable mass transfer. Implemented modifications significantly influence basic properties of merging double compact objects, sometimes in non-intuitive way. For one of tested models with restricted CE development criteria local merger rate density for BH-BH systems increased due to emergence of a new dominant formation scenario without any CE phase. We find that the changes in highly uncertain assumptions on RLOF physics may significantly affect (i) local merger rate density, (ii) shape of the mass and mass ratio distributions, and (iii) dominant evolutionary formation (with and without CE) scenarios of LIGO/Virgo sources. Our results demonstrate that without sufficiently strong constraints on RLOF physics, one is not able to draw fully reliable conclusions about the population of double compact object systems based on population synthesis studies.

astro-ph.HE↗

arXiv2020

Non-interacting black hole binaries with Gaia and LAMOST

Until recently, black holes (BHs) could be discovered only through accretion from other stars in X-ray binaries, or in merging double compact objects. Improvements in astrometric and spectroscopic measurements have made it possible to detect BHs also in non-interacting BH binaries (nBHB) through a precise analysis of the companion's motion. In this study, using an updated version of the Startrack binary-star population modelling code and a detailed model of the Milky Way (MW) galaxy we calculate the expected number of detections for Gaia and LAMOST surveys. We develop a formalism to convolve the binary population synthesis output with a realistic stellar density distribution, star-formation history (SFH), and chemical evolution for the MW, which produces a probability distribution function of the predicted compact-binary population over the MW. This avoids the additional statistical uncertainty which is introduced by methods which Monte Carlo sample from binary population synthesis output to produce one potential specific realisation of the MW compact-binary distribution, and our method is also comparatively fast to such Monte Carlo realisations. Specifically, we predict $\sim41$-$340$ nBHBs to be observed by Gaia, although the numbers may drop to $\sim10$-$70$ if the recent ($\lesssim100\;$ Myr) star formation is low ($\sim1\;M_\odot$/yr ). For LAMOST we predict $\lesssim14$ detectable nBHBs, which is lower partially because its field-of-view covers just $\sim6\%$ of the Galaxy.

astro-ph.HE↗

arXiv2020

The most ordinary formation of the most unusual double black hole merger

LIGO/Virgo Collaboration reported the detection of the most massive black hole - black hole (BH-BH) merger up to date with component masses of 85 Msun and 66 Msun (GW190521). Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud (>200 Msun; e.g. R136a) and employing newly estimated uncertainties on pulsational pair-instability mass-loss (that allow for possibility of forming BHs with mass up to 90Msun) we show that it is trivial to form such massive BH-BH mergers through the classical isolated binary evolution (with no assistance from either dynamical interactions or exotica). A binary consisting of two massive (180+150 Msun) Population II stars (Z=0.0001) evolves through a stable Roche lobe overflow and common envelope episode. Both exposed stellar cores undergo direct core-collapse and form massive BHs while avoiding pair-instability pulsation mass-loss or total disruption. LIGO/Virgo observations show that the merger rate density of light BH-BH mergers (both components: <50 Msun) is of the order of 10-100 Gpc^-3 yr^-1, while GW190521 indicates that the rate of heavier mergers is 0.02-0.43 Gpc^-3 yr^-1. Our model (with standard assumptions about input physics) but extended to include 200 Msun stars and allowing for the possibility of stellar cores collapsing to 90 Msun BHs produces the following rates: 63 Gpc^-3 yr^-1 for light BH-BH mergers and 0.04 Gpc^-3 yr^-1 for heavy BH-BH mergers. We do not claim that GW190521 was formed by an isolated binary, but it appears that such a possibility can not be excluded.

astro-ph.HE↗

arXiv2020

Formation of low-spinning 100 Msun black holes

It is speculated that a merger of two massive stellar-origin BHs in a dense stellar environment may lead to the formation of a massive BH in the pair-instability mass gap (50-135 Msun). Such a merger-formed BH is expected to typically have a high spin (a=0.7). If such a massive BH acquires another BH it may lead to another merger detectable by LIGO/Virgo in gravitational waves. Acquiring a companion may be hindered by gravitational-wave kick/recoil, which accompanies the first merger and may quickly remove the massive BH from its parent globular or nuclear cluster. We test whether it is possible for a massive merger-formed BH in the pair-instability gap to be retained in its parent cluster and have low spin. Such a BH would be indistinguishable from a primordial BH. We employed results from numerical relativity calculations of black hole mergers to explore the range of gravitational-wave recoil velocities for various combinations of merging BH masses and spins. We compared merger-formed massive BH speeds with typical escape velocities from globular and nuclear clusters. We show that a globular cluster is highly unlikely to form and retain a 100 Msun BH if the spin of the BH is low (a<0.3) as such BHs acquire high recoil speeds (>200 km/s) that exceed typical escape speeds from globular clusters (50 km/s). However, a very low-spinning (a=0.1) and massive (100 Msun) BH could be formed and retained in a galactic nuclear star cluster. Even though such massive merger-formed BHs with such low spins acquire high speeds during formation (400 km/s), they may avoid ejection since massive nuclear clusters have high escape velocities (300-500 km/s). A future detection of a massive BH in the pair-instability mass gap with low spin would therefore not be proof of the existence of primordial BHs, which are sometimes claimed to have low spins and arbitrarily high masses.

astro-ph.HE↗

arXiv2020

Phase-dependent study of near-infrared disk emission lines in LB-1

The mass, origin and evolutionary stage of the binary system LB-1 has been the subject of intense debate, following the claim that it hosts an $\sim$70$M_{\odot}$ black hole, in stark contrast with the expectations for stellar remnants in the Milky Way. We conducted a high-resolution, phase-resolved spectroscopic study of the near-infrared Paschen lines in this system, using the 3.5-m telescope at Calar Alto Observatory. We find that Pa$β$ and Pa$γ$ (after proper subtraction of the stellar absorption component) are well fitted with a standard double-peaked model, typical of disk emission. We measured the velocity shifts of the red and blue peaks at 28 orbital phases: the line center has an orbital motion in perfect antiphase with the stellar motion, and the radial velocity amplitude ranges from 8 to 13 km/s for different choices of lines and profile modelling. We interpret this curve as proof that the disk is tracing the orbital motion of the primary, ruling out the circumbinary disk and the hierarchical triple scenarios. The phase-averaged peak-to-peak half-separation (proxy for the projected rotational velocity of the outer disk) is $\sim$70 km s$^{-1}$, larger than the stellar orbital velocity and also inconsistent with a circumbinary disk. From those results, we infer a primary mass 4--8 times higher than the secondary mass. Moreover, we show that the ratio of the blue and red peaks (V/R intensity ratio) has a sinusoidal behaviour in phase with the secondary star, which can be interpreted as the effect of external irradiation by the secondary star on the outer disk. Finally, we briefly discuss our findings in the context of alternative scenarios recently proposed for LB-1. Definitive tests between alternative solutions will require further astrometric data from $Gaia$.

astro-ph.SR↗

arXiv2020

BSE versus StarTrack: implementations of new wind, remnant-formation, and natal-kick schemes in NBODY7 and their astrophysical consequences

The masses of stellar-remnant black holes (BH), as a result of their formation via massive single- and binary-stellar evolution, is of high interest in this era of gravitational-wave detection from binary black hole (BBH) and binary neutron star (BNS) mergers. Here we present new developments in the N-body evolution program NBODY7 in regards to its stellar-remnant formation and related schemes. We demonstrate that the newly-implemented stellar-wind and remnant-formation schemes in the NBODY7 code's BSE sector, such as the 'rapid' and the 'delayed' supernova (SN) schemes along with an implementation of pulsational-pair-instability and pair-instability supernova (PPSN/PSN), now produces neutron star (NS) and BH masses that agree nearly perfectly, over large ranges of zero-age-main sequence (ZAMS) mass and metallicity, with those from the StarTrack population-synthesis program. We also demonstrate the new implementations of various natal-kick mechanisms on NSs and BHs such as the 'convection-asymmetry-driven', 'collapse-asymmetry-driven', and 'neutrino-emission-driven' kicks, in addition to a fully consistent implementation of the standard, fallback-dependent, momentum-conserving natal kick. We find that the SN material fallback causes the convection-asymmetry kick to effectively retain similar number and mass of BHs in clusters as for the standard, momentum-conserving kick. The collapse-asymmetry kick would cause nearly all BHs to retain in clusters irrespective of remnant formation model and metallicity, whereas the inference of a large number of BHs in GCs would potentially rule out the neutrino-driven kick mechanism. Pre-SN mergers of massive primordial binaries would cause BH masses to deviate from the single-star ZAMS mass-remnant mass relation. Such mergers, at low metallicities, can produce low-spinning BHs within the PSN mass gap that can be retained in a stellar cluster.

astro-ph.SR↗

arXiv2019

A wide star-black-hole binary system from radial-velocity measurements

All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M$_{\odot}$$^{1-4}$. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries$^{5,6}$. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying H$α$ emission line require the presence of a dark companion with a mass of $68^{+11}_{-13}$ M$_{\odot}$, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes$^{7,8}$, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories$^{9-11}$.

astro-ph.SR↗

arXiv2019

The Impact of Nuclear Physics Uncertainties on Galactic Chemical Evolution Predictions

Modeling the evolution of the elements in the Milky Way is a multidisciplinary and challenging task. In addition to simulating the 13 billion years evolution of our Galaxy, chemical evolution simulations must keep track of the elements synthesized and ejected from every astrophysical site of interest (e.g., supernova, compact binary merger). The elemental abundances of such ejecta, which are a fundamental input for chemical evolution codes, are usually taken from theoretical nucleosynthesis calculations performed by the nuclear astrophysics community. Therefore, almost all chemical evolution predictions rely on the nuclear physics behind those calculations. In this proceedings, we highlight the impact of nuclear physics uncertainties on galactic chemical evolution predictions. We demonstrate that nuclear physics and galactic evolution uncertainties both have a significant impact on interpreting the origin of neutron-capture elements in our Solar System. Those results serve as a motivation to create and maintain collaborations between the fields of nuclear astrophysics and galaxy evolution.

astro-ph.GA↗

arXiv2019

Populations of stellar mass Black holes from binary systems

In large and complicated stellar systems like galaxies it is difficult to predict the number and characteristics of a black hole population. Such populations may be modelled as an aggregation of homogeneous (i.e. having uniform star formation history and the same initial chemical composition) stellar populations. Using realistic evolutionary models we predict the abundances and properties of black holes formed from binaries in these environments. We show that the black hole population will be dominated by single black holes originating from binary disruptions and stellar mergers. Furthermore, we discuss how black hole populations are influenced by such factors as initial parameters, metallicity, initial mass function, and natal kick models. As an example application of our results, we estimate that about 26 microlensing events to happen every year in the direction of the Galactic Bulge due to black holes in a survey like OGLE-IV. Our results may be used to perform in-depth studies related to realistic black hole populations, e.g. observational predictions for space survey missions like Gaia, or Einstein Probe. We prepared a publicly available database with the raw data from our simulations to be used for more in-depth studies.

astro-ph.HE↗

arXiv2019

Multiband gravitational-wave event rates and stellar physics

Joint gravitational-wave detections of stellar-mass black-hole binaries by ground- and space-based observatories will provide unprecedented opportunities for fundamental physics and astronomy. We present a semianalytic method to estimate multiband event rates by combining selection effects of ground-based interferometers (like LIGO/Virgo) and space missions (like LISA). We forecast the expected number of multiband detections first by using information from current LIGO/Virgo data, and then through population synthesis simulations of binary stars. We estimate that few to tens of LISA detections can be used to predict mergers detectable on the ground. Conversely, hundreds of events could potentially be extracted from the LISA data stream using prior information from ground detections. In general, the merger signal of binaries observable by LISA is strong enough to be unambiguously identified by both current and future ground-based detectors. Therefore third-generation detectors will not increase the number of multiband detections compared to LIGO/Virgo. We use population synthesis simulations of isolated binary stars to explore some of the stellar physics that could be constrained with multiband events, and we show that specific formation pathways might be overrepresented in multiband events compared to ground-only detections.

astro-ph.HE↗

arXiv2019

The observed vs total population of ULXs

We have analyzed how anisotropic emission of radiation affects the observed sample of ultraluminous X-ray sources (ULXs) by performing simulations of the evolution of stellar populations, employing recent developments in stellar and binary physics, and by utilizing a geometrical beaming model motivated by theory and observation. Whilst ULXs harboring black hole accretors (BH ULXs) are typically emitting isotropically, the majority of ULXs with neutron star accretors (NS ULXs) are found to be beamed. These findings confirm previous assertions that a significant fraction of ULXs are hidden from view due to a substantial misalignment of the emission beam and the line-of-sight. We find the total number of NS ULXs in regions with constant star formation, solar metallicity, and ages above ~1 Gyr to be higher than the BH ULXs, although observationally both populations are comparable. For lower metallicities BH ULX dominate both the total and observed ULX populations. As far as burst star-formation is concerned, young ULX populations are dominated by BH ULXs, but this changes as the population ages and, post star-formation, NS ULXs dominate both the observed and total population of ULXs. We also compare our simulation output to a previous analytical prediction for the relative ratio of BH to NS ULXs in idealized flux-limited observations and find broad agreement for all but the lowest metallicities. In so doing we find that in such surveys the observed ULX population should be heavily dominated by black-hole systems rather than by systems containing neutron stars.

astro-ph.HE↗

arXiv2019

Neutron Star Mergers Might not be the Only Source of r-Process Elements in the Milky Way

Probing the origin of r-process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H]\,$>-1$ in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times ($\propto~t^{-1}$) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions ($\propto~t^{-1.5}$) or power laws combined with a strong burst of mergers before the onset of Type~Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type~Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this extra site could be responsible for roughly 50% of the Eu production in the early universe, before the onset of Type~Ia supernovae. Rare classes of supernovae could be this additional r-process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern.

astro-ph.HE↗

arXiv2019

Nucleosynthetic Yields from Neutron Stars Accreting in Binary Common Envelopes

Massive-star binaries can undergo a phase where one of the two stars expands during its advanced evolutionary stage as a giant and envelops its companion, ejecting the hydrogen envelope and tightening its orbit. Such a common envelope phase is required to tighten the binary orbit in the formation of many of the observed X-ray binaries and merging compact binary systems. In the formation scenario for neutron star binaries, the system might pass through a phase where a neutron star spirals into the envelope of its giant star companion. These phases lead to mass accretion onto the neutron star. Accretion onto these common-envelope-phase neutron stars can eject matter that has undergone burning near to the neutron star surface. This paper presents nucleosynthetic yields of this ejected matter, using population synthesis models to study the importance of these nucleosynthetic yields in a galactic chemical evolution context. Depending on the extreme conditions in temperature and density found in the accreted material, both proton-rich and neutron-rich nucleosynthesis can be obtained, with efficient production of neutron rich isotopes of low Z material at the most extreme conditions, and proton rich isotopes, again at low Z, in lower density models. Final yields are found to be extremely sensitive to the physical modeling of the accretion phase. We show that neutron stars accreting in binary common envelopes might be a new relevant site for galactic chemical evolution, and therefore more comprehensive studies are needed to better constrain nucleosynthesis in these objects.

astro-ph.SR↗

arXiv2019

Black holes, gravitational waves and fundamental physics: a roadmap

The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.

gr-qc↗

arXiv2018

Wind Roche Lobe Overflow as a way to make type Ia supernova from the widest symbiotic systems

Symbiotic stars are interacting binaries with one of the longest orbital periods. Since they can contain a massive white dwarf with a high accretion rate they are considered a possible type Ia supernovae (SNe Ia) progenitors. Among symbiotic binaries there are systems containing a Mira donor, which can have orbital periods of a few tens of years and more. This subclass of symbiotic stars due to their very large separation usually was not considered promising SNe Ia progenitors. We analysed evolution of one of the well studied symbiotic star with a Mira donor, V407 Cyg. We showed that the standard evolution model predicts that the system will not become a SN Ia. However, by simply adding a Wind Roche Lobe Overflow as one of the mass transfer modes we predict that the white dwarf in V407 Cyg will reach the Chandrasekhar limit in 40-200 Myr.

astro-ph.SR↗