arXiv subjects

Ruediger Pakmor

Publications and source records attributed to Ruediger Pakmor.

At least 37 records · Page 2Linked to original sources

arXiv2025

A super-Chandrasekhar mass type Ia supernova progenitor at 49 pc set to detonate in 23 Gyr

Double white dwarf binaries are a leading explanation to the origin of type Ia supernovae, but no system exceeding the Chandrasekhar mass limit (1.4 M$_\odot$) has been found that will explode anywhere close to a Hubble time. Here, we present the super-Chandrasekhar mass double white dwarf WDJ181058.67+311940.94 whose merger time ($22.6\pm1.0$ Gyr) is of the same order as a Hubble time. The mass of the binary is large, combining to $1.555\pm0.044$ M$_\odot$, while being located only 49 pc away. We predict that the binary will explode dynamically via a double detonation destroying both stars just before they merge, appearing as a subluminous type Ia supernova with a peak apparent magnitude of about $m_V=-16$ (200,000 times brighter than Jupiter). The observationally-derived birthrate of super-Chandrasekhar mass double white dwarfs is now at least $6.0\times10^{-4}$ yr$^{-1}$ and the observed rate of type Ia supernovae in the Milky Way from such systems is approximately $4.4\times10^{-5}$ yr$^{-1}$, while the predicted type Ia supernova rate in the Milky Way from all progenitor channels is about sixty times larger. Hence, WDJ181058.67+311940.94 mitigates the observed deficit of massive double white dwarfs witnessed in volume-complete populations, but further evidence is required to determine the majority progenitors of type Ia supernovae.

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arXiv2025

Magnetic Field Amplification during Stellar Collisions between Low-Mass Stars

Blue straggler stars in stellar clusters are a subset of stars that are bluer and appear younger than other cluster members, seemingly straggling behind in their evolution. They offer a unique opportunity to understand the stellar dynamics and populations within their hosts. In the collisional formation scenario, a persistent challenge is the excessive angular momentum in the collision product. The consequent significant mass loss during transition to a stable state leads to a star with too low a mass to be a blue straggler, unless it spins down efficiently. While many proposed spin-down mechanisms involve boosted angular momentum loss via magnetic braking within the collision product, the existence or strength of these magnetic fields has not been confirmed. Here, we report three-dimensional magnetohydrodynamical simulations of collisions between two low-mass main-sequence stars and investigate magnetic field amplification. Magnetic field energy is amplified by a factor of $10^{8}-10^{10}$, resulting in the magnetic field strength of $10^{7}-10^{8}$G at the core of the collision product, independent of collision parameters. The surface magnetic field strengths have increased up to $10-10^{4}$ G. In addition, a distinctly flattened, rotating gas structure appears around the collision products in off-axis collisions, which may be a hint of possible disk formation. Such significant magnetic amplification and potential disk formation suggest the possibility of efficient spin-down of collision products via magnetic braking and magnetic disk locking, which can result in their appearance as blue stragglers.

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arXiv2024

Non-LTE radiative transfer simulations: Improved agreement of the double detonation with normal Type Ia supernovae

The double detonation is a widely discussed explosion mechanism for Type Ia supernovae, whereby a helium shell detonation ignites a secondary detonation in the carbon/oxygen core of a white dwarf. Even for modern models that invoke relatively small He shell masses, many previous studies have found that the products of the helium shell detonation lead to discrepancies with normal Type Ia supernovae, such as strong Ti II absorption features, extremely red light curves and too large a variation with viewing direction. It has been suggested that non local thermodynamic equilibrium (non-LTE) effects may help to reduce these discrepancies with observations. Here we carry out full non-LTE radiative transfer simulations for a recent double detonation model with a relatively small helium shell mass of 0.05 M$_\odot$. We construct 1D models representative of directions in a 3D explosion model to give an indication of viewing angle dependence. The full non-LTE treatment leads to improved agreement between the models and observations. The light curves become less red, due to reduced absorption by the helium shell detonation products, since these species are more highly ionised. Additionally, the expected variation with observer direction is reduced. The full non-LTE treatment shows promising improvements, and reduces the discrepancies between the double detonation models and observations of normal Type Ia supernovae.

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arXiv2024

Magnetic dynamos in galaxy clusters: the crucial role of galaxy formation physics at high redshifts

Observations of Faraday rotation and synchrotron emission in galaxy clusters imply large-scale magnetic fields with $μ\mathrm{G}$ strengths possibly extending back to $z=4$. Non-radiative cosmological simulations of galaxy clusters show a comparably slow magnetic field growth that only saturates at late times. We include galaxy formation physics and find a significantly accelerated magnetic field growth. We identify three crucial stages in the magnetic field evolution. 1) At high redshift, the central dominant galaxy serves as the prime agent that magnetizes not only its immediate vicinity but also most of the forming protocluster through a combination of a small-scale dynamo induced by gravitationally driven, compressive turbulence and stellar and active galactic nuclei feedback that distribute the magnetic field via outflows. 2) This process continues as other galaxies merge into the forming cluster in subsequent epochs, thereby transporting their previously amplified magnetic field to the intracluster medium (ICM) through ram pressure stripping and galactic winds. 3) At lower redshift, gas accretion and frequent cluster mergers trigger additional small-scale dynamo processes, thereby preventing the decay of the magnetic field and fostering the increase of the magnetic coherence scale. We show that the magnetic field observed today in the weakly collisional ICM is consistently amplified on collisional scales. Initially, this occurs in the collisional interstellar medium during protocluster assembly, and later in the ICM on the magnetic coherence scale, which always exceeds the particle mean free path, supporting the use of magneto-hydrodynamics for studying the cluster dynamo. We generate synthetic Faraday rotation measure observations of protoclusters, highlighting the potential for studying magnetic field growth during the onset of cluster formation at cosmic dawn.

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arXiv2024

Expected insights on type Ia supernovae from LISA's gravitational wave observations

The nature of progenitors of Type Ia supernovae has long been debated, primarily due to the elusiveness of the progenitor systems to traditional electromagnetic observation methods. We argue that gravitational wave observations with the upcoming Laser Interferometer Space Antenna (LISA) offer the most promising way to test one of the leading progenitor scenarios - the double-degenerate scenario, which involves a binary system of two white dwarf stars. In this study we review published results, supplementing them with additional calculations for the context of Type Ia supernovae. We discuss the fact that LISA will be able to provide a complete sample of double white dwarf Type Ia supernova progenitors with orbital periods shorter than 16-11 minutes (gravitational wave frequencies above 2-3 millihertz). Such a sample will enable a statistical validation of the double-degenerate scenario by simply counting whether LISA detects enough double white dwarf binaries to account for the measured Type Ia merger rate in Milky Way-like galaxies. Additionally, we illustrate how LISA's capability to measure the chirp mass will set lower bounds on the primary mass, revealing whether detected double white dwarf binaries will eventually end up as a Type Ia supernova. We estimate that the expected LISA constraints on the Type Ia merger rate for the Milky Way will be 4-9%. We also discuss the potential gravitational wave signal from a Type Ia supernova assuming a double-detonation mechanism and explore how multi-messenger observations could significantly advance our understanding of these transient phenomena.

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arXiv2024

Brightest Cluster Galaxy Offsets in Cold Dark Matter

The distribution of offsets between the brightest cluster galaxies of galaxy clusters and the centroid of their dark matter distributions is a promising probe of the underlying dark matter physics. In particular, since this distribution is sensitive to the shape of the potential in galaxy cluster cores, it constitutes a test of dark matter self-interaction on the largest mass scales in the universe. We examine these offsets in three suites of modern cosmological simulations; IllustrisTNG, MillenniumTNG and BAHAMAS. For clusters above $10^{14}\rm{M_\odot}$, we examine the dependence of the offset distribution on gravitational softening length, the method used to identify centroids, redshift, mass, baryonic physics, and establish the stability of our results with respect to various nuisance parameter choices. We find that offsets are overwhelmingly measured to be smaller than the minimum converged length scale in each simulation, with a median offset of $\sim1\rm{kpc}$ in the highest resolution simulation considered, TNG300-1, which uses a gravitational softening length of $1.48\rm{kpc}$. We also find that centroids identified via source extraction on smoothed dark matter and stellar particle data are consistent with the potential minimum, but that observationally relevant methods sensitive to cluster strong gravitational lensing scales, or those using gas as a tracer for the potential can overestimate offsets by factors of $\sim10$ and $\sim30$, respectively. This has the potential to reduce tensions with existing offset measurements which have served as evidence for a nonzero dark matter self-interaction cross section.

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arXiv2024

Ground-based and JWST Observations of SN 2022pul: II. Evidence from Nebular Spectroscopy for a Violent Merger in a Peculiar Type-Ia Supernova

We present an analysis of ground-based and JWST observations of SN~2022pul, a peculiar "03fg-like" (or "super-Chandrasekhar") Type Ia supernova (SN Ia), in the nebular phase at 338d post explosion. Our combined spectrum continuously covers 0.4--14 $μ$m and includes the first mid-infrared spectrum of an 03fg-like SN Ia. Compared to normal SN Ia 2021aefx, SN 2022pul exhibits a lower mean ionization state, asymmetric emission-line profiles, stronger emission from the intermediate-mass elements (IMEs) argon and calcium, weaker emission from iron-group elements (IGEs), and the first unambiguous detection of neon in a SN Ia. Strong, broad, centrally peaked [Ne II] line at 12.81 $μ$m was previously predicted as a hallmark of "violent merger'' SN Ia models, where dynamical interaction between two sub-$M_{ch}$ white dwarfs (WDs) causes disruption of the lower mass WD and detonation of the other. The violent merger scenario was already a leading hypothesis for 03fg-like SNe Ia; in SN 2022pul it can explain the large-scale ejecta asymmetries seen between the IMEs and IGEs and the central location of narrow oxygen and broad neon. We modify extant models to add clumping of the ejecta to better reproduce the optical iron emission, and add mass in the innermost region ($< 2000$ km s$^{-1}$) to account for the observed narrow [O I]~$λ\lambda6300$, 6364 emission. A violent WD-WD merger explains many of the observations of SN 2022pul, and our results favor this model interpretation for the subclass of 03fg-like SN Ia.

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arXiv2024

Simulating the tidal disruption of stars by stellar-mass black holes using moving-mesh hydrodynamics

In the centers of dense star clusters, close encounters between stars and compact objects are likely to occur. We study tidal disruption events of main-sequence (MS) stars by stellar-mass black holes (termed $μ$TDEs), which can shed light on the processes occurring in these clusters, including being an avenue in the mass growth of stellar-mass BHs. Using the moving-mesh hydrodynamics code \texttt{AREPO}, we perform a suite of hydrodynamics simulations of partial $μ$TDEs of realistic, \texttt{MESA}-generated MS stars by varying the initial mass of the star ($0.5\,{\rm M}_{\rm \odot}$ and $1\,{\rm M}_{\rm \odot}$), the age of the star (zero-age, middle-age and terminal-age), the mass of the black hole ($10\,{\rm M}_{\rm \odot}$ and $40\,{\rm M}_{\rm \odot}$) and the impact parameter (yielding almost no mass loss to full disruption). We then examine the dependence of the masses, spins, and orbital parameters of the partially disrupted remnant on the initial encounter parameters. We find that the mass lost from a star decreases exponentially with increasing distance of approach and that a $1\,{\rm M}_{\rm \odot}$ star loses lesser mass than a $0.5\,{\rm M}_{\rm \odot}$. Moreover, a more evolved star is less susceptible to mass loss. Tidal torques at the closest approach spin up the remnant by factors of $10^2$--$10^4$ depending on the impact parameter. The remnant star can be bound (eccentric) or unbound (hyperbolic) to the black hole: hyperbolic orbits occur when the star's central density concentration is relatively low and the black hole-star mass ratio is high, which is the case for the disruption of a $0.5\,{\rm M}_{\rm \odot}$ star. Finally, we provide best-fit analytical formulae for a range of parameters that can be incorporated into cluster codes to model star-black hole interaction more accurately.

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arXiv2024

Cosmological evolution of metallicity correlation functions from the Auriga simulations

We study the cosmological evolution of the two-point correlation functions of galactic gas-phase metal distributions using the 28 simulated galaxies from the Auriga Project. Using mock observations of the $z = 0$ snapshots to mimic our past work, we show that the correlation functions of the simulated mock observations are well matched to the correlation functions measured from local galaxy surveys. This comparison suggests that the simulations capture the processes important for determining metal correlation lengths, the key parameter in metallicity correlation functions. We investigate the evolution of metallicity correlations over cosmic time using the true simulation data, showing that individual galaxies undergo no significant systematic evolution in their metal correlation functions from $z\sim 3$ to today. In addition, the fluctuations in metal correlation length are correlated with but lag ahead fluctuations in star formation rate. This suggests that re-arrangement of metals within galaxies occurs at a higher cadence than star formation activity, and is more sensitive to the changes of environment, such as galaxy mergers, gas inflows / outflows, and fly-bys.

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arXiv2024

Magnetic field amplification in cosmological zoom simulations from dwarf galaxies to galaxy groups

Magnetic fields are ubiquitous in the Universe. Recently, cosmological simulations of galaxies have successfully begun to incorporate magnetic fields and their evolution in galaxies and their haloes. However, so far they have mostly focused on Milky Way-like galaxies. Here we analyse a sample of high resolution cosmological zoom simulations of disc galaxies in haloes with mass $M_\mathrm{200c}$ from $10^{10}\,\mathrm{M}_\odot$ to $10^{13}\,\mathrm{M}_\odot$, simulated with the Auriga galaxy formation model. We show that with sufficient numerical resolution the magnetic field amplification and saturation is converged. The magnetic field strength reaches equipartition with turbulent energy density for galaxies in haloes with $M_\mathrm{200c}\gtrsim 10^{11.5}\,\mathrm{M_\odot}$. For galaxies in less massive haloes, the magnetic field strength saturates at a fraction of equipartition that decreases with decreasing halo mass. For our lowest mass haloes, the magnetic field saturates significantly below $10\%$ of equipartition. We quantify the resolution we need to obtain converged magnetic field strengths and discuss our resolution requirements also in the context of the IllustrisTNG cosmological box simulations. We show that, at $z=0$, rotation-dominated galaxies in our sample exhibit for the most part an ordered large scale magnetic field, with fewer field reversals in more massive galaxies. Finally, we compare the magnetic fields in our cosmological galaxies at $z=0$ with simulations of isolated galaxies in a collapsing halo setup. Our results pave the way for detailed studies of cosmic rays and other physical processes in similar cosmological galaxy simulations that crucially depend on the strength and structure of magnetic fields.

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arXiv2023

Sampling the Faraday rotation sky of TNG50: Imprint of the magnetised circumgalactic medium around Milky Way-like galaxies

Faraday rotation measure (RM) is arguably the most practical observational tracer of magnetic fields in the diffuse circumgalactic medium (CGM). We sample synthetic Faraday rotation skies of Milky Way-like galaxies in TNG50 of the IllustrisTNG project by placing an observer inside the galaxies at a solar circle-like position. Our synthetic RM grids emulate specifications of current and upcoming surveys; the NRAO VLA Sky Survey (NVSS), the Polarisation Sky Survey of the Universe's Magnetism (POSSUM), and a future Square Kilometre Array (SKA1-mid) polarisation survey. It has been suggested that magnetic fields regulate the survival of high-velocity clouds. However, there is only a small number of observational detections of magnetised clouds thus far. In the first part of the paper, we test conditions for the detection of magnetised circumgalactic clouds. Based on the synthetic RM samplings of clouds in the simulations, we predict upcoming polarimetric surveys will open opportunities for the detection of even low-mass and distant clouds. In the second part of the paper, we investigate the imprint of the CGM in the all-sky RM distribution. We test whether the RM variation produced by the CGM is correlated with global galaxy properties, such as distance to a satellite, specific star formation rate, neutral hydrogen covering fraction, and accretion rate to the supermassive black hole. We argue that the observed fluctuation in the RM measurements on scales less than 1 degree, which has been considered an indication of intergalactic magnetic fields, might in fact incorporate a significant contribution of the Milky Way CGM.

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arXiv2023

Trading oxygen for iron I: the [O/Fe] -- specific star formation rate relation of galaxies

Our current knowledge of star-forming metallicity relies primarily on gas-phase oxygen abundance measurements. This may not allow one to accurately describe differences in stellar evolution and feedback driven by variations in iron abundance. $α$-elements (such as oxygen) and iron are produced by sources that operate on different timescales and the link between them is not straightforward. We explore the origin of the [O/Fe] - specific SFR (sSFR) relation, linking chemical abundances to galaxy formation timescales. This relation is followed by star-forming galaxies across redshifts according to cosmological simulations and basic theoretical expectations. Its apparent universality makes it suitable for trading the readily available oxygen for iron abundance. The relation is determined by the relative iron production efficiency of core-collapse and type Ia supernovae and the delay time distribution of the latter -- uncertain factors that could be constrained empirically with the [O/Fe]-sSFR relation. We compile and homogenise a literature sample of star-forming galaxies with observational iron abundance determinations to place first constraints on the [O/Fe]-sSFR relation over a wide range of sSFR. The relation shows a clear evolution towards lower [O/Fe] with decreasing sSFR and a flattening above log(sSFR/yr)>-9. The result is broadly consistent with expectations, but better constraints are needed to inform the models. We independently derive the relation from old Milky Way stars and find a remarkable agreement between the two, as long as the recombination-line absolute oxygen abundance scale is used in conjunction with stellar metallicity measurements.

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arXiv2023

Simulation-guided galaxy evolution inference: A case study with strong lensing galaxies

Understanding the evolution of galaxies provides crucial insights into a broad range of aspects in astrophysics, including structure formation and growth, the nature of dark energy and dark matter, baryonic physics, and more. It is, however, infeasible to track the evolutionary processes of individual galaxies in real time given their long timescales. As a result, galaxy evolution analyses have been mostly based on ensembles of galaxies that are supposed to be from the same population according to usually basic and crude observational criteria. We propose a new strategy of evaluating the evolution of an individual galaxy by identifying its descendant galaxies as guided by cosmological simulations. As a proof of concept, we examined the evolution of the total mass distribution of a target strong lensing galaxy at $z=0.884$ using the proposed strategy. We selected 158 galaxies from the IllustrisTNG300 simulation that we identified as analogs of the target galaxy. We followed their descendants and found 11 observed strong lensing galaxies that match in stellar mass and size with the descendants at their redshifts. The observed and simulated results are discussed, although no conclusive assessment is made given the low statistical significance due to the small sample size. Nevertheless, the test confirms that our proposed strategy is already feasible with existing data and simulations. We expect it to play an even more important role in studying galaxy evolution as more strong lens systems and larger simulations become available with the advent of next-generation survey programs and cosmological simulations.

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arXiv2023

Helium as a signature of the double detonation in Type Ia supernovae

The double detonation is a widely discussed mechanism to explain Type Ia supernovae from explosions of sub-Chandrasekhar mass white dwarfs. In this scenario, a helium detonation is ignited in a surface helium shell on a carbon/oxygen white dwarf, which leads to a secondary carbon detonation. Explosion simulations predict high abundances of unburnt helium in the ejecta, however, radiative transfer simulations have not been able to fully address whether helium spectral features would form. This is because helium can not be sufficiently excited to form spectral features by thermal processes, but can be excited by collisions with non-thermal electrons, which most studies have neglected. We carry out a full non-local thermodynamic equilibrium (non-LTE) radiative transfer simulation for an instance of a double detonation explosion model, and include a non-thermal treatment of fast electrons. We find a clear He I λ 10830 feature which is strongest in the first few days after explosion and becomes weaker with time. Initially this feature is blended with the Mg II λ 10927 feature but over time separates to form a secondary feature to the blue wing of the Mg II λ 10927 feature. We compare our simulation to observations of iPTF13ebh, which showed a similar feature to the blue wing of the Mg II λ 10927 feature, previously identified as C I. Our simulation shows a good match to the evolution of this feature and we identify it as high velocity He I λ 10830. This suggests that He I λ 10830 could be a signature of the double detonation scenario.

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arXiv2023

Close Encounters of Star - Black Hole Binaries with Single Stars

Multi-body dynamical interactions of binaries with other objects are one of the main driving mechanisms for the evolution of star clusters. It is thus important to bring our understanding of three-body interactions beyond the commonly employed point-particle approximation. To this end we here investigate the hydrodynamics of three-body encounters between star-black hole (BH) binaries and single stars, focusing on the identification of final outcomes and their long-term evolution and observational properties, using the moving-mesh hydrodynamics code AREPO. This type of encounters produces five types of outcomes: stellar disruption, stellar collision, weak perturbation of the original binary, binary member exchange, and triple formation. The two decisive parameters are the binary phase angle, which determines which two objects meet at the first closest approach, and the impact parameter, which sets the boundary between violent and non-violent interactions. When the impact parameter is smaller than the semimajor axis of the binary, tidal disruptions and star-BH collisions frequently occur when the BH and the incoming star first meet, while the two stars mostly merge when the two stars meet first instead. In both cases, the BHs accrete from an accretion disk at super-Eddington rates, possibly generating flares luminous enough to be observed. The stellar collision products either form a binary with the BH or remain unbound to the BH. Upon collision, the merged stars are hotter and larger than main sequence stars of the same mass at similar age. Even after recovering their thermal equilibrium state, stellar collision products, if isolated, would remain hotter and brighter than main sequence stars until becoming giants.

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arXiv2023

Triple evolution: an important channel in the formation of type Ia supernovae

Type Ia supernovae (SNe Ia) are thought to be the result of thermonuclear explosions in white dwarfs (WDs). Commonly considered formation pathways include two merging WDs (the double degenerate channel), and a single WD accreting material from a H or He donor (the single degenerate channel). Since the predicted SNe Ia rates from WD in binaries are thought to be insufficient to explain the observed SNe Ia rate, it is important to study similar interactions in higher-order multiple star systems such as triple systems. We use the evolutionary population synthesis code Multiple Stellar Evolution (MSE) to study stellar evolution, binary interactions and gravitational dynamics of the triple-star systems. Also, unlike previous studies, prescriptions are included to simultaneously take into account the single and double degenerate channels, and we consider triples across the entire parameter space (including those with tight inner binaries). We explore the impact of typically ignored or uncertain physics such as fly-bys and CE prescription parameters on our results. The majority of systems undergo circular mergers to explode as SNe Ia, while eccentric collisions contribute to $0.4-4$ per cent of SNe Ia events. The time-integrated SNe Ia rate from the triple channel is found to be $(3.60 \pm 0.04) \times {10^{-4}}\,\mathrm{M_{\odot}}^{-1}$ which is, surprisingly, similar to that of the isolated binary channel where the SNe Ia rate is $(3.2 \pm 0.1) \times {10^{-4}}\,\mathrm{M_{\odot}}^{-1}$. This implies that triples, when considering their entire parameter space, yield an important contribution to the overall SNe Ia rate.

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arXiv2023

Gravitational wave emission from dynamical stellar interactions

We are witnessing the dawn of gravitational wave (GW) astronomy. With currently available detectors, observations are restricted to GW frequencies in the range between ${\sim} 10\,\mathrm{Hz}$ and $10\,\mathrm{kHz}$, which covers the signals from mergers of compact objects. The launch of the space observatory LISA will open up a new frequency band for the detection of stellar interactions at lower frequencies. In this work, we predict the shape and strength of the GW signals associated with common-envelope interaction and merger events in binary stars, and we discuss their detectability. Previous studies estimated these characteristics based on semi-analytical models. In contrast, we used detailed three-dimensional magnetohydrodynamic simulations to compute the GW signals. We show that for the studied models, the dynamical phase of common-envelope events and mergers between main-sequence stars lies outside of the detectability band of the LISA mission. We find, however, that the final stages of common-envelope interactions leading to mergers of the stellar cores fall into the frequency band in which the sensitivity of LISA peaks, making them promising candidates for detection. These detections can constrain the enigmatic common-envelope dynamics. Furthermore, future decihertz observatories such as DECIGO or BBO would also be able to observe this final stage and the post-merger signal, through which we might be able to detect the formation of Thorne-Żytkow objects.

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arXiv2023

Close Encounters of Tight Binary Stars with Stellar-mass Black Holes

Strong dynamical interactions among stars and compact objects are expected in a variety of astrophysical settings, such as star clusters and the disks of active galactic nuclei. Via a suite of 3D hydrodynamics simulations using the moving-mesh code AREPO, we investigate the formation of transient phenomena and their properties in close encounters between an $2M_{\odot}$ or $20M_{\odot}$ equal-mass circular binary star and single $20M_{\odot}$ black hole (BH). Stars can be disrupted by the BH during dynamical interactions, naturally producing electromagnetic transient phenomena. Encounters with impact parameters smaller than the semimajor axis of the initial binary frequently lead to a variety of transients whose electromagnetic signatures are qualitatively different from those of ordinary disruption events involving just two bodies. These include the simultaneous or successive disruptions of both stars and one full disruption of one star accompanied by successive partial disruptions of the other star. On the other hand, when the impact parameter is larger than the semimajor axis of the initial binary, the binary is either simply tidally perturbed or dissociated into bound and unbound single stars ("micro-Hills" mechanism). The dissociation of $20M_{\odot}$ binaries can produce a runaway star and an active BH moving away from one another. Also, the binary dissociation can either produce an interacting binary with the BH, or a non-interacting, hard binary; both could be candidates of BH high- and low-mass X-ray binaries. Hence our simulations especially confirm that strong encounters can lead to the formation of the (generally difficult to form) BH low-mass X-ray binaries.

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