arXiv subjects

Hagai B. Perets

Publications and source records attributed to Hagai B. Perets.

At least 37 records · Page 2Linked to original sources

arXiv2025

Evolution of Triple Stars

Triple stellar systems, consisting of three gravitationally bound stars, play a fundamental role in a wide array of astrophysical processes, from stellar evolution to the formation of exotic objects and gravitational wave sources. This review provides a comprehensive overview of the dynamics and evolution of triple stellar systems, highlighting their crucial role in shaping stellar populations and driving diverse astrophysical phenomena. We begin by discussing the observed properties of triples, including their frequency and orbital configurations, emphasizing the challenges in characterizing these systems. We then delve into the intricate dynamics of triples, exploring hierarchical, secular, quasi-secular, and chaotic regimes, with particular attention to the von Zeipel-Lidov-Kozai (vZLK) mechanism and its extensions. The interplay between stellar evolution and triple dynamics is examined, including mass loss, mass transfer, common-envelope evolution, and supernovae effects. We discuss the development and applications of triple population synthesis codes, demonstrating their power in modeling complex evolutionary scenarios and predicting observational outcomes. Finally, we emphasize the unique role of triple evolution in producing a wide range of astrophysical phenomena, from interacting binaries and stellar mergers to explosive transients and gravitational wave sources, underscoring the importance of triple systems in our understanding of stars and stellar populations.

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arXiv2025

On the Ordering of Exoplanet Systems

We present a comprehensive analysis of planetary radii ordering within multi-planet systems, namely their ordinal position with respect to their size in a given system, utilizing data from the NASA Exoplanet Archive. In addition, we consider not only the ordinal positions but also the specific period ratios and radius ratios of planetary pairs in multi-planet systems. We explore various dependencies on stellar host type and metallicity, as well as planetary types, and explore the differences between planetary systems with different planet multiplicities and different planetary pairs in the same system. Focusing on Kepler systems with two to four planets, we account for observational biases and uncover a robust trend of smaller inner planets. This trend is particularly pronounced in inner pairs of three-planet systems and exhibits variations in stellar metallicity and planet multiplicity. Notably, we find that the distribution of inner-to-outer planet radii ratios depends on the system's metallicity, suggesting a link between initial conditions and the resulting system architecture. Interestingly, planet pairs in resonance do not exhibit significantly different size ratios compared to non-resonant pairs, challenging current theoretical expectations, again, possibly suggesting that initially resonant systems could have been later destabilized. Our findings align with planet formation and migration models where larger planets form farther out and migrate inward. Importantly, we emphasize the significance of planet ordering as a novel and crucial observable for constraining planet formation and evolution models. The observed patterns offer unique insights into the complex interplay of formation, migration, and dynamical interactions shaping planetary systems.

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arXiv2025

Thermonuclear explosion criteria for direct and indirect collisions of CO white dwarfs: a study of the impact-parameter threshold for detonation

The physical collisions of two white dwarfs (WDs) (i.e. not slow mergers) have been shown to produce type-Ia-like supernovae (SNe) explosions. Most studies of WD collisions have focused on zero impact-parameter (direct) collisions, which can also be studied in 2D. However, the vast majority of WD collisions arising from any evolutionary channels suggested to date are expected to be indirect, i.e. have a non-negligible impact parameter upon collision. Here, we use one of the highest resolution 3D simulations to date (making use of the AREPO code) in order to explore both direct and indirect collisions and the conditions in which they give rise to a detonation and the production of a luminous SNe. Using our simulations, we find a detonation criterion that can provide the critical impact parameter for an explosion to occur, depending on the density profile of the colliding WDs, their composition, and their collision velocities. We find that the initial velocity has a significant impact on the amount of 56Ni production from the explosion. Furthermore, the production of the 56Ni also depends on numerical modeling aspects.

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arXiv2025

Too fast to be single: Tidal evolution and photometric identification of stellar and planetary companions

Many stars, including those in binary or multiple systems, exhibit modified rotational evolution due to tidal interactions. While magnetic braking slows rotation in single stars, close binaries experience synchronization from tidal forces, resulting in high spin rates. Thus, fast rotators often signify synchronized binaries or planetary systems. We analyze stellar rotation in the Kepler field to photometrically identify non-single systems. Establishing an initial rotation-temperature relationship for individual stars via young clusters, we confirm our findings through magnitude excess and prior binary star system studies. Stars rotating faster than this relationship display a bimodal distribution in peculiar velocity, indicative of non-single or young stars. Leveraging this, we separate non-single stars when peculiar velocity is measurable, or estimate likelihood for those without. Our method identifies 2229 potential non-single star systems with rotation periods exceeding 3 days. For ultra-fast rotators ($P_{rot} < 3 days$), we compile a catalog of 1518 ultra-short-period binary candidates, often part of hierarchical triples, reinforcing rapid spin's association with multiplicity. Applying our method to planet-host stars uncovers Kepler-1184 as a potential circumbinary system and identifies Kepler-493 and Kepler-957 potentially synchronized by close-in planets, with three others as potential false positives. Analysis of known non-single stars reveals clear tidal effects: period synchronization, orbit circularization, and a minimal pericenter constraint for binaries ($r_p \propto (P_{orb}/P_{rot})^{0.77}$). These findings offer insights into tidal evolution, provide a robust method for identifying stellar multiplicity, and have implications for stellar evolution, binary formation, and exoplanet dynamics.

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arXiv2025

The formation of mini-AGN disks around IMBHs and their dynamical implications

This study explores the formation and implications of mini-active galactic nuclei (mAGN) disks around intermediate-mass black holes (IMBHs) embedded in gas-rich globular/nuclear clusters (GCs). We examine the parameter space for stable mAGN disks, considering the influence of IMBH mass, disk radius, and gas density on disk stability. The dynamics of stars and black holes within the mAGN disk are modeled, with a focus on gas-induced migration and gas dynamical friction. These dynamical processes can lead to several potentially observable phenomena, including the alignment of stellar orbits into the disk plane, the enhancement of gravitational wave mergers (particularly IMRIs and EMRIs), and the occurrence of mili/centi-tidal disruption events (mTDEs/cTDEs) with unique observational signatures. We find that gas hardening can significantly accelerate the inspiral of binaries within the disk, potentially leading to a frequency shift in the emitted gravitational waves. Additionally, we explore the possibility of forming accreting IMBH systems from captured binaries within the mAGN disk, potentially resulting in the formation of ultraluminous X-ray sources (ULXs). The observational implications of such accreting systems, including X-ray emission, optical signatures, and transient phenomena, are discussed. Furthermore, we investigate the possibility of large-scale jets emanating from gas-embedded IMBHs in GCs. While several caveats and uncertainties exist, our work highlights the potential for mAGN disks to provide unique insights into IMBH demographics, accretion physics, and the dynamics of GCs.

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arXiv2025

Planet formation and long-term stability in a very eccentric stellar binary

Planets orbiting one of the two stars in a binary are vulnerable to gravitational perturbations from the other star. Particularly, highly eccentric companion stars risk disrupting planetary orbits, such as in the extreme system TOI 4633 where close encounters between the companion and a gas giant planet in the habitable zone make it one of the most fragile systems discovered so far. Here, we report that TOI 4633's planet likely survived these encounters throughout the system's age by orbiting retrograde relative to the binary, stabilised by the Coriolis force. Using direct $N$-body simulations, we show it otherwise tends to collide with the binary stars or becomes free-floating after getting ejected. A retrograde planetary orbit has profound implications for TOI 4633's formation and evolution, suggesting an extraordinary history where its eccentric companion was likely randomly captured after planet formation in a single-star system. Alternatively, if stars and planet are born in situ from the same gas clump, we show the planet must have formed at sub-snow-line distances, contrary to the conventional core-accretion model. Our study highlights the importance of considering the long-term stability ($\gtrsim\rm Gyr$) of planets in eccentric binaries and demonstrates that the mere existence in such dynamically hostile environments places strong constraints on their orbital configuration and formation.

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arXiv2024

Accurate and Robust Stellar Rotation Periods catalog for 82771 Kepler stars using deep learning

We propose a new framework to predict stellar properties from light curves. We analyze the light-curve data from the Kepler space mission and develop a novel tool for deriving the stellar rotation periods for main-sequence stars. Using this tool, we provide rotation periods for more than 80K stars. Our model, LightPred, is a novel deep-learning model designed to extract stellar rotation periods from light curves. The model utilizes a dual-branch architecture combining Long Short-Term Memory (LSTM) and Transformer components to capture temporal and global data features. We train LightPred on self-supervised contrastive pre-training and simulated light curves generated using a realistic spot model. Our evaluation demonstrates that LightPred outperforms classical methods like the Autocorrelation Function (ACF) in terms of accuracy and average error. We apply LightPred to the Kepler dataset, generating the largest catalog to date. Using error analysis based on learned confidence and consistency metric, we were able to filter the predictions and remove stellar types with variability which is different than spot-induced variability. Our analysis shows strong correlations between error levels and stellar parameters. Additionally, we confirm tidal synchronization in eclipsing binaries with orbital periods shorter than 10 days. Our findings highlight the potential of deep learning in extracting fundamental stellar properties from light curves, opening new avenues for understanding stellar evolution and population demographics.

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arXiv2024

New versus past silica crush curve experiments: application to Dimorphos benchmarking impact simulations

Crush curves are of fundamental importance to numerical modeling of small and porous astrophysical bodies. The empirical literature often measures them for silica grains, and different studies have used various methods, sizes, textures, and pressure conditions. Here we review past studies and supplement further experiments in order to develop a full and overarching understanding of the silica crush curve behavior. We suggest a new power-law function that can be used in impact simulations of analog materials similar to micro-granular silica. We perform a benchmarking study to compare this new crush curve to the parametric quadratic crush curve often used in other studies, based on the study case of the DART impact onto the asteroid Dimorphos. We find that the typical quadratic crush curve parameters do not closely follow the silica crushing experiments, and as a consequence they under (over) estimate compression close (far) from the impact site. The new crush curve presented here, applicable to pressures between a few hundred Pa and up to 1.1 GPa, might therefore be more precise. Additionally, it is not calibrated by case-specific parameters, and can be used universally for comet- or asteroid-like bodies, given an assumed composition similar to micro-granular silica.

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arXiv2024

Adaptive Habitability of Exoplanets: Thriving Under Extreme Environmental Change

The dynamic nature of life's ability to thrive in diverse and changing planetary environments suggests that habitability and survival depend on the evolutionary path and life adaptation to environmental conditions. Here we explore such "adaptive habitability" through astro-ecological models. We study the interplay between temperature adaptation and environmental fluctuations, particularly those induced by solar activity and orbital dynamics. We present a simplified ecological-evolutionary model to investigate the limits of life's adaptability on a planetary scale. By incorporating complexities such as multiple niches, migration, species interactions, and realistic temperature variations, we demonstrate the potential for adaptive habitability in the face of both gradual and abrupt environmental changes. Through simulations encompassing monotonic, periodic, and secular dynamical evolution-induced temperature profiles, we identify critical thresholds for survival and extinction, highlighting the importance of phenotypic variance and dispersal rates in adapting to varying environmental conditions. These findings underscore the significance of considering temporal variations in assessing exoplanet habitability and expanding the search space for potentially habitable worlds.

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arXiv2024

Capture of stars into gaseous discs around massive black holes: Alignment, circularization and growth

The majority of massive black holes (MBHs) likely hosted gas discs during their lifetimes. These could either be long-lived active galactic nuclei (AGN) discs, or shorter-lived discs formed following singular gas infall events, as was likely the case in our own Galactic Center. Stars and compact objects in such environments are therefore expected to interact with the gaseous disc as they go through it, and potentially become aligned and fully embedded within it. The interactions of embedded stars with the gas could give rise to a plethora of physical processes affecting the stars, including growth through accretion of gas, migration in the disc, stellar captures, and mergers with other stars. The impact of such processes strongly depends on the population of stars that eventually align with the disc and become embedded in it. Here we make use of analytic tools to analyze the alignment process, accounting for both geometric drag and gas dynamical friction. We find that up to $\sim 50\%$ of main sequence stars and stellar mass black holes in the central 0.1 pc can align with AGN discs in the Galactic Center and similar galactic nuclei. The orbits of aligned stars are typically circularized and are prograde with respect to the AGN disc. Furthermore, alignment and accretion are intimately linked, and the capture of stars by an AGN disc can potentially explain the origin of the young stellar disc in the Galactic Center with a top-heavy mass function, even without the need for a star-formation event.

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arXiv2024

Three-Body Binary Formation in Clusters: Analytical Theory

Binary formation in clusters through triple encounters between three unbound stars, 'three-body' binary formation, is one of the main dynamical formation processes of binary systems in dense environments. In this paper, we use an analytical probabilistic approach to study the process for the equal mass case and calculate a probability distribution for the orbital parameters of three-body-formed binaries, as well as their formation rate. For the first time, we give closed-form analytical expressions to the full orbital parameter distribution, accounting for both energy and angular momentum conservation. This calculation relies on the sensitive dependence of the outcomes of three-body scatterings on the initial conditions: here we compute the rate of three-body binaries from ergodic interactions, which allow for an analytical derivation of the distribution of orbital parameters of the binaries thus created. We find that soft binaries are highly favoured in this process and that these binaries have a super-thermal eccentricity distribution, while the few hard three-body binaries have an eccentricity distribution much closer to thermal. The analytical results predict and reproduce simulation results of three-body scattering experiments in the literature well.

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arXiv2024

Soft no more: gas shielding protects soft binaries from disruption in gas-rich environments

Binaries in dense environments are traditionally classified as soft or hard based on their binding energy relative to the kinetic energy of surrounding stars. Heggie's law suggests that stellar encounters tend to soften soft binaries and harden hard binaries, altering their separations. However, interactions with gas in such environments can significantly modify this behavior. This study investigates the impact of gas on binary softening and its consequences. We find that gas interactions can actually harden binaries, extending the soft-hard boundary to larger separations. This introduces a "shielding radius" within which binaries are likely to harden due to gas interactions, surpassing the traditional soft-hard limit. Consequently, a notable portion of binaries initially classified as "soft" may become "hard" when both gas and stars are considered. We propose a two-stage formation process for hard binaries: initial soft binary formation, either dynamically or through gas-assisted capture, followed by gas-induced hardening before eventual disruption. In environments with low gas density but high gas content, the shielding radius could exceed the typical hard-soft limit by an order of magnitude, leading to a significant fraction of originally soft binaries effectively becoming hard. Conversely, in high gas-density environments, gas-induced hardening may dominate, potentially rendering the entire binary population hard. Gas hardening emerges as a crucial factor in shaping binary populations in gas-rich settings, such as clusters, star-forming regions, and possibly AGN disks. This highlights the complex interplay between gas dynamics and stellar interactions in binary evolution within dense environments.

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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

Upper Limits of $^{44}$Ti Decay Emission in Four Nearby Thermonuclear Supernova Remnants

To identify progenitors and investigate evidence of He burning, we searched for decay radiation of freshly synthesized $^{44}$Ti in four young nearby thermonuclear supernova remnants: Kepler, SN 1885, G1.9+0.3 and SN 1006, by analysing the up-to-date NuSTAR archival data. No apparent flux excess from the 68 and 78 keV line emissions accompanying decay was detected above the power law continuum applied for the remnants and the absorbed stray light. By comparing the inferred upper limits of the line flux and the initial $^{44}$Ti masses with a wide variety of supernova nucleosynthesis models, we placed constraints on the supernova progenitors. We derived the first NuSTAR line flux upper limit for Kepler and ruled out most of the double-detonation scenarios with a thick He layer under low density. We estimated, for the first time, the upper limit for SN 1885, which is high because of the large distance yet still remains consistent with the He shell detonation. The new flux and mass limit of G1.9+0.3 derived from a longer total exposure is lower than the results from previous studies and evidently excludes explosive burning of He-rich matter. The relatively advanced age and the large spatial extent of SN 1006 have prevented meaningful constraints.

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arXiv2024

Morphology and Mach Number Dependence of Subsonic Bondi-Hoyle Accretion

We carry out three-dimensional computations of the accretion rate onto an object (of size $R_{\rm sink}$ and mass $m$) as it moves through a uniform medium at a subsonic speed $v_{\infty}$. The object is treated as a fully-absorbing boundary (e.g. a black hole). In contrast to early conjectures, we show that when $R_{\rm sink}\ll R_{A}=2Gm/v^2$ the accretion rate is independent of $v_{\infty}$ and only depends on the entropy of the ambient medium, its adiabatic index, and $m$. Our numerical simulations are conducted using two different numerical schemes via the Athena++ and Arepo hydrodynamics solvers, which reach nearly identical steady-state solutions. We find that pressure gradients generated by the isentropic compression of the flow near the accretor are sufficient to suspend much of the surrounding gas in a near-hydrostatic equilibrium, just as predicted from the spherical Bondi-Hoyle calculation. Indeed, the accretion rates for steady flow match the Bondi-Hoyle rate, and are indicative of isentropic flow for subsonic motion where no shocks occur. We also find that the accretion drag may be predicted using the Safronov number, $Θ=R_{A}/R_{\rm sink}$, and is much less than the dynamical friction for sufficiently small accretors ($R_{\rm sink}\ll R_{A}$).

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arXiv2024

Transients from ONe White-Dwarf - Neutron-Star/Black-Hole Mergers

We conduct the first 3D hydrodynamic simulations of oxygen-neon white dwarf-neutron star/black hole mergers (ONe WD-NS/BH mergers). Such mergers constitute a significant fraction, and may even dominate, the inspiral rates of all WD-NS binaries. We post-process our simulations to obtain the nuclear evolution of these systems and couple the results to a supernova spectral synthesis code to obtain the first light curves and spectra for these transients. We find that the amount of $^{56}$Ni synthesised in these mergers grows as a strong function of the WD mass, reaching typically $0.05$ and up to $0.1\,{\rm M}_\odot$ per merger. Photodisintegration leads to similar amounts of $^4$He and about a ten times smaller amount of $^1$H. The nuclear yields from these mergers, in particular those of $^{55}$Mn, may contribute significantly to Galactic chemical evolution. The transients expected from ONe WD-NS mergers are dominantly red/infrared, evolve on month-long timescales and reach bolometric magnitudes of up to -16.5. The current surveys must have already detected these transients or are, alternatively, putting strong constraints on merger scenarios. The properties of the expected transients from WD-NS mergers best agree with faint type Iax supernovae. The Vera Rubin Observatory (LSST) will be detecting up to thousands of merging ONe WD-NS systems per year. We simulate a subset of our models with 2D axisymmetric FLASH code to investigate why they have been challenging for previous studies. We find that the likely main challenge has been effectively modelling the nuclear statistical equilibrium regime in such mergers.

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arXiv2023

Dynamical evolution of second-generation circumstellar/protoplanetary disks in evolved wide binary systems

In mass-transferring wide binary stellar systems, the companion star can capture some of the mass released in wind by the primary evolved star, and form an accretion disk. Such accretion disks could evolve to form disks of comparable properties to protoplanetary disks and may enable the formation of new planets and/or the interactions, re-growth, and re-migration of pre-existing planets in the newly formed disks. We study the formation and the dynamical evolution of such "second generation" (SG) protoplanetary disks in evolved wide binary systems, with primaries in the mass range of 1-8 Solar mass. We follow their evolution from the asymptotic giant branch (AGB) phase of the red giant stellar donor until its evolution to become a white dwarf. We perform 1D semi-analytical numerical simulations for several binary systems varying the mass of the evolved stellar donor and the initial orbital distance, taking into account the changing mass-loss rates and the binary orbital expansion due to mass-loss. We calculate the radial density profile of the formed SG accretion disk and its temperature profile, considering a non-stationary viscosity profile which depends on the radial temperature profile. We find that SG circumstellar disks evolve to form a long-lived stable structure over the lifetime of the donor star, and we show that we can consistently produce the observed conditions and accretion rate inferred for the Mira evolved wide-binary system. The quasi-steady state radial surface density profiles are comparable with the typical range of masses and densities of observed (regular "first-generation") protoplanetary disks. This suggests that realistic SG disks can give rise to a second phase of planet formation and dynamics in old wide binary systems.

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arXiv2023

Resonant Dynamical Friction Around a Super-Massive Black Hole: Analytical Description

We derive an analytical model for the so-called phenomenon of `resonant dynamical friction', where a disc of stars around a super-massive black hole interacts with a massive perturber, so as to align its inclination with the disc's orientation. We show that it stems from a singular behaviour of the orbit-averaged equations of motion, which leads to a rapid alignment of the argument of the ascending node $Ω$ of each of the disc stars, with that of the perturber, $Ω_{\rm p}$, with a phase-difference of $90^\circ$. This phenomenon occurs for all stars whose maximum possible $\dotΩ$ (maximised over all values of $Ω$ for all the disc stars), is greater than $\dotΩ_{\rm p}$; this corresponds approximately to all stars whose semi-major axes are less than twice that of the perturber. The rate at which the perturber's inclination decreases with time is proportional to its mass and is shown to be much faster than Chandrasekhar's dynamical friction. We find that the total alignment time is inversely proportional to the root of the perturber's mass. This persists until the perturber enters the disc. The predictions of this model agree with a suite of numerical $N$-body simulations which we perform to explore this phenomenon, for a wide range of initial conditions, masses, \emph{etc.}, and are an instance of a general phenomenon. Similar effects could occur in the context of planetary systems, too.

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