arXiv subjects

Zsolt Regály

Publications and source records attributed to Zsolt Regály.

18 recordsLinked to original sources

arXiv2026

Limits on eccentricity excitation by tidally enhanced mass loss in Wolf-Rayet binaries

Wolf--Rayet (WR) binaries frequently display high orbital eccentricities, and their eccentricity distributions are inconsistent with that of high-mass Galactic field binaries at high significance. This raises the question of whether these eccentricities are primordial or generated during the WR phase. We test whether a tidal enhancement of the mass-loss rate can pump the eccentricity of a WR binary on a sample of 14 WR systems. We model each system as a variable-mass two-body problem and integrate the orbital evolution over the WR lifetime under three prescriptions: i) adiabatic isotropic mass loss, ii) equilibrium-tide circularization, and iii) a tidally enhanced, pericenter-focused mass loss in which the companion-facing reduction of the effective surface gravity raises the local Eddington parameter. Adiabatic mass loss can not change eccentricity and affects only the orbital separation, which increases by up to a factor of ~20. Tidal circularization is effective only in the tightest orbits, which are compatible with the observed systems only for high mass-loss rates, a regime in which any primordial eccentricity is erased. The tidally enhanced mass loss channel pumps the eccentricity by only $\leq10^{-4}$, three to four orders of magnitude below the observed values, and therefore cannot be the responsible mechanism. We conclude that the high eccentricities of WR binaries are predominantly primordial rather than generated by mass loss during the WR phase.

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arXiv2025

Life in the dark: Potential urability of moons of rogue planets

Free-floating planets are thought to be numerous in the Galaxy and may retain their moons after ejection from their natal systems. If those satellites acquire or preserve orbital eccentricity, tidal dissipation could provide a long-lasting internal heat source, potentially creating urable environments (capable of enabling abiogenesis) in the absence of stellar radiation. We explore (i) whether moons remain bound to planets expelled by a core-collapse supernova, (ii) how the explosion reshapes their orbits, and (iii) under which circumstances tidal heating can sustain urable subsurface oceans. We carried out three-dimensional N-body simulations with an 8th-order Runge-Kutta scheme, modelling homologous stellar mass loss for progenitors of 10 M$_{\odot}$. Post-explosion orbital elements of single moons and resonant moon systems were analysed, and tidal heating power was estimated with a constant phase-lag model for several tidal dissipation functions and moon densities. All simulated moons survive the supernova and remain bound to their planets. The explosion excites moon eccentricities up to $\sim7\times10^{-4}$ and $\sim3\times10^{-3}$ for single moons of planets with circular and eccentric orbits, respectively. For resonant pairs, an eccentricity of $\leq2\times10^{-2}$ is preserved. The semi-major axis of the moons changes by $\leq0.2\%$. For 12-15\% of cases -- preferentially moons at $a\leq15\,R_{\mathrm{planet}}$ and with $e\geq10^{-3}$ -- the specific tidal heating power lies between 0.1 and 10 times what is estimated on Europa or Enceladus, sufficient to maintain liquid oceans beneath an ice crust. Eccentricity damping timescales exceed the age of the Solar System for $a\geq10\,R_{\mathrm{planet}}$, implying billions of years of continuous heating on the moons. Such worlds represent promising targets for future searches for extraterrestrial life.

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arXiv2025

Dust-driven vortex cascades originating at water snow regions: A pathway to planetesimal formation

The origin of observed planetary systems, including our Solar System, as well as their diversity, is still an open question. Streaming instability (SI) is an important mechanism for the formation of gravitationally bound planetesimals, which can grow to form planetary embryos and eventually planets. Snow lines in a protoplanetary disk can assist this process, as they can form pressure maxima and promote both dust accumulation and growth. Since the sublimation of a volatile is gradual due to opacity changes, a snow line in a protoplanetary disk is in fact a radially extended "snow region" of constant temperature. It has been shown that dust can influence disk viscosity through the adsorption of charged particles, and even a minor perturbation in the gas can trigger the excitation of multiple small-scale Rossby vortices. Here, we investigate the possibility of Rossby vortex excitation and rapid planetesimal formation at temperature substructures associated with the snow regions, using global 2D gas-dust coupled hydrodynamic simulations that include dust feedback and self-gravity. We find that an initial temperature substructure in a protoplanetary disk can seed a rapid cascade of long-lived, self-sustaining Rossby vortices. The vortices accumulate significant amount of dust and the local conditions are favorable for SI as well as gravitational collapse. However, the vortex formation via this mechanism requires sufficient decoupling between dust and gas, and such conditions may not be met early on when the disk is gas-rich, resulting in a delayed onset of vortex formation. The self-sustaining Rossby vortices offer exceptionally favorable conditions for dust growth and the formation of planetesimals, as well as a possible pathway for the rapid formation of planetary cores.

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arXiv2024

Prominent mid-infrared excess of the dwarf planet (136472) Makemake discovered by JWST/MIRI indicates ongoing activity

We report on the discovery of a very prominent mid-infrared (18-25 μm) excess associated with the trans-Neptunian dwarf planet (136472) Makemake. The excess, detected by the MIRI instrument of the James Webb Space Telescope, along with previous measurements from the Spitzer and Herschel space telescopes, indicates the occurrence of temperatures of about 150 K, much higher than what solid surfaces at Makemake's heliocentric distance could reach by solar irradiation. We identify two potential explanations: a continuously visible, currently active region, powered by subsurface upwelling and possibly cryovolcanic activity, covering <1% of Makemake's surface, or an as yet undetected ring containing very small carbonaceous dust grains, which have not been seen before in trans-Neptunian or Centaur rings. Both scenarios point to unprecedented phenomena among trans-Neptunian objects and could greatly impact our understanding of these distant worlds.

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arXiv2024

Origin of Ca II emission around polluted white dwarfs

Dozens of white dwarfs with anomalous metal polluted atmospheres (DZ WDs) are known to host dust and gas discs. The line profiles of the Ca II triplet emitted by the gas discs show significant asymmetry. Several minor planets have been discovered orbiting such WDs. The most challenging burden of modelling gas discs around DZ WDs is to simultaneously explain the asymmetry and metal pollution of the WD's atmosphere, while staying consistent with other aspects of the observations, like the morphology of the Ca II lines. This paper aims to construct a self-consistent model to explain the simultaneous WD pollution and Ca II line asymmetry over at least three years. In our model an asteroid disintegrates in an eccentric orbit, periodically entering below the WD's Roche limit. The resulting debris sublimates at a temperature of 1500 K, producing gas that viscously spreads to form a disc. The evolution of the discs is studied over a period of 1.2 years using two-dimensional hydrodynamic simulations. Synthetic Ca II line profiles are calculated using the surface mass density and velocity distributions provided by the simulations, for the first time taking into account the asymmetric velocity distribution in the discs. An asteroid disintegrating in an eccentric orbit gives rise to the formation of an asymmetric disc and asymmetric Ca II triplet emission. Our model can explain the periodic reversal of the red- and blueshifted peak of the Ca II lines caused by the precession of the disc on timescales of 10.6 to 177.4 days. Our work suggests that the persistence of Ca II asymmetry over decades and its periodic change in the peaks can be explained in two scenarios: a) the asteroid disrupts on a short timescale (couple of orbits), and the gas has a low viscosity range (0.001<alpha<0.05), b) the asteroid disrupts on a timescale of a year, and the viscosity of the gas is required to be high, alpha=0.1.

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arXiv2023

Double neutron star formation via consecutive type II supernova explosions

Since the discovery of the first double neutron star (DNS) system, the number of these exotic binaries has reached fifteen. Here we investigate a channel of DNS formation in binary systems with components above the mass limit of type II supernova explosion (SN II), i.e. 8 MSun. We apply a spherically symmetric homologous envelope expansion model to account for mass loss, and follow the dynamical evolution of the system numerically with a high-precision integrator. The first SN occurs in a binary system whose orbital parameters are pre-defined, then, the homologous expansion model is applied again in the newly formed system. Analysing 1 658 880 models we find that DNS formation via subsequent SN II explosions requires a fine-tuning of the initial parameters. Our model can explain DNS systems with a separation greater than 2.95 au. The eccentricity of the DNS systems spans a wide range thanks to the orbital circularisation effect due to the second SN II explosion. The eccentricity of the DNS is sensitive to the initial eccentricity of the binary progenitor and the orbital position of the system preceding the second explosion. In agreement with the majority of the observations of DNS systems, we find the system centre-of mass velocities to be less than 60 km/s. Neutron stars that become unbound in either explosion gain a peculiar velocity in the range of 0.02 - 240 km/s. In our model, the formation of tight DNS systems requires a post-explosion orbit-shrinking mechanism, possibly driven by the ejected envelopes.

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arXiv2022

A gap at 1 au in the disk of DI Cha A revealed by infrared interferometry

DI Cha A is K0-type pre-main sequence star, the brightest component of a quadruple stellar system. Here we report on a detailed study of this star based on archival VLTI/MIDI and VLTI/PIONIER infrared interferometric observations, as well as optical--infrared photometric monitoring from ground-based and space-born instruments. We determined the structure of the circumstellar disk by fitting simultaneously the interferometric visibilities and the spectral energy distribution, using both analytical models and the radiative transfer code RADMC-3D. The modeling revealed that the radial density distribution of the disk appears to have a gap between 0.21 and 3.0 au. The inner ring, whose inner size coincides with the sublimation radius, is devoid of small, submicrometer-sized dust grains. The inner edge of the outer disk features a puffed-up rim, typically seen in intermediate-mass stars. Grain growth, although less progressed, was also detected in the outer disk. The inner ring is variable at mid-infrared wavelengths on both daily and annual timescales, while the star stays remarkably constant in the optical, pointing to geometrical or accretion changes in the disk as possible explanation for the flux variations.

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arXiv2021

Increased isolation mass for pebble accreting planetary cores in pressure maxima of protoplanetary discs

The growth of a pebble accreting planetary core is stopped when reaching its \textit{isolation mass} that is due to a pressure maximum emerging at the outer edge of the gap opened in gas. This pressure maximum traps the inward drifting pebbles stopping the accretion of solids onto the core. On the other hand, a large amount of pebbles ($\sim 100M_\oplus$) should flow through the orbit of the core until reaching its isolation mass. The efficiency of pebble accretion increases if the core grows in a dust trap of the protoplanetary disc. Dust traps are observed as ring-like structures by ALMA suggesting the existence of global pressure maxima in discs that can also act as planet migration traps. This work aims to reveal how large a planetary core can grow in such a pressure maximum by pebble accretion. In our hydrodynamic simulations, pebbles are treated as a pressureless fluid mutually coupled to the gas via drag force. Our results show that in a global pressure maximum the pebble isolation mass for a planetary core is significantly larger than in discs with power-law surface density profile. An increased isolation mass shortens the formation time of giant planets.

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arXiv2020

Torques felt by solid accreting planets

The solid material of protoplanetary discs forms an asymmetric pattern around a low-mass planet (M_p<=10M_Earth) due to the combined effect of dust-gas interaction and the gravitational attraction of the planet. Recently, it has been shown that although the total solid mass is negligible compared to that of gas in protoplanetary discs, a positive torque can be emerged by a certain size solid species. The torque magnitude can overcome that of gas which may result in outward planetary migration. In this study, we show that the accretion of solid species by the planet strengthens the magnitude of solid torque being either positive or negative. We run two-dimensional, high-resolution (1.5Kx3K) global hydrodynamic simulations of an embedded low-mass planet in a protoplanetary disc. The solid material is handled as a pressureless fluid. Strong accretion of well-coupled solid species by a M_p<0.3M_Earth protoplanet results in the formation of such a strongly asymmetric solid pattern close to the planet that the positive solid torque can overcome that of gas by two times. However, the accretion of solids in the pebble regime results in increased magnitude negative torque felt by protoplanets and strengthened positive torque for Earth-mass planets. For M_p>=3M_Earth planets the magnitude of the solid torque is positive, however, independent of the accretion strength investigated. We conclude that the migration of solid accreting planets can be substantially departed from the canonical type-I prediction.

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arXiv2020

Outbursts in Global Protoplanetary Disk Simulations

While accreting through a circumstellar disk, young stellar objects are observed to undergo sudden and powerful accretion events known as FUor or EXor outbursts. Although such episodic accretion is considered to be an integral part of the star formation process, the triggers and mechanisms behind them remain uncertain. We conducted global numerical hydrodynamics simulations of protoplanetary disk formation and evolution in the thin-disk limit, assuming both magnetically layered and fully magnetorotational instability (MRI)-active disk structure. In this paper, we characterize the nature of the outbursts occurring in these simulations. The instability in the dead zone of a typical layered disk results in "MRI outbursts". We explore their progression and their dependence on the layered disk parameters as well as cloud core mass. The simulations of fully MRI-active disks showed an instability analogous to the classical thermal instability. This instability manifested at two temperatures--above approximately 1400 K and 3500 K--due to the steep dependence of Rosseland opacity on the temperature. The origin of these thermally unstable regions is related to the bump in opacity resulting from molecular absorption by water vapor and may be viewed as a novel mechanism behind some of the shorter duration accretion events. Although we demonstrated local thermal instability in the disk, more investigations are needed to confirm that a large-scale global instability will ensue. We conclude that the magnetic structure of a disk, its composition, as well as the stellar mass, can significantly affect the nature of episodic accretion in young stellar objects.

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arXiv2020

Global Protoplanetary Disk Simulations: Dead Zone Formation and FUor Outbursts

We conducted global hydrodynamic simulations of protoplanetary disk evolution with an adaptive Shakura-Sunyaev α prescription to represent the layered disk structure, and starting with the collapse phase of the molecular cloud. With the canonical values of model parameters, self-consistent dead zones formed at the scale of a few au. The instabilities associated with the dead zone and corresponding outbursts, similar to FUor eruptions, were also observed in the simulations.

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arXiv2019

Dynamical Gaseous Rings in Global Simulations of Protoplanetary Disk Formation

Global numerical simulations of protoplanetary disk formation and evolution were conducted in thin-disk limit, where the model included magnetically layered disk structure, a self-consistent treatment for the infall from cloud core as well as the smallest possible inner computational boundary. We compared the evolution of a layered disk with a fully magnetically active disk. We also studied how the evolution depends on the parameters of the layered disk model - the MRI triggering temperature and active layer thickness - as well as the mass of the prestellar cloud core. With the canonical values of parameters a dead zone formed within the inner $\approx$ 15 au region of the magnetically layered disk. The dead zone was not a uniform structure and long-lived, axisymmetric, gaseous rings ubiquitously formed within this region due to the action of viscous torques. The rings showed a remarkable contrast in the disk environment as compared to a fully magnetically active disk and were characterized by high surface density and low effective viscosity. Multiple gaseous rings could form simultaneously in the dead zone region which were highly dynamical and showed complex, time-dependent behavior such as inward migration, vortices, gravitational instability and large-scale spiral waves. An increase in MRI triggering temperature had only marginal effects, while changes in active layer thickness as well as the initial cloud core mass had significant effects on the structure and evolution of the inner disk. Dust with large fragmentation barrier could be trapped in the rings, which may play a key role in planet formation.

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arXiv2019

Water delivery to the TRAPPIST-1 planets

Three of the seven rocky planets (e, f, and g) in TRAPPIST-1 system orbit in the habitable zone of the host star. Therefore, water can be in liquid state at their surface being essential for life. Recent studies suggest that these planets formed beyond the snow line in a water-rich region. The initial water reservoir can be lost during the planet formation due to the stellar activity of the infant low-mass star. However, a potential subsequent water delivery event, like the late heavy bombardment (LHB) in the Solar System, can replenish planetary water reservoirs. To study this water delivery process, we set up a simple model in which an additional 5 M_Earth--50 M_Earth planet is embedded in a water-rich asteroid belt beyond the snow line of TRAPPIST-1. Asteroids perturbed out from the chaotic zone of the putative planet can enter into the inner system and accreted by the known planets. Our main finding is that the larger is the orbital distance of planet, the higher is the amount of water delivered to the planet by an LHB-like event.

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arXiv2018

Dynamics of Haumea's dust ring

The particle dynamics of the recently observed ring around dwarf planet Haumea is numerically investigated. The point mass gravitational force, a second degree and order gravity field, and the solar radiation pressure as the main perturbations are considered. The quasi-stationary state of the ring varies for different micron-sized grains and depends also on the spin-orbit resonances between the rotation rate of the main body and the orbital period of the dust particles. The simulations confirm the variable radial width of the ring observed during the transit ingress and egress. Results show that the micron sized grains, initially on circular orbits, become eccentric and form an apse-aligned ring at the observed radial distance near to the 3:1 spin-orbit resonance. It is also demonstrated that this coincidence is only apparent and independent of 3:1 resonance.

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arXiv2014

Transient chaos and fractal structures in planetary feeding zones

The circular restricted three body problem is investigated in the context of accretion and scattering processes. In our model a large number of identical non-interacting mass-less planetesimals are considered in planar case orbiting a star-planet system. This description allows us to investigate in dynamical systems approach the gravitational scattering and possible captures of the particles by the forming planetary embryo. Although the problem serves a large variety of complex motion, the results can be easily interpreted because of the low dimensionality of the phase space. We show that initial conditions define isolated regions of the disk, where accretion or escape of the planetesimals occur, these have, in fact, a fractal structure. The fractal geometry of these "basins" implies that the dynamics is very complex. Based on the calculated escape rates and escape times, it is also demonstrated that the planetary accretion rate is exponential for short times and follows a power-law for longer integration. A new numerical calculation of the maximum mass that a planet can reach (described by the expression of the isolation mass) is also derived.

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arXiv2014

The Fly's Eye Camera System -- an instrument design for large étendue time-domain survey

In this paper we briefly summarize the design concepts of the Fly's Eye Camera System, a proposed high resolution all-sky monitoring device which intends to perform high cadence time domain astronomy in multiple optical passbands while still accomplish a high étendue. Fundings have already been accepted by the Hungarian Academy of Sciences in order to design and build a Fly's Eye device unit. Beyond the technical details and the actual scientific goals, this paper also discusses the possibilities and yields of a network operation involving $\sim10$ sites distributed geographically in a nearly homogeneous manner. Currently, we expect to finalize the mount assembly -- that performs the sidereal tracking during the exposures -- until the end of 2012 and to have a working prototype with a reduced number of individual cameras sometimes in the spring or summer of 2013.

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arXiv2013

The Fly's Eye Camera System -- an instrument design for large étendue time-domain survey

In this paper we briefly summarize the design concepts of the Fly's Eye Camera System, a proposed high resolution all-sky monitoring device which intends to perform high cadence time domain astronomy in multiple optical passbands while still accomplish a high étendue. Fundings have already been accepted by the Hungarian Academy of Sciences in order to design and build a Fly's Eye device unit. Beyond the technical details and the actual scientific goals, this paper also discusses the possibilities and yields of a network operation involving ~10 sites distributed geographically in a nearly homogeneous manner. Currently, we expect to finalize the mount assembly -- that performs the sidereal tracking during the exposures -- until the end of 2012 and to have a working prototype with a reduced number of individual cameras sometimes in the spring or summer of 2013.

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arXiv2011

Transit timing variations in the HAT-P-13 planetary system

In this Letter we present observations of recent HAT-P-13b transits. The combined analysis of published and newly obtained transit epochs shows evidence for significant transit timing variations since the last publicly available ephemerides. Variation of transit timings result in a sudden switch of transit times. The detected full range of TTV spans ~0.015 days, which is significantly more than the known TTV events exhibited by hot Jupiters. If we have detected a periodic process, its period should be at least ~3 years because there are no signs of variations in the previous observations. This argument makes unlikely that the measured TTV is due to perturbations by HAT-P-13c.

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