arXiv subjects

R. Sfair

Publications and source records attributed to R. Sfair.

At least 19 recordsLinked to original sources

arXiv2026

The dynamically hazardous asteroid 2025 TV$_{10}$. A new co-orbital asteroid of Venus

Twenty co-orbital asteroids of Venus are currently known, several of which may evolve into potentially hazardous asteroids (PHAs) over timescales of thousands of years. We report the identification and first dynamical characterization of 2025 TV$_{10}$, a newly discovered Venus co-orbital asteroid, and assess its potential collisional hazard to Earth. We performed numerical simulations of a large number of asteroid clones, and we studied their close encounters with Venus and Earth. The asteroid may leave its co-orbital orbit on timescales of 1200 yr. The orbit of 2025 TV$_{10}$ is one of the closest of the absolute minimum of the "minimum orbital intersection distance" (MOID) with Earth for the Venus co-orbital asteroids known to date. Owing to its orbital parameters, 2025 TV$_{10}$ represents one of the most dynamically hazardous members of the Venus co-orbital population identified to date. Its faint magnitude and restricted observability windows make future observations challenging but essential for constraining its orbit.

astro-ph.EP↗

arXiv2026

Constraining the size, shape, and albedo of the large Trans-Neptunian Object (28978) Ixion with multi-chord stellar occultations

Trans-Neptunian objects (TNOs) are among the most primitive remnants of the early Solar System, and constraining their sizes, shapes, albedos, and surface properties is essential for understanding their origin and evolution. Stellar occultations provide highly accurate size and shape measurements for TNOs, while photometry constrains their albedo and surface colours. (28978) Ixion is one of the largest TNOs and a prominent Plutino, making it a key target for comparative studies. We aim to constrain Ixion's projected size, shape, absolute magnitude, geometric albedo, and surface colours, and to search for evidence of an atmosphere or circum-object material. We analysed stellar occultation campaigns by Ixion conducted between 2020 and 2023 within the Lucky Star collaboration, comprising 51 observations from eight events, including 30 positive detections. Five multi-chord events enabled a global limb fit and an accurate reconstruction of Ixion's projected shape. The occultations reveal a slightly elongated limb well represented by a single projected ellipse, yielding an area-equivalent diameter of $D_{\mathrm{equiv}} = 696.78^{+10.75}_{-8.87}$ km and an apparent oblateness $\epsilon' = 0.081^{+0.004}_{-0.010}$. Typical radial residuals of order $\sim$10 km indicate a largely stable shape across epochs, with modest epoch-dependent variations. Calibrated photometric data yield an absolute magnitude of $H_V = 3.845 \pm 0.006$, a phase-curve slope of $\beta = 0.1301 \pm 0.0078$ mag deg$^{-1}$, and a visible geometric albedo of $p_V = 0.106^{+0.003}_{-0.003}$, with colours consistent with moderately red TNO surfaces. No atmosphere or circum-object material is detected down to our sensitivity limits. The best-sampled event also allows a precise measurement of the angular diameter of the occulted star Gaia DR3 4056440205544338944, $\theta_\star = 0.670 \pm 0.010$ mas.

astro-ph.EP↗

arXiv2025

The rings of (2060) Chiron: Evidence of an evolving system

The centaur (2060) Chiron has long been a candidate for hosting material in orbit, based on occultation and photometric and spectroscopic data. Here, we present a multichord stellar occultation observed on 2023 September 10 UT that reveals new and complex structures surrounding Chiron. High-cadence light curves show multiple secondary events that are best explained (when compared with a multishell interpretation) with a system of three confined rings located at average radii of 273, 325, and 438 km, the outermost of which lies beyond Chiron's Roche limit. The rings appear coplanar, with a mean pole orientation of {\lambda} = 151{\deg} +/- 4{\deg} and \b{eta} = 20{\deg} +/- 6{\deg}. A broader, disklike structure extends from about 200 to 800 km, and a newly detected faint feature is observed at ~1380 km. Chiron thus appears as the fourth small solar system body known for hosting a ring system. Comparisons with previous occultation events that have occurred since 1994 show that these features are not permanent. With these observations, we may witness for the first time the ongoing formation and evolution of a ring system.

astro-ph.EP↗

arXiv2025

Particle dynamics in TOI-178 planetary system

The TOI-178 system hosts six planets with five of them locked in a :4:6:9:12 Laplace resonance chain. We perform N-body simulations to investigate the dynamics of test particles in this system. We observe that co-orbital regions around each planet are approximately 30\% wider than predicted by classical theory for planets in the resonance chain, while TOI-178b, which lies outside the chain, shows a 52\% enhancement. The region between TOI-178e and TOI-178f reveals Kirkwood gap-like structures created by mean-motion resonances with TOI-178f (4:3, 5:4, 6:5) and TOI-178g (5:3), where particle clearing occurs on 500-year timescales. An extended integration of the innermost region (0.015-0.025 au) shows periodic inclination oscillations with period 196 years, coincident with TOI-178b's own oscillation period, with maximum amplitude occurring near the 3:2 resonance location. These structures are consistent with the system's resonant architecture and provide a baseline characterization that enables future comparative studies of similar phenomena in other multi-planet systems with resonant configurations.

astro-ph.EP↗

arXiv2025

The invisible threat: assessing the collisional hazard posed by the undiscovered Venus co-orbital asteroids

Currently, 20 co-orbital asteroids of Venus are known, with only one with an eccentricity below 0.38. This is most likely caused by observational biases since asteroids with larger eccentricities may approach the Earth and are easier to detect. We aim to assess the possible threat that the yet undetected population of Venus co-orbitals may pose to Earth, and investigate their detectability from Earth and space observatories. We used semi-analytical models of the 1:1 mean-motion resonance with Venus and numerical simulations to monitor close encounters with Earth on several co-orbital cycles. We analyzed observability windows and brightness variations for potential Venus co-orbitals as viewed from ground-based telescopes to assess their future detection feasibility with next-generation survey capabilities. There is a range of orbits with e < 0.38, larger at lower inclinations, for which Venus' co-orbitals can pose a collisional hazard to Earth. Current ground-based observations are constrained by periodic observing windows and solar elongation limitations, though the Rubin Observatory may detect some of these objects during favorable configurations. Space missions based on Venus' orbits may be instrumental in detecting Venus' co-orbitals at low eccentricities.

astro-ph.EP↗

arXiv2025

The Moon as a possible source for Earth's co-orbital bodies

There is a growing number of Earth's co-orbital bodies being discovered. At least five of them are known to be temporarily in quasi-satellite orbits. One of those, 469219 Kamo'oalewa, was identified as possibly having the same composition as the Moon. We explore the conditions necessary for lunar ejecta to evolve into Earth's co-orbital bodies, with particular attention to the formation of quasi-satellite orbits. We systematically investigate the parameter space of ejection velocity and geographic launch location across the entire lunar surface. The study employs numerical simulations of the four-body problem (Sun-Earth-Moon-particle) with automated classification methodology for identifying all co-orbital states. Particles are ejected from randomly distributed points covering the entire lunar surface with velocities ranging from 1.0 to 2.6 times the Moon's escape velocity. Trajectories co-orbital to Earth are found to be a common outcome, with approximately 6.15% of all simulated particles evolving into Earth co-orbital motion and 1.92% specifically exhibiting quasi-satellite behavior. We identify an optimal ejection velocity (1.2 times escape velocity) for quasi-satellite production, yielding over 6% conversion efficiency at this specific velocity. The spatial distribution of successful ejections shows a strong preference for the equatorial regions of the trailing hemisphere. Collisions with Earth or the Moon occur for only 4% of the sample. Our results strengthen the plausibility of lunar origin for Earth's co-orbital bodies, including quasi-satellites like Kamo'oalewa and 2024 PT5. We identify both "prompt" and "delayed" co-orbital formation mechanisms, with a steady-state production regime that could explain the presence of lunar-derived objects in Earth's co-orbital regions despite the infrequent occurrence of major lunar impacts capable of launching meter-scale fragments.

astro-ph.EP↗

arXiv2025

Dust production through collisions between small bodies: an application to the G-ring arc

The G-ring arc of Saturn, confined by the 7:6 corotation eccentric resonance with Mimas, is primarily composed of micrometric particles. These particles, significantly influenced by the solar radiation pressure, are subject to rapid depletion. This study investigates a mechanism for dust replenishment in the arc, specifically analyzing collisions between macroscopic bodies and the satellite Aegaeon. Utilizing N-body and Smoothed particle hydrodynamics (SPH) simulations, we assess the dust generation from these impacts, with a focus on the most likely collision parameters derived from the N-body simulations. Our findings indicate that, while collisions among macroscopic bodies are inefficient for dust production, impacts involving Aegaeon are substantially more effective, providing conservative lower limits for dust generation. This mechanism, in conjunction with the natural decay processes and continuous dust generation from impacts, potentially keeps the arc population over thousands of years with a possible variation in brightness.

astro-ph.EP↗

arXiv2025

Stellar occultation observations of (38628) Huya and its satellite: a detailed look into the system

The physical and orbital parameters of Trans-Neptunian Objects (TNOs) provide valuable information about the Solar System's formation and evolution. In particular, the characterization of binaries provides insights into the formation mechanisms that may be playing a role at such large distances from the Sun. Studies show two distinct populations, and (38628) Huya occupies an intermediate position between the unequal-size binaries and those with components of roughly equal sizes. In this work, we predicted and observed three stellar occultation events by Huya. Huya and its satellite - S/2012 (38628) 1 - were detected during occultations in March 2021 and again in June 2023. Additionally, an attempt to detect Huya in February 2023 resulted in an additional single-chord detection of the secondary. A spherical body with a minimum diameter of D = 165 km can explain the three single-chord observations and provide a lower limit for the satellite size. The astrometry of Huya's system, as derived from the occultations and supplemented by observations from the Hubble Space Telescope and Keck Observatory, provided constraints on the satellite orbit and the mass of the system. Therefore, assuming the secondary is in an equatorial orbit around the primary, the limb fitting was constrained by the satellite orbit position angle. The system density, calculated by summing the most precise measurement of Huya's volume to the spherical satellite average volume, is $\rho_{1}$ = 1073 $\pm$ 66 kg m$^{-3}$. The density that the object would have assuming a Maclaurin equilibrium shape with a rotational period of 6.725 $\pm$ 0.01 hours is $\rho_{2}$ = 768 $\pm$ 42 kg m$^{-3}$. This difference rules out the Maclaurin equilibrium assumption for the main body shape.

astro-ph.EP↗

arXiv2023

Dynamics around the binary system (65803) Didymos

Didymos and Dimorphos are primary and secondary, respectively, asteroids who compose a binary system that make up the set of Near Earth Asteroids (NEAs). They are targets of the Double Asteroid Redirection Test (DART), the first test mission dedicated to study of planetary defense, for which the main goal is to measure the changes caused after the secondary body is hit by a kinect impactor. The present work intends to conduct a study, through numerical integrations, on the dynamics of massless particles distributed in the vicinity of the two bodies. An approximate shape for the primary body was considered as a model of mass concentrations (mascons) and the secondary was considered as a massive point. Our results show the location and size of stable regions, and also their lifetime.

astro-ph.EP↗

arXiv2022

Milliarcsecond astrometry for the Galilean moons using stellar occultations

A stellar occultation occurs when a Solar System object passes in front of a star for an observer. This technique allows the determination of sizes and shapes of the occulting body with kilometer precision. Also, this technique constrains the occulting body's positions, albedos, densities, etc. In the context of the Galilean moons, these events can provide their best ground-based astrometry, with uncertainties in the order of 1 mas ($\sim$ 3 km at Jupiter's distance during opposition). We organized campaigns and successfully observed a stellar occultation by Io (JI) in 2021, one by Ganymede (JIII) in 2020, and one by Europa (JII) in 2019, with stations in North and South America. Also, we re-analyzed two previously published events, one by Europa in 2016 and another by Ganymede in 2017. Then, we fit the known 3D shape of the occulting satellite and determine its center of figure. That resulted in astrometric positions with uncertainties in the milliarcsecond level. The positions obtained from these stellar occultations can be used together with dynamical models to ensure highly accurate orbits of the Galilean moons. These orbits can help plan future space probes aiming at the Jovian system, such as JUICE by ESA and Europa Clipper by NASA, and allow more efficient planning of flyby maneuvers.

astro-ph.EP↗

arXiv2022

The main perturbing objects on the orbits of (616) Prometheus and (617) Pandora

The dynamical evolution of the Prometheus and Pandora pair of satellites is chaotic, with a short 3.3 years Lyapunov time. It is known that the anti-alignment of the apses line of Prometheus and Pandora, which occurs every 6.2 years, is a critical configuration that amplifies their chaotic dynamical evolution. However, the mutual interaction between Prometheus and Pandora is not enough to explain the longitudinal lags observed by the Hubble Space Telescope. The main goal of the current work is to identify the main contributors to the chaotic dynamical evolution of the Prometheus-Pandora pair beyond themselves. Therefore, in this work, we first explore the sensibility of this dynamical system to understand it numerically and then build numerical experiments to reach our goals. We identified that almost all major satellites of the Saturn system play a significant role in the evolution of Prometheus' and Pandora's orbits.

astro-ph.EP↗

arXiv2021

Refined physical parameters for Chariklo's body and rings from stellar occultations observed between 2013 and 2020

The Centaur (10199) Chariklo has the first rings system discovered around a small object. It was first observed using stellar occultation in 2013. Stellar occultations allow the determination of sizes and shapes with kilometre accuracy and obtain characteristics of the occulting object and its vicinity. Using stellar occultations observed between 2017 and 2020, we aim at constraining Chariklo's and its rings physical parameters. We also determine the rings' structure, and obtain precise astrometrical positions of Chariklo. We predicted and organised several observational campaigns of stellar occultations by Chariklo. Occultation light curves were measured from the data sets, from which ingress and egress times, and rings' width and opacity were obtained. These measurements, combined with results from previous works, allow us to obtain significant constraints on Chariklo's shape and rings' structure. We characterise Chariklo's ring system (C1R and C2R), and obtain radii and pole orientations that are consistent with, but more accurate than, results from previous occultations. We confirmed the detection of W-shaped structures within C1R and an evident variation of radial width. The observed width ranges between 4.8 and 9.1 km with a mean value of 6.5 km. One dual observation (visible and red) does not reveal any differences in the C1R opacity profiles, indicating ring particle's size larger than a few microns. The C1R ring eccentricity is found to be smaller than 0.022 (3-sigma), and its width variations may indicate an eccentricity higher than 0.005. We fit a tri-axial shape to Chariklo's detections over eleven occultations and determine that Chariklo is consistent with an ellipsoid with semi-axes of 143.8, 135.2 and 99.1 km. Ultimately, we provided seven astrometric positions at a milliarcseconds accuracy level, based on Gaia EDR3, and use it to improve Chariklo's ephemeris.

astro-ph.EP↗

arXiv2020

Stability and Evolution of Fallen Particles Around the Surface of Asteroid (101955) Bennu

In this work, we study the dynamics of particles around Bennu. The goal is to understand the stability, evolution, and final outcome of the simulated particles around the asteroid. According to the results, the particle sizes can be divided into two main groups depending on their behavior. Particles smaller than a centimeter are quickly removed from the system by solar radiation pressure, while the dynamics of particles larger than a few centimeters is dominated by the gravitational field of Bennu. Because of its shape and spin period, Bennu has eight equilibrium points around it. The structure of the phase space near its equatorial surface is directly connected to these equilibrium points. Therefore, we performed numerical simulations to obtain information about the orbital evolution near the equilibrium points. The results show that most of the particles larger than a few centimeters fall in the equatorial region close to the Kingfisher area or close to the region diametrically opposite to it. In contrast, almost none of these particles fall in the equatorial region close to the Osprey area. In addition, we also performed computational experiments considering a spherical cloud of particles initially orbiting Bennu. Most of the particles in prograde orbits fall on the surface within our integration period, which was limited to 1.14 years. The particles preferentially fall near high-altitude regions at low equatorial latitudes and close to the north pole. The mid-latitudes are those more depleted of falls, as in the Nightingale and Sandpiper areas.

astro-ph.EP↗

arXiv2020

Stellar occultations enable milliarcsecond astrometry for Trans-Neptunian objects and Centaurs

Trans-Neptunian objects (TNOs) and Centaurs are remnants of our planetary system formation, and their physical properties have invaluable information for evolutionary theories. Stellar occultation is a ground-based method for studying these small bodies and has presented exciting results. These observations can provide precise profiles of the involved body, allowing an accurate determination of its size and shape. The goal is to show that even single-chord detections of TNOs allow us to measure their milliarcsecond astrometric positions in the reference frame of the Gaia second data release (DR2). Accurated ephemerides can then be generated, allowing predictions of stellar occultations with much higher reliability. We analyzed data from stellar occultations to obtain astrometric positions of the involved bodies. The events published before the Gaia era were updated so that the Gaia DR2 catalog is the reference. Previously determined sizes were used to calculate the position of the object center and its corresponding error with respect to the detected chord and the International Celestial Reference System (ICRS) propagated Gaia DR2 star position. We derive 37 precise astrometric positions for 19 TNOs and 4 Centaurs. Twenty-one of these events are presented here for the first time. Although about 68\% of our results are based on single-chord detection, most have intrinsic precision at the submilliarcsecond level. Lower limits on the diameter and shape constraints for a few bodies are also presented as valuable byproducts. Using the Gaia DR2 catalog, we show that even a single detection of a stellar occultation allows improving the object ephemeris significantly, which in turn enables predicting a future stellar occultation with high accuracy. Observational campaigns can be efficiently organized with this help, and may provide a full physical characterization of the involved object.

astro-ph.EP↗

arXiv2020

Asteroid triple system 2001 SN263 : surfaces characteristics and dynamical environment

The (153591) 2001 SN263 asteroid system, target of the first Brazilian interplanetary space mission, is one of the known three triple systems within the population of NEAs. One of the mission objectives is to collect data about the formation of this system. The analysis of these data will help in the investigation of the physical and dynamical structures of the components (Alpha, Beta and Gamma) of this system, in order to find vestiges related to its origin. In this work, we assume the irregular shape of the 2001 SN263 system components as uniform density polyhedra and computationally investigate the gravitational field generated by these bodies. The goal is to explore the dynamical characteristics of the surface and environment around each component. Then, taking into account the rotational speed, we analyze their topographic features through the quantities geometric altitude, tilt, geopotential, slope, surface accelerations, among others. Additionally, the investigation of the environment around the bodies made it possible to construct zero-velocity curves, which delimit the location of equilibrium points. The Alpha component has a peculiar number of 12 equilibrium points, all of them located very close to its surface. In the cases of Beta and Gamma, we found four equilibrium points not so close to their surfaces. Then, performing numerical experiments around their equilibrium points, we identified the location and size of just one stable region, which is associated with an equilibrium point around Beta. Finally, we integrated a spherical cloud of particles around Alpha and identified the location on the surface of Alpha were the particles have fallen.

astro-ph.EP↗

arXiv2020

Dynamical Environment and Surface Characteristics of Asteroid (16) Psyche

Radar observations show that (16) Psyche is one of the largest and most massive asteroids of the M-class located in the main belt, with a diameter of approximately 230 km. This fact makes Psyche a unique object since observations indicated an iron-nickel composition. It is believed that this body may be what was left of a metal core of an early planet that would have been fragmented over millions of years due to violent collisions. In this work we study a variety of dynamical aspects related to the surface, as well as, the environment around this asteroid. We use computational tools to explore the gravitational field generated by this body, assuming constant values for its density and rotation period. We then determine a set of physical and dynamical characteristics over its entire surface. The results include the geometric altitude, geopotential altitude, tilt, slope, among others. We also explore the neighborhood around the asteroid (16) Psyche, so that the location and linear stability of the equilibrium points were found. We found four external equilibrium points, two of them linearly stable. We confirmed the stability of these points by performing numerical simulations of massless particles around the asteroid, which also showed an asymmetry in the size of the stable regions. In addition, we integrate a cloud of particles in the vicinity of (16) Psyche in order to verify in which regions of its surface the particles are most likely to collide.

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arXiv2019

Cosmology from an exponential dependence on the trace of the energy-momentum tensor -- Numerical approach and cosmological tests

In this paper, we present the cosmological scenario obtained from $f(R,T)$ gravity by using an exponential dependence on the trace of the energy-momentum tensor. With a numerical approach applied to the equations of motion, we show several precise fits and the respective cosmological consequences. As a matter of completeness, we also analyzed cosmological scenarios where this new version of $f(R,T)$ is coupled with a real scalar field. In order to find analytical cosmological parameters, we used a slow-roll approximation for the evolution of the scalar field. This approximation allowed us to derived the Hubble and the deceleration parameters whose time evolutions describe the actual phase of accelerated expansion, and corroborate with our numerical investigations. Therefore, the analytical parameters unveil the viability of this proposal for $f(R,T)$ in the presence of an inflaton field.

gr-qc↗

arXiv2019

The 2014-2015 Brazilian Mutual Phenomena campaign for the Jovian satellites and improved results for the 2009 events

Progress in astrometry and orbital modelling of planetary moons in the last decade enabled better determinations of their orbits. These studies need accurate positions spread over extended periods. We present the results of the 2014-2015 Brazilian campaign for 40 mutual events from 47 observed light curves by the Galilean satellites plus one eclipse of Amalthea by Ganymede. We also reanalysed and updated results for 25 mutual events observed in the 2009 campaign. All telescopes were equipped with narrow-band filters centred at 889 nm with a width of 15 nm to eliminate the scattered light from Jupiter. The albedos' ratio was determined using images before and after each event. We simulated images of moons, umbra, and penumbra in the sky plane, and integrated their fluxes to compute albedos, simulate light curves and fit them to the observed ones using a chi-square fitting procedure. For that, we used the complete version of the Oren-Nayer reflectance model. The relative satellite positions mean uncertainty was 11.2 mas ($\sim$35 km) and 10.1 mas ($\sim$31 km) for the 2014-2015 and 2009 campaigns respectively. The simulated and observed \textsc{ascii} light curve files are freely available in electronic form at the \textit{Natural Satellites DataBase} (NSDB). The 40/25 mutual events from our 2014-2015/2009 campaigns represent a significant contribution of 17%/15% in comparison with the PHEMU campaigns lead by the IMCCE. Besides that, our result for the eclipse of Amalthea is only the 4$^{th}$ such measurement ever published after the three ones observed by the 2014-2015 international PHEMU campaign. Our results are suitable for new orbital/ephemeris determinations for the Galilean moons and Amalthea.

astro-ph.EP↗