arXiv subjects

Bruno Sicardy

Publications and source records attributed to Bruno Sicardy.

At least 19 recordsLinked to original sources

arXiv2026

Size, shape, density, and atmospheric limit of (50000) Quaoar revealed from 14 years of stellar occultation

We present results from 28 stellar occultations by the large Trans-Neptunian Object (50000) Quaoar registered between 2018 and 2025. By performing a joint analysis of this occultation data-set, along with other 9 published events, we were able to fit an oblate ellipsoid shape, with equatorial semi-axes, a and b of 566.1+2.5-2.2 km, and a polar semi-axis, c, of 511.2+3.6-3.7 km. It provides an equivalent volumetric diameter of 1094.4 +/- 4.6 km and polar oblateness of 0.097 +/- 0.011. Considering an absolute magnitude of H = 2.79 +/- 0.35, we derive a geometric albedo of pV = 0.125 +/- 0.038. We have derived new upper limits to the surface pressure of a CH4 atmosphere of 0.15 nbar (1-sigma) and 0.65 nbar (3-sigma). We also provide a table with the 36 new astrometric positions for Quaoar. Using the new system mass derived from Weywot's orbit around Quaoar, we calculated a density of 1.760 +/- 0.109 g/cm3. Moreover, from the derived size and rotation period (8.8394 +/- 0.0002 hours (Ortiz et al. 2003)), we calculate that, if Quaoar is in Maclaurin hydrostatic equilibrium state, it would have a density of 1.859 +/- 0.200 g/cm3. This result, within the error bars, is compatible with the value we found. Therefore, this work shows that Quaoar can be a Maclaurin object, being eligible as a dwarf planet.

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arXiv2026

Central flashes during stellar occultations. Effects of diffraction, interferences, and stellar diameter

Central flashes occur during stellar occultations by solar system objects. We catalog diffraction effects on the flash with point-like stars, monochromatic waves and spherical transparent atmosphere. Diffraction involves the Huygens principle, the Sommerfeld lemma and the stationary phase method, while finite stellar diameter cases involve Clausius' theorem. For point-like stars, the central flash shape is that of the classical Poisson spot, but with larger height. For tenuous atmospheres that cannot focus the stellar rays at shadow center, the flash is amplified by the factor (R_0/r_0)^2 compared to the Poisson spot, where R0 and r0 are the object and the shadow radii, respectively. For denser atmospheres that can focus the rays at shadow center, the flash peaks at 2[(pi*R/lambda_F})^2]*phi0, where R is the central flash layer radius, lambda_F is the Fresnel scale and phi0 is the flux that would be observed at shadow center without focusing. For isothermal atmospheres with scale height H, the height is 2(R*H)(pi/lambda_F)^2. Fringes surrounding the central flash are separated by lambda_P=lambda_F^2/R, related to the separation between the primary and secondary stellar images. For a projected stellar diameter D*>lambda_P, the flash is described by complete elliptic integrals, and has full width at half maximum of 1.14D* and peak value 8H/D*. For Earth-based occultations by Pluto and Triton observed in the visible with point-like stars, diffraction causes flashes with very large heights ~10e4-10e5, spread over a very small meter-sized region in the shadow plane. In practice, the flash is usually smoothed by the stellar diameter, but still reaches high values of ~50 and ~200 during Pluto and Triton occultations, respectively. Diffraction dominates when using millimeter wavelengths or longer. Effects of departure from sphericity, atmospheric waves and stellar limb darkening are discussed.

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arXiv2026

Rings around irregular bodies. II. Numerical simulations of the 1/3 spin-orbit resonance confinement and applications to Chariklo

Rings have been found around Chariklo, Haumea and Quaoar, three small objects of the Solar System. All these rings are observed near the second-order spin-orbit resonances (SORs) 1/3 or 5/7 with the central body, suggesting an active confinement mechanism by these resonances. Our goal is to understand how collisional rings can be confined near second-order SORs in spite of the fact that they force self-intersecting streamlines.We use full 3D numerical simulations that treat rings of inelastically colliding particles orbiting non-axisymmetric central bodies, characterized by a dimensionless mass anomaly parameter mu. While most of our simulations ignore self-gravity, a few runs include gravitational interactions between particles, providing preliminary results on the effect of self-gravity on the ring confinement. The 1/3 SOR can confine ring material, by transferring the forced resonant mode into free Lindblad modes. We derive a criterion ensuring that the 1/3 SOR counteracts viscous spreading. Assuming meter-sized ring particles, and tau~1, this requires a threshold value mu > 1e-3 in Chariklo's case. The confinement is not permanent as a slow outward leakage of particles is observed in our simulations. This leakage can be halted by an outside moonlet with a mass of ~1e-7 - 1e-6 relative to Chariklo, corresponding to subkilometer-sized objects. With self-gravity, the ring viscosity nu increases by a factor of few in low-tau rings due to gravitational encounters. For large tau, self-gravity wakes enhance nu by a factor of ~100 compared to a non-gravitating ring, requiring ~10-fold larger mu since the threshold value increases proportional to square-root of nu.

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arXiv2025

Rings around irregular bodies I. Structure of the resonance mesh, applications to Chariklo, Haumea and Quaoar

Three ring systems have been discovered to date around small irregular objects of the solar system (Chariklo, Haumea and Quaoar). For the three bodies, material is observed near the second-order 1/3 Spin-Orbit Resonance (SOR) with the central object, and in the case of Quaoar, a ring is also observed near the second-order resonance 5/7 SOR. This suggests that second-order SORs may play a central role in ring confinement. This paper aims at better understanding this role from a theoretical point of view. It also provides a basis to better interpret the results obtained from N-body simulations and presented in a companion paper. A Hamiltonian approach yields the topological structure of phase portraits for SORs of orders from one to five. Two cases of non-axisymmetric potentials are examined: a triaxial ellipsoid characterized by an elongation parameter C22 and a body with mass anomaly mu, a dimensionless parameter that measures the dipole component of the body's gravitational field. The estimated triaxial shape of Chariklo shows that its corotation points are marginally unstable, those of Haumea are largely unstable, while those of Quaoar are safely stable. The topologies of the phase portraits show that only first- (aka Lindblad) and second-order SORs can significantly perturb a dissipative collisional ring. We calculate the widths, the maximum eccentricities and excitation time scales associated with first- and second-order SORs, as a function of C22 and mu. Applications to Chariklo, Haumea and Quaoar using mu ~ 0.001 show that the first- and second-order SORs caused by their triaxial shapes excite large (>~ 0.1) orbital eccentricities on the particles, making the regions inside the 1/2 SOR inhospitable for rings. Conversely, the 1/3 and 5/7 SORs caused by mass anomalies excite moderate eccentricities (<~ 0.01), and are thus a more favorable place for the presence of a ring.

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arXiv2025

JWST occultation reveals unforeseen complexity in Chariklo's ring system

Ring systems have been discovered around several small bodies in the outer Solar System through stellar occultations. While such measurements provide key information about ring geometry and dynamical interactions, little is known about their origins, lifetimes, evolutionary pathways, or compositions. Here we report near-infrared observations with the James Webb Space Telescope (JWST) of a stellar occultation by (10199) Chariklo, a Centaur known to host a double-ring system. Our JWST measurements show that Chariklo's inner dense ring has become significantly more opaque than in previous observations, pointing to ongoing replenishment processes or dynamical restructuring. In contrast, the outer ring exhibits a much weaker near-infrared occultation signature than seen in earlier visible-light detections. This discrepancy may reflect material loss, suggesting that the outer ring could be transient, or may arise from wavelength-dependent opacity. These scenarios, which are not mutually exclusive, point to an unprecedented level of complexity in small-body ring systems, distinct from those observed around any other minor bodies in the Solar System.

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arXiv2025

Size and Shape of Jupiter Trojan (2207) Antenor from Stellar Occultations

Librating around the Lagrange L5, the Jupiter's Trojan (2207) Antenor has been observed in recent years and its rotational light curve suggests it to be a very likely binary asteroid candidate. From stellar occultations, we report results from three events from Europe and North America to estimate the 2D apparent size and shape of Jupiter's Trojan (2207) Antenor. For the best-fitted ellipse in the sky-plane, we determined that Antenor has a 2D apparent equatorial radius of 54.30 +/- 0.99 km at the moment of the occultations, with an apparent oblateness of 0.144 +/- 0.051. We highlight the positive detection from 2021 June 12, which shows an intriguing feature that can be interpreted as a very large topographical feature (of about 11 km) of the body or that can provide further evidence that this object is, in fact, a close or contact binary. We also determine astrometric positions, with uncertainties of a few milliarcseconds (mas) for our preferred solutions.

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arXiv2025

Investigating the formation of small Solar System objects using stellar occultations by satellites: present, future and its use to update satellite orbits

The history of the outer solar system is intrinsically related to the Giant Planets migration. A massive disk of material within a radius of 30~au was scattered during the planetary migration, creating different dynamic populations in the Transneptunian region. They were formed in a collisional environment when massive collisions allowed them to grow and form much smaller moons than the primary body. The dynamical group, known as the Cold Classicals, was formed in a sparse disk from 42 to about 47~au and did not suffer much from planet migration. Observations show that many of Cold Classical are binary, consistent with the streaming instability process. The stellar occultation technique, with a spatial resolution of a few kilometres, can be used to search for binaries where other techniques are unable to do so, and to characterise the known satellites of Trans-Neptunian Objects (TNO), constraining their formation scenarios. We review here the first stellar occultations by TNO's satellites (besides Charon), discuss the methods used to detect these events. We also fit new orbital elements and system mass for Vanth (Orcus/1) and Weywot (Quaoar/1), finding reasonable solutions for pure Keplerian orbits. Finally, we discuss the prospects regarding the stellar occultations by TNO binaries and their implications for the study of the history of the Solar System.

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arXiv2024

Origins of rings in the solar system

Until about a decade ago, ring systems were only known to exist around giant planets. Each one of the four giant planets harbours its own distinctive and unique system of rings and inner satellites. The past decade has been marked by the unexpected discoveries of dense rings around small objects of the outer solar system: the Centaur object Chariklo (and possibly Chiron), the dwarf planet Haumea and the trans-Neptunian object Quaoar. In the latter case, an additional surprise came from the fact that Quaoar's rings orbit well beyond the Roche limit of the body. Here, we address the possible origins and evolution of these ring systems.

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arXiv2024

Stellar occultations by Trans-Neptunian Objects

Stellar occultations provide a powerful tool to explore objects of the outer solar system. The Gaia mission now provides milli-arcsec accuracy on the predictions of these events and makes possible observations that were previously unthinkable. Occultations return kilometric accuracies on the three-dimensional shape of bodies irrespective of their geocentric distances, with the potential of detecting topographic features along the limb. From the shape, accurate values of albedo can be derived, and if the mass is known, the bulk density is pinned down, thus constraining the internal structure and equilibrium state of the object. Occultations are also extremely sensitive to tenuous atmospheres, down to the nanobar level. They allowed the monitoring of Pluto's and Triton's atmospheres in the last three decades, constraining their seasonal evolution. They may unveil in the near future atmospheres around other remote bodies of the solar system. Since 2013, occultations have led to the surprising discovery of ring systems around the Centaur object Chariklo, the dwarf planet Haumea and the large trans-Neptunian object Quaoar, while revealing dense material around the Centaur Chiron. This suggests that rings are probably much more common features than previously thought. Meanwhile, they have raised new dynamical questions concerning the confining effect of resonances forced by irregular objects on ring particles. Serendipitous occultations by km-sized trans-Neptunian or Oort objects has the potential to provide the size distribution of a population that suffered few collisions until now, thus constraining the history of primordial planetesimals in the 1-100 km range.

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arXiv2024

Binary asteroid candidates in Gaia DR3 astrometry

Asteroids with companions constitute an excellent sample for studying the collisional and dynamical evolution of minor planets. The currently known binary population were discovered by different complementary techniques that produce, for the moment, a strongly biased distribution, especially in a range of intermediate asteroid sizes (approximately 20 to 100 km) where both mutual photometric events and high-resolution adaptive optic imaging are poorly efficient. A totally independent technique of binary asteroid discovery, based on astrometry, can help to reveal new binary systems and populate a range of sizes and separations that remain nearly unexplored. In this work, we describe a dedicated period detection method and its results for the Gaia DR3 data set. This method looks for the presence of a periodic signature in the orbit post-fit residuals. After conservative filtering and validation based on statistical and physical criteria, we are able to present a first sample of astrometric binary candidates, to be confirmed by other observation techniques such as photometric light curves and stellar occultations.

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arXiv2022

Study of atmospheres in the solar system, from stellar occultation or planetary transit

Stellar occultations and transits occur when a planetary body passes in front of a star (including our Sun). For objects with an atmosphere, refraction plays an essential role to explain the drops of flux and the aureoles observed during these events. This can be used to derived key parameters of the atmospheres, such as their density, pressure and temperature profiles, as well as the presence of atmospheric gravity waves and zonal winds. Here we derive from basic principles the equations that rule the ray propagation in planetary atmospheres, and we show how they can be used to derive the physical parameters of these atmospheres.

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arXiv2021

Pluto's atmosphere in plateau phase since 2015 from a stellar occultation at Devasthal

A stellar occultation by Pluto was observed on 6 June 2020 with the 1.3-m and 3.6-m telescopes located at Devasthal, Nainital, India, using imaging systems in the I and H bands, respectively. From this event, we derive a surface pressure for Pluto's atmosphere of $p_{\rm surf}= 12.23^{+0.65}_{-0.38} $~$μ$bar. This shows that Pluto's atmosphere is in a plateau phase since mid-2015, a result which is in excellent agreement with the Pluto volatile transport model of Meza et al. (2019}. This value does not support the pressure decrease reported by independent teams, based on occultations observed in 2018 and 2019, see Young et al. (2021} and Arimatu et al. (2020), respectively.

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arXiv2021

Understanding the trans-Neptunian Solar system: Reconciling the results of serendipitous stellar occultations and the inferences from the cratering record

The most pristine remnants of the Solar system's planet formation epoch orbit the Sun beyond Neptune, the small bodies of the trans-Neptunian object populations. The bulk of the mass is in ~100 km objects, but objects at smaller sizes have undergone minimal collisional processing, with New Horizons recently revealing that ~20 km effective diameter body (486958) Arrokoth appears to be a primordial body, not a collisional fragment. This indicates bodies at these sizes (and perhaps smaller) retain a record of how they were formed, and are the most numerous record of that epoch. However, such bodies are impractical to find by optical surveys due to their very low brightnesses. Their presence can be inferred from the observed cratering record of Pluto and Charon, and directly measured by serendipitous stellar occultations. These two methods produce conflicting results, with occultations measuring roughly ten times the number of ~km bodies inferred from the cratering record. We use numerical models to explore how these observations can be reconciled with evolutionary models of the outer Solar system. We find that models where the initial size of bodies decreases with increasing semimajor axis of formation, and models where the surface density of bodies increases beyond the 2:1 mean-motion resonance with Neptune can produce both sets of observations, though comparison to various observational tests favours the former mechanism. We discuss how to evaluate the astrophysical plausibility of these solutions, and conclude extended serendipitous occultation surveys with broad sky coverage are the most practical approach.

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arXiv2020

A Single-chord Stellar Occultation by the Extreme Trans-Neptunian Object (541132) Leleākūhonua

A stellar occultation by the extreme large-perihelion trans-Neptunian object (541132) Leleākūhonua (also known by the provisional designation of 2015 TG387) was predicted by the Lucky Star project and observed with the Research and Education Collaborative Occultation Network on 2018 October 20 UT. A single detection and a nearby nondetection provide constraints for the size and albedo. When a circular profile is assumed, the radius is $r={110}_{-10}^{+14}$ km, corresponding to a geometric albedo ${p}_{V}={0.21}_{-0.05}^{+0.03}$, for an adopted absolute magnitude of H V = 5.6, typical of other objects in dynamically similar orbits. The occultation also provides a high-precision astrometric constraint.

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arXiv2020

Resonances in non-axisymmetric gravitational potentials

We study sectoral resonances of the form $jκ= m(n-Ω)$ around a non-axisymmetric body with spin rate $Ω$, where $κ$ and $n$ are the epicyclic frequency and mean motion of a particle, respectively, where $j>0$ and $m$ ($<0$ or $>0$) are integers, $j$ being the resonance order. This describes $n/Ω\sim m/(m-j)$ resonances inside and outside the corotation radius,as well as prograde and retrograde resonances. Results are: (1) the kinematics of a periodic orbit depends only on $(m',j')$, the irreducible (relatively prime) version of $(m,j)$. In a rotating frame, the periodic orbit has $j'$ braids, $|m'|$ identical sectors and $|m'|(j'-1)$ self-crossing points; (2) thus, Lindblad resonances (with $j=1$) are free of self-crossing points; (3) resonances with same $j'$ and opposite $m'$ have the same kinematics, and are called $twins$; (4) the order of a resonance at a given $n/Ω$ depends on the symmetry of the potential. A potential that is invariant under a $2π/k$-rotation creates only resonances with $m$ multiple of $k$; (5) resonances with same $j$ and opposite $m$ have the same kinematics and same dynamics, and are called $true~twins$; (6) A retrograde resonance ($n/Ω< 0$) is always of higher order than its prograde counterpart ($n/Ω> 0$); (7) the resonance strengths can be calculated in a compact form with the classical operators used in the case of a perturbing satellite. Applications to Chariklo and Haumea are made.

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arXiv2019

The rings of Neptune

In 1984, three telescopes in South America recorded an occultation of a star near Neptune. It was attributed to the existence of a partial ring or ring arc. The existence of ring arcs around Neptune was confirmed during subsequent years via other occultation experiments and by the Voyager 2 spacecraft. The Voyager observations established that the Neptunian arcs are concentrations of particles embedded within Neptune's narrow Adams ring, the outermost of six tenuous rings discovered by Voyager and discussed here. Four ring arcs were identified: the trailing arc Fraternité, a double-component arc Egalité, dubbed Egalité 1 and 2, Liberté, and the leading arc Courage. The arcs varied in extent from $\sim$ 1$^\circ$ to $\sim$ 10$^\circ$, and together were confined to a longitude range of 40$^\circ$, with typical radial widths of $\sim$ 15 km and optical depth of order 0.1. The properties of the dusty component of Neptune's rings are also discussed in this chapter. Although the arcs should have been destroyed in a few months time through differential Keplerian motion, they appeared to persist at least throughout the Voyager era, and well beyond. However, observations from Earth (both with the Hubble Space Telescope and ground-based adaptive optics) show an evolution in the last three decades, with the disappearance of both Courage and Liberté by 2009. This chapter reviews the constraints provided on the mean motion of the arcs and Galatea, the satellite possibly responsible for the arc confinement. This in turn constrains the various models that have been proposed to explain the arc longevity.

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arXiv2019

The dynamics of rings around Centaurs and Trans-Neptunian Objects

Since 2013, dense and narrow rings are known around the small Centaur object Chariklo and the dwarf planet Haumea. Dense material has also been detected around the Centaur Chiron, although its nature is debated. This is the first time ever that rings are observed elsewhere than around the giant planets, suggesting that those features are more common than previously thought. The origins of those rings remain unclear. In particular, it is not known if the same generic process can explain the presence of material around Chariklo, Chiron, Haumea, or if each object has a very different history. Nonetheless, a specific aspect of small bodies is that they may possess a non-axisymmetric shape (topographic features and or elongation) that are essentially absent in giant planets. This creates strong resonances between the spin rate of the object and the mean motion of ring particles. In particular, Lindblad-type resonances tend to clear the region around the corotation (or synchronous) orbit, where the particles orbital period matches that of the body. Whatever the origin of the ring is, modest topographic features or elongations of Chariklo and Haumea explain why their rings should be found beyond the outermost 1/2 resonance, where the particles complete one revolution while the body completes two rotations. Comparison of the resonant locations relative to the Roche limit of the body shows that fast rotators are favored for being surrounded by rings. We discuss in more details the phase portraits of the 1/2 and 1/3 resonances, and the consequences of a ring presence on satellite formation.

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arXiv2019

The future of stellar occultations by distant solar system bodies: perspectives from the Gaia astrometry and the deep sky surveys

Distant objects in the solar system are crucial to better understand the history and evolution of its outskirts. The stellar occultation technique allows the determination of their sizes and shapes with kilometric accuracy, a detailed investigation of their immediate vicinities, as well as the detection of tenuous atmospheres. The prediction of such events is a key point in this study, and yet accurate enough predictions are available to a handful of objects only. In this work, we briefly discuss the dramatic impact that both the astrometry from the Gaia space mission and the deep sky surveys -- the Large Synoptic Survey Telescope in particular -- will have on the prediction of stellar occultations and how they may influence the future of the study of distant small solar system bodies through this technique.

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