arXiv subjects

Aurélien Crida

Publications and source records attributed to Aurélien Crida.

At least 19 recordsLinked to original sources

arXiv2026

Saturn's rings age I.: Reconsideration of the exposure age

At the end of the Cassini mission, Saturn's rings have been claimed to be spectacularly young compared to the age of the Solar System: their unusual ice-rich composition corresponds to initially pure ice rings polluted by interplanetary dust particles for 100 to 400 Myr. Since then, this exposure age has been commonly accepted as the real age of the rings. In this paper, we review the processes that are involved in determining the exposure age. We aim to see how the exposure age depends on various parameters and how relevant it is to define the real rings age. First, a new expression for the gravitational focusing onto planar rings, important parameter but crudely defined in the literature, is derived. Then, an analytical formula describing how the dust fraction varies with time in static or viscously evolving rings is provided, including possible vaporisation at impact. Finally, we introduce a cleaning process from space weathering to possibly alter dust and reduce its amount to make rings look younger than they are. We first found that the gravitational focusing is 5 times less important than previously thought, which automatically increases the exposure age from 0.5 to 2 Gyr. Moreover, depending on the impact properties (vaporisation rate, space weathering efficiency), several billion years can easily be reached. Finally, we find that the dust fraction in the rings converges towards a finite value which, in particular with an efficient space weathering mechanism, can be close to the observed one in the current rings. In this case, neither the age nor the initial composition of the rings can be derived, and the measure of the dust fraction and bombardment rate only constrains the physical parameters of the impacts and the efficiency of the space weathering. As long as the latter parameters are not known, the exposure age argument in favour of young rings is completely undercut.

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arXiv2025

The reflex instability: exponential growth of a large-scale $m=1$ mode in astrophysical discs

We report the finding of a linear, non-axisymmetric, global instability in gas discs around stars, which may be relevant to other astrophysical discs. It takes the form of an $m=1$ mode that grows in the disc density distribution while the star-barycentre distance rises exponentially with a characteristic timescale that is orders of magnitude longer than the orbital period. We present results of hydrodynamical simulations with various codes and numerical methods, using either barycentric or stellocentric reference frames, with or without the disc's self gravity: all simulations consistently show an unstable mode growing exponentially. The instability disappears if, and only if, the reflex motion of the star due to the disc's asymmetry is not taken into account in the simulations. For this reason we refer to this instability as the reflex instability. We identify a feedback loop as a possible origin, whereby the acceleration of the star excites the eccentricity of the disc, yielding an $m=1$ mode in the density distribution which, in turn, pulls the star. The growth timescale of the instability decreases with increasing disc mass and is a few hundred orbits for disc-to-star mass ratios of a few percent. If truly physical, and not due to a numerical artifact that would be common to all the codes we have employed, the reflex instability could have a dramatic impact on protoplanetary discs evolution and planetary formation.

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arXiv2025

On inertial forces (indirect terms) in problems with a central body

Gravitational systems in astrophysics often comprise a body -- the primary -- that far outweights the others, and which is taken as the centre of the reference frame. A fictitious acceleration, also known as the indirect term, must therefore be added to all other bodies in the system to compensate for the absence of motion of the primary. In this paper, we first stress that there is not one indirect term but as many indirect terms as there are bodies in the system that exert a gravitational pull on the primary. For instance, in the case of a protoplanetary disc with two planets, there are three indirect terms: one arising from the whole disc, and one per planet. We also highlight that the direct and indirect gravitational accelerations should be treated in a balanced way: the indirect term from one body should be applied to the other bodies in the system that feel its direct gravitational acceleration, and only to them. We point to situations where one of those terms is usually neglected however, which may lead to spurious results. These ideas are developed here for star-disc-planets interactions, for which we propose a recipe for the force to be applied onto a migrating planet, but they can easily be generalized to other astrophysical systems.

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arXiv2025

The Gaia spectroscopic catalogue of exoplanets and host stars

Complete, accurate, and precise catalogues of exoplanet host star (EHS) properties are essential to deriving high-quality exoplanet parameters. This paper aims at homogeneously parameterising EHS and their exoplanets, using Gaia and GSP-spec data. For 2573 EHS, we computed their L*, R*, and M*, with no prior assumption from stellar evolution models. Their Galactic positions, kinematic and orbital properties were derived. We re-scaled Mp and Rp of 3556 exoplanets, fully consistently with the stellar data. These new stellar Teff, L*, and R* are in rather good agreement with literature values but are more precise. In particular, R* are derived with typically less than 3% uncertainty; this reduces the uncertainty on Rp significantly and allows for a finer analysis of the decrease in the number of planets around the evaporation valley. Larger differences, however, were found for M*. We note that the EHS population is rather diverse in terms of the chemical and Galactic properties, although they are all found in the Solar vicinity, close to the Local spiral arm. Most EHS belong to the thin disc, but some older thick disc and halo members have also been identified. For the less massive planets, the average Rp increases with the metallicity of the host star. For giant planets, a dichotomy between dense and inflated planets is found. Denser planets tend to be more massive as the metallicity of the host star increases, while inflated planets are more massive for less metallic hosts. If confirmed, this bimodality implies that the diversity of giant exoplanets depends on their Galactic birth locus, with dense giant planets being more numerous than inflated ones when [M/H]>~1.5 times Solar, as in the central Milky Way regions. The Gaia spectroscopic catalogue of exoplanets and their host stars is large, homogeneous, and precise. It would be a useful added-value for planetary studies.

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arXiv2025

Understanding the formation of Saturn's regular moons in the context of giant planet moons formation scenarios

This article explores the different formation scenarios of the Kronian moons system in the context of a highly dissipative Saturn, with the objective of identifying the most likely of these scenarios. First, we review the diversity of objects - moons and rings - orbiting solar system giant planets, and the diversity of their architectures, which formation scenarios must reproduce. We then identify in this broader context the specific features of the Saturn system, such as the particularly large spectrum of its moon masses, the uniqueness of Titan and the presence of both dense and tenuous rings, before discussing the applicability of the different giant planet moon formation scenarios to the Saturn case. We discuss each of the most relevant scenarios and their respective merits. Finally, we tentatively propose a "favorite" scenario and we identify the key observations to be made by future space missions and/or Earth-based telescopic observations to validate this scenario or possibly alternative ones.

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arXiv2024

The Origin and Evolution of Saturn: A Post-Cassini Perspective

The Saturn System has been studied in detail by the Cassini-Huygens Mission. A major thrust of those investigations has been to understand how Saturn formed and evolved and to place Saturn in the context of other gas giants and planetary systems in general. Two models have been proposed for the formation of the giant planets,the core accretion model and the disk instability model. The heavy element enrichment, core size, and internal structure of Saturn, compared to Jupiter strongly favor the core accretion model as for Jupiter. Two features of the core accretion model that are distinct from the disk instability model are the growth of a core with a mass several times that of the Earth, followed by runaway collapse of gas onto the core once a mass threshold is reached. The heavy element core grows slowly over millions of years through accretion of cm-m sized pebbles, even larger bodies, and moon sized embryos in the gaseous disk. The abundance pattern of heavy elements is thus a key constraint on formation models. C, N, S, and P at Saturn are presently known to varying degree of uncertainty. The He to H ratio in the atmosphere is crucial for understanding heat balance, interior processes, and planetary evolution, but present values at Saturn range from low to high, allowing for a wide range of possibilities. While the very low values are favored to explain excess luminosity, high values might indicate presence of layered convection in the interior, resulting in slow cooling. Additional insight into Saturn's formation comes from the unique data on the rings from Cassini's Grand Finale orbits. While the solar system is the only analog for the extra solar systems, detection of the alkali metals and water in giant exoplanets is useful for understanding the formation and evolution of Saturn, where such data are presently lacking.

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arXiv2024

The Solar System could have formed in a low-viscosity disc: A dynamical study from giant planet migration to the Nice model

In the context of low-viscosity protoplanetary discs (PPDs), the formation scenarios of the Solar System should be revisited. In particular, the Jupiter-Saturn pair has been shown to lock in the 2:1 mean motion resonance while migrating generally inwards, making the Grand Tack scenario impossible. We explore what resonant chains of multiple giant planets can form in a low-viscosity disc, and whether these configurations can evolve into forming the Solar System in the post gas disc phase. We used hydrodynamical simulations to study the migration of the giant planets in a disc with viscosity $α=10^{-4}$. After a transition phase to a gas-less configuration, we studied the stability of the obtained resonant chains through their interactions with a disc of leftover planetesimals by performing N-body simulations using rebound. The gaps open by giant planets are wider and deeper for lower viscosity, reducing the damping effect of the disc and thus weakening resonant chains. Exploring numerous configurations, we found five stable resonant chains of four or five planets. In a thin PPD, the four giant planets revert their migration and migrate outwards. After disc dispersal, under the influence of a belt of planetesimals, some resonant chains undergo an instability phase while others migrate smoothly over a billion years. For three of our resonant chains, about 1% of the final configurations pass the four criteria to fit the Solar System. The most successful runs are obtained for systems formed in a cold PPD with a massive planetesimal disc. This work provides a fully consistent study of the dynamical history of the Solar System's giant planets, from the protoplanetary disc phase up to the giant planet instability. Although building resonant configurations is difficult in low-viscosity discs, we find it possible to reproduce the Solar System from a cold, low-viscosity protoplanetary disc.

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arXiv2023

Environmental transition: overview of actions to reduce the environmental footprint of astronomy

To keep current global warming below 1.5°C compared with the pre-industrial era, measures must be taken as quickly as possible in all spheres of society. Astronomy must also make its contribution. In this proceeding, and during the workshop to which it refers, different levers of actions are discussed through various examples: individual efforts, laboratory-level actions, impact evaluation and mitigation in major projects, institutional level, and involvement through collectives.

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arXiv2023

Presence of liquid water during the evolution of exomoons orbiting ejected free-floating planets

Free-floating planets (FFPs) can result from dynamical scattering processes happening in the first few million years of a planetary system's life. Several models predict the possibility, for these isolated planetary-mass objects, to retain exomoons after their ejection. The tidal heating mechanism and the presence of an atmosphere with a relatively high optical thickness may support the formation and maintenance of oceans of liquid water on the surface of these satellites. In order to study the timescales over which liquid water can be maintained, we perform dynamical simulations of the ejection process and infer the resulting statistics of the population of surviving exomoons around free-floating planets. The subsequent tidal evolution of the moons' orbital parameters is a pivotal step to determine when the orbits will circularize, with a consequential decay of the tidal heating. We find that close-in ($a \lesssim 25 $R$_{\rm J}$) Earth-mass moons with CO$_2$-dominated atmospheres could retain liquid water on their surfaces for long timescales, depending on the mass of the atmospheric envelope and the surface pressure assumed. Massive atmospheres are needed to trap the heat produced by tidal friction that makes these moons habitable. For Earth-like pressure conditions ($p_0$ = 1 bar), satellites could sustain liquid water on their surfaces up to 52 Myr. For higher surface pressures (10 and 100 bar), moons could be habitable up to 276 Myr and 1.6 Gyr, respectively. Close-in satellites experience habitable conditions for long timescales, and during the ejection of the FFP remain bound with the escaping planet, being less affected by the close encounter.

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arXiv2022

Tilting Uranus via the migration of an ancient satellite

Context. The 98°-obliquity of Uranus is commonly attributed to giant impacts that occurred at the end of the planetary formation. This picture, however, is not devoid of weaknesses. Aims. On a billion-year timescale, the tidal migration of the satellites of Jupiter and Saturn has been shown to strongly affect their spin-axis dynamics. We aim to revisit the scenario of tilting Uranus in light of this mechanism. Methods. We analyse the precession spectrum of Uranus and identify the candidate secular spin-orbit resonances that could be responsible for the tilting. We determine the properties of the hypothetical ancient satellite required for a capture and explore the dynamics numerically. Results. If it migrates over 10 Uranus' radii, a single satellite with minimum mass 4e-4 Uranus' mass is able to tilt Uranus from a small obliquity and make it converge towards 90°. In order to achieve the tilting in less than the age of the Solar System, the mean drift rate of the satellite must be comparable to the Moon's current orbital expansion. Under these conditions, simulations show that Uranus is readily tilted over 80°. Beyond this point, the satellite is strongly destabilised and triggers a phase of chaotic motion for the planet's spin axis. The chaotic phase ends when the satellite collides into the planet, ultimately freezing the planet's obliquity in either a prograde, or plainly retrograde state (as Uranus today). Spin states resembling that of Uranus can be obtained with probabilities as large as 80%, but a bigger satellite is favoured, with mass 1.7e-3 Uranus' mass or more. Yet, a smaller ancient satellite is not categorically ruled out, and there is room for improving this basic scenario in future studies. Interactions among several pre-existing satellites is a promising possibility.

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arXiv2020

Probing the impact of varied migration and gas accretion rates for the formation of giant planets in the pebble accretion scenario

The final orbital position of growing planets is determined by their migration speed, which is essentially set by the planetary mass. Small mass planets migrate in type I migration, while more massive planets migrate in type II migration, which is thought to depend mostly on the viscous evolution rate of the disc. A planet is most vulnerable to inward migration before it reaches type II migration and can lose a significant fraction of its semi-major axis at this stage. We investigated the influence of different disc viscosities, the dynamical torque and gas accretion from within the horseshoe region as mechanisms for slowing down planet migration. Our study confirms that planets growing in low viscosity environments migrate less, due to the earlier gap opening and slower type II migration rate. We find that taking the gas accretion from the horseshoe region into account allows an earlier gap opening and this results in less inward migration of growing planets. Furthermore, this effect increases the planetary mass compared to simulations that do not take the effect of gas accretion from the horseshoe region. Moreover, combining the effect of the dynamical torque with the effect of gas accretion from the horseshoe region, significantly slows down inward migration. Taking these effects into account could allow the formation of cold Jupiters (a > 1 au) closer to the water ice line region compared to previous simulations that did not take these effects into account. We thus conclude that gas accretion from within the horseshoe region and the dynamical torque play crucial roles in shaping planetary systems.

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arXiv2020

The heavy-elements heritage of the falling sky

Recent dynamical analysis based on Gaia data have revealed major accretion events in Milky Way's history. Nevertheless, our understanding of the primordial Galaxy is hindered because the bona fide identification of the most metal-poor and correspondently oldest accreted stars remains challenging. Contrary to alpha-elements, neutron-capture elements present unexplained large abundance spreads for low metallicity stars, that could result from a mixture of formation sites. We have analysed the abundances of yttrium, europium, magnesium and iron in Milky Way satellite galaxies, field halo stars and globular clusters. The chemical information has been complemented with orbital parameters based on Gaia data. In particular, the orbit's average inclination has been considered. The [Y/Eu] abundance behaviour with respect to the [Mg/Fe] turnovers for satellite galaxies of different masses reveals that higher luminosity systems, for which the [Mg/Fe] abundance declines at higher metallicities, present enhanced [Y/Eu] abundances, particularly in the [Fe/H] regime between -2.25 and -1.25 dex. In addition, the analysis has uncovered a chemo-dynamical correlation for both globular clusters and field stars of the Galactic halo, accounting for about half of the [Y/Eu] abundance spread. [Y/Eu] under-abundances typical of protracted chemical evolutions, are preferentially observed in polar-like orbits, pointing to a possible anisotropy in the accretion processes. Our results strongly suggest that the observed [Y/Eu] abundance spread in the Milky Way halo could result from a mixture of systems with different masses. They also highlight that both nature and nurture are relevant to the Milky Way formation, since its primordial epochs, opening new pathways for chemical diagnostics of our Galaxy building up.

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arXiv2019

From the stellar properties of HD219134 to the internal compositions of its transiting exoplanets

The harvest of exoplanet discoveries has opened the area of exoplanet characterisation. But this cannot be achieved without a careful analysis of the host star parameters. The system of HD219134 hosts two transiting exoplanets and at least two additional non-transiting exoplanets. We used the VEGA/CHARA interferometer to measure the angular diameter of HD219134, leading to a stellar radius of $R_{\star}=0.726\pm0.014 R_{\odot}$. We also derived the stellar density from the transits light curves ($ρ_{\star}=1.82\pm0.19 ρ_{\odot}$), which finally gives a direct estimate of the mass ($M_{\star}=0.696\pm0.078 M_{\odot}$) with a correlation of 0.46 between $R_{\star}$ and $M_{\star}$. This new mass is smaller than that derived from the C2kSMO stellar evolutionary model, which provides a mass range of 0.755$-$0.810 ($\pm 0.040$) $M_{\odot}$. This allows us to infer the mass, radius and density of the two transiting exoplanets of the system. We then use an inference model to obtain the internal parameters of these two transiting exoplanets. Moreover, we find that planet $b$ and $c$ have smaller radii than previously estimated ($1.500\pm0.057$ and $1.415\pm0.049 R_{\oplus}$, respectively); this clearly puts these planets out of the gap in the exoplanetary radii distribution and validates their super-Earth nature. Planet $b$ is more massive than planet $c$, but possibly less dense. We investigate whether this could be caused by partial melting of the mantle and find that tidal heating due to non-zero eccentricity of planet $b$ may be powerful enough. The system of HD219134 constitutes a very valuable benchmark for both stellar physics and exoplanetary science. The direct determination of the stellar density, radius and mass should be more extensively applied to provide accurate exoplanets properties and calibrate stellar models.

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arXiv2019

Quasi-static contraction during runaway gas accretion onto giant planets

Gas-giant planets, like Jupiter and Saturn, acquire massive gaseous envelopes during the approximately 3 Myr-long lifetimes of protoplanetary discs. In the core accretion scenario, the formation of a solid core of around 10 Earth masses triggers a phase of rapid gas accretion. Previous 3D grid-based hydrodynamical simulations found runaway gas accretion rates corresponding to approximately 10 to 100 Jupiter masses per Myr. Such high accretion rates would result in all planets with larger-than-10-Earth-mass cores forming Jupiter-like planets, in clear contrast to the ice giants in the Solar System and the observed exoplanet population. In this work, we use 3D hydrodynamical simulations, that include radiative transfer, to model the growth of the envelope on planets with different masses. We find that gas flows rapidly through the outer part of the envelope, but this flow does not drive accretion. Instead, gas accretion is the result of quasi-static contraction of the inner envelope, which can be orders of magnitude smaller than the mass flow through the outer atmosphere. For planets smaller than Saturn, we measure moderate gas accretion rates that are below 1 Jupiter mass per Myr. Higher mass planets, however, accrete up to 10 times faster and do not reveal a self-driven mechanism that can halt gas accretion. Therefore, the reason for the final masses of Saturn and Jupiter remains difficult to understand, unless their completion coincided with the dissipation of the Solar Nebula.

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arXiv2018

Mass, radius, and composition of the transiting planet 55 Cnc e : using interferometry and correlations

The characterization of exoplanets relies on that of their host star. However, stellar evolution models cannot always be used to derive the mass and radius of individual stars, because many stellar internal parameters are poorly constrained. Here, we use the probability density functions (PDFs) of directly measured parameters to derive the joint PDF of the stellar and planetary mass and radius. Because combining the density and radius of the star is our most reliable way of determining its mass, we find that the stellar (respectively planetary) mass and radius are strongly (respectively moderately) correlated. We then use a generalized Bayesian inference analysis to characterize the possible interiors of 55 Cnc e. We quantify how our ability to constrain the interior improves by accounting for correlation. The information content of the mass-radius correlation is also compared with refractory element abundance constraints. We provide posterior distributions for all interior parameters of interest. Given all available data, we find that the radius of the gaseous envelope is $0.08 \pm 0.05 R_p$. A stronger correlation between the planetary mass and radius (potentially provided by a better estimate of the transit depth) would significantly improve interior characterization and reduce drastically the uncertainty on the gas envelope properties.

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arXiv2018

Mass, radius, and composition of the transiting planet 55 Cnc e : using interferometry and correlations --- A quick update

In a recent paper (Crida et al. 2018, accepted on April 19), we presented a method to derive the mass and radius of a transiting exoplanet and their intrinsic correlation, that we applied to 55 Cnc e. We wrote: "More precise observations of the transit would be very useful in this particular case and would allow to increase significantly the gain on the planetary density precision." Three months later, Bourrier et al. (2018) published new observations of the system. Additionally, Gaia's DR2 was released on April 25, 2018. The purpose of this research note is solely to implement these precise new data in our pipeline to provide an up-to-date result of our model. We find $R_p = 1.947\pm 0.038\,R_\oplus$ and $M_p = 8.59\pm 0.43\,M_\oplus$. We also double the precision on the planetary density ($ρ_p = 1.164\pm 0.062\,ρ_\oplus$), which allows us to refine the estimates for all interior parameters of interest. In particular, we now find that the radius of the gaseous envelope is $0.03\pm 0.02\,R_p$.

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arXiv2018

Capture into first-order resonances and long-term stability of pairs of equal-mass planets

Massive planets form within the lifetime of protoplanetary disks and undergo orbital migration due to planet-disk interactions. When the first planet reaches the inner edge of the disk its migration stops and the second planet is locked in resonance. We detail the resonant trapping comparing semi-analytical formulae and numerical simulations in the case of two equal-mass coplanar planets trapped in first order resonances. We describe the family of resonant stable equilibrium points (zero-amplitude libration orbits) using expanded and non-expanded Hamiltonians. We show that during convergent migration the planets evolve along these families of equilibria. Eccentricity damping from the disk leads to a final equilibrium configuration that we predict analytically. The fact that observed multi-exoplanetary systems are rarely seen in resonances suggests that the resonant configurations achieved by migration can become unstable after the removal of the disk. We probe the stability of the resonances as a function of planetary mass. To do this, we fictitiously increase the masses of the planets, adiabatically maintaining the low-amplitude libration regime until instability. We discuss two hypotheses for instability: low-order secondary resonance of the libration frequency with a fast synodic frequency, and minimal approach distance between planets. We show that secondary resonances do not seem to impact resonant systems at low-amplitude of libration. Resonant systems are more stable than non-resonant ones for a given minimal distance at close encounters, but we show that the latter still play the decisive role in the destabilisation of resonant pairs. We show that, as the planetary mass increases and the minimal distance between planets gets smaller in terms of mutual Hill radius, the region of stability around the resonance center shrinks, until the equilibrium point itself becomes unstable.

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arXiv2018

Rings in the Solar System: a short review

Rings are ubiquitous around giant planets in our Solar System. They evolve jointly with the nearby satellite system. They could form either during the giant planet formation process or much later, as a result of large scale dynamical instabilities either in the local satellite system, or at the planetary scale. We review here the main characteristics of rings in our solar system, and discuss their main evolution processes and possible origin. We also discuss the recent discovery of rings around small bodies.

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