arXiv subjects

Jean-François Gonzalez

Publications and source records attributed to Jean-François Gonzalez.

At least 37 records · Page 2Linked to original sources

arXiv2020

Spirals, shadows & precession in HD 100453 -- I. The orbit of the binary

In recent years, several protoplanetary discs have been observed to exhibit spirals, both in scattered light and (sub)millimetre continuum data. The HD 100453 binary star system hosts such a disc around its primary. Previous work has argued that the spirals were caused by the gravitational interaction of the secondary, which was assumed to be on a circular orbit, coplanar with the disc (meaning here the large outer disc, as opposed to the very small inner disc). However, recent observations of the CO gas emission were found incompatible with this assumption. In this paper, we run SPH simulations of the gas and dust disc for seven orbital configurations taken from astrometric fits and compute synthetic observations from their results. Comparing to high-resolution ALMA $^{12}$CO data, we find that the best agreement is obtained for an orbit with eccentricity $e=0.32$ and semi-major axis $a=207$ au, inclined by $61^\circ$ relative to the disc plane. The large misalignment between the disc and orbit planes is compatible with the tidal evolution of a circumprimary disc in an eccentric, unequal-mass binary star.

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arXiv2020

On the cavity size in circumbinary discs

How does the cavity size in circumbinary discs depend on disc and binary properties? We investigate by simulating disc cavities carved by binary companions using smoothed particle hydrodynamics (SPH). We find that a cavity is quickly opened on the dynamical time, while the cavity size is set on the viscous time. In agreement with previous findings, we find long term cavity sizes of 2-5 times the binary semi-major axis, increasing with eccentricity and decreasing with disc aspect ratio. When considering binaries inclined with respect to the disc we find three regimes: i) discs that evolve towards a coplanar orbit have a large cavity, slightly smaller than that of an initially coplanar disc; ii) discs that evolve towards a polar orbit by breaking have a small cavity, equal in size to that of an initially polar disc; iii) discs that evolve towards a polar orbit via warping have an intermediate-sized cavity. We find typical gas depletions inside the cavity of $\gtrsim 2$ orders of magnitude in surface density.

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arXiv2020

Evolution of porous dust grains in protoplanetary discs -- I. Growing grains

One of the main problems in planet formation, hampering the growth of small dust to planetesimals, is the so-called radial-drift barrier. Pebbles of cm to dm sizes are thought to drift radially across protoplanetary discs faster than they can grow to larger sizes, and thus to be lost to the star. To overcome this barrier, drift has to be slowed down or stopped, or growth needs to be sped up. In this paper, we investigate the role of porosity on both drift and growth. We have developed a model for porosity evolution during grain growth and applied it to numerical simulations of protoplanetary discs. We find that growth is faster for porous grains, enabling them to transition to the Stokes drag regime, decouple from the gas, and survive the radial-drift barrier. Direct formation of small planetesimals from porous dust is possible over large areas of the disc.

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arXiv2019

Size and density sorting of dust grains in SPH simulations of protoplanetary disc II: Fragmentation

Grain growth and fragmentation are important processes in building up large dust aggregates in protoplanetary discs. Using a 3D two-phase (gas-dust) SPH code, we investigate the combined effects of growth and fragmentation of a multi-phase dust with different fragmentation thresholds in a time-evolving disc. We find that our fiducial disc, initially in a fragmentation regime, moves toward a pure-growth regime in a few thousands years. Timescales change as a function of the disc and dust properties. When fragmentation is efficient, it produces, in different zones of the disc, Fe/Si and rock/ice ratios different from those predicted when only pure growth is considered. Chemical fractionation and the depletion/enrichment in iron observed in some chondrites can be linked to the size-density sorting and fragmentation properties of precursor dusty grains . We suggest that aggregation of chondritic components could have occurred where/when fragmentation was not efficient if their aerodynamical sorting has to be preserved. Chondritic components would allow aerodynamical sorting in a fragmentation regime only if they have similar fragmentation properties. We find that, in the inner disc, and for the same interval of time, fragmenting dust can grow larger when compared to the size of grains predicted by pure growth. This counter-intuitive behaviour is due to the large amount of dust which piles up in a fragmenting zone followed by the rapid growth that occurs when this zone transitions to a pure growth regime. As an important consequence, dust can overcome the radial-drift barrier within a few thousands years.

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arXiv2017

Self-induced dust traps: overcoming planet formation barriers

Planet formation is thought to occur in discs around young stars by the aggregation of small dust grains into much larger objects. The growth from grains to pebbles and from planetesimals to planets is now fairly well understood. The intermediate stage has however been found to be hindered by the radial-drift and fragmentation barriers. We identify a powerful mechanism in which dust overcomes both barriers. Its key ingredients are i) backreaction from the dust onto the gas, ii) grain growth and fragmentation, and iii) large-scale gradients. The pile-up of growing and fragmenting grains modifies the gas structure on large scales and triggers the formation of pressure maxima, in which particles are trapped. We show that these self-induced dust traps are robust: they develop for a wide range of disc structures, fragmentation thresholds and initial dust-to-gas ratios. They are favored locations for pebbles to grow into planetesimals, thus opening new paths towards the formation of planets.

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arXiv2017

Size and density sorting of dust grains in SPH simulations of protoplanetary discs

The size and density of dust grains determine their response to gas drag in protoplanetary discs. Aerodynamical (size x density) sorting is one of the proposed mechanisms to explain the grain properties and chemical fractionation of chondrites. However, the efficiency of aerodynamical sorting and the location in the disc in which it could occur are still unknown. Although the effects of grain sizes and growth in discs have been widely studied, a simultaneous analysis including dust composition is missing. In this work we present the dynamical evolution and growth of multicomponent dust in a protoplanetary disc using a 3D, two-fluid (gas+dust) Smoothed Particle Hydrodynamics (SPH) code. We find that the dust vertical settling is characterised by two phases: a density-driven phase which leads to a vertical chemical sorting of dust and a size-driven phase which enhances the amount of lighter material in the midplane. We also see an efficient radial chemical sorting of the dust at large scales. We find that dust particles are aerodynamically sorted in the inner disc. The disc becomes sub-solar in its Fe/Si ratio on the surface since the early stage of evolution but sub-solar Fe/Si can be also found in the outer disc-midplane at late stages. Aggregates in the disc mimic the physical and chemical properties of chondrites, suggesting that aerodynamical sorting played an important role in determining their final structure.

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arXiv2016

Grand challenges in protoplanetary disc modelling

The Protoplanetary Discussions conference --- held in Edinburgh, UK, from 7th --11th March 2016 --- included several open sessions led by participants. This paper reports on the discussions collectively concerned with the multiphysics modelling of protoplanetary discs, including the self-consistent calculation of gas and dust dynamics, radiative transfer and chemistry. After a short introduction to each of these disciplines in isolation, we identify a series of burning questions and grand challenges associated with their continuing development and integration. We then discuss potential pathways towards solving these challenges, grouped by strategical, technical and collaborative developments. This paper is not intended to be a review, but rather to motivate and direct future research and collaboration across typically distinct fields based on \textit{community driven input}, to encourage further progress in our understanding of circumstellar and protoplanetary discs.

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arXiv2016

Effects of photophoresis on the dust distribution in a 3D protoplanetary disc

Photophoresis is a physical process based on momentum exchange between an illuminated dust particle and its gaseous environment. Its net effect in protoplanetary discs (PPD) is the outward transport of solid bodies from hot to cold regions. This process naturally leads to the formation of ring-shaped features where dust piles up. In this work, we study the dynamical effects of photophoresis in PPD by including the photophoretic force in the two-fluid (gas+dust) smoothed particle hydrodynamics (SPH) code developed by Barrière-Fouchet et al. (2005). We find that the conditions of pressure and temperature encountered in the inner regions of PPD result in important photophoretic forces, which dramatically affect the radial motion of solid bodies. Moreover, dust particles have different equilibrium locations in the disc depending on their size and their intrinsic density. The radial transport towards the outer parts of the disc is more efficient for silicates than for iron particles, which has important implications for meteoritic composition. Our results indicate that photophoresis must be taken into account in the inner regions of PPD to fully understand the dynamics and the evolution of the dust composition.

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arXiv2015

ALMA images of discs: are all gaps carved by planets?

Protoplanetary discs are now routinely observed and exoplanets, after the numerous indirect discoveries, are starting to be directly imaged. To better understand the planet formation process, the next step is the detection of forming planets or of signposts of young planets still in their disc, such as gaps. A spectacular example is the ALMA science verification image of HL Tau showing numerous gaps and rings in its disc. To study the observability of planet gaps, we ran 3D hydrodynamical simulations of a gas and dust disc containing a 5 M J gap-opening planet and characterised the spatial distribution of migrating, growing and fragmenting dust grains. We then computed the corresponding synthetic images for ALMA. For a value of the dust fragmentation threshold of 15 m s --1 for the collisional velocity, we identify for the first time a self-induced dust pile up in simulations taking fragmentation into account. This feature, in addition to the easily detected planet gap, causes a second apparent gap that could be mistaken for the signature of a second planet. It is therefore essential to be cautious in the interpretation of gap detections.

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arXiv2014

Photophoresis in protoplanetary disks: a numerical approach

It is widely accepted that rocky planets form in the inner regions of protoplanetary disks (PPD) about 1 - 10 AU from the star. However, theoretical calculations show that when particles reach the size for which the radial migration is the fastest they tend to be accreted very efficiently by the star. This is known as the radial-drift barrier. We explore the photophoresis in the inner regions of PPD as a possible mechanism for preventing the accretion of solid bodies onto the star. Photophoresis is the thermal creep induced by the momentum exchange of an illuminated solid particle with the surrounding gas. Recent laboratory experiments predict that photophoresis would be able to stop the inward drift of macroscopic bodies (from 1 mm to 1 m in size). This extra force has been included in our two-fluid (gas+dust) SPH code in order to study its efficiency. We show that the conditions of pressure and temperature encountered in the inner regions of PPD result in strong dynamical effects on the dust particles due to photophoresis. Our simulations show that there is a radial and a vertical sorting of the dust grains according to their sizes and their intrinsic densities. Thus, our calculations support the fact that photophoresis is a mechanism which can have a strong effect on the morphology of the inner regions of PPD, ultimately affecting the fate of planetesimals.

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arXiv2013

Growing dust grains in protoplanetary discs - I. Radial drift with toy growth models

In a series of papers, we present a comprehensive analytic study of the global motion of growing dust grains in protoplanetary discs, addressing both the radial drift and the vertical settling of the particles. Here we study how the radial drift of dust particles is affected by grain growth. In a first step, toy models in which grain growth can either be constant, accelerate or decelerate are introduced. The equations of motion are analytically integrable and therefore the grains dynamics is easy to understand. The radial motion of growing grains is governed by the relative efficiency of the growth and migration processes which is expressed by the dimensionless parameter Lambda, as well as the exponents for the gas surface density and temperature profiles, denoted p and q respectively. When Lambda is of order unity, growth and migration are strongly coupled, providing the most efficient radial drift. For the toy models considered, grains pile up when -p+q+1/2<0. Importantly, we show the existence of a second process which can help discs to retain their solid materials. For accelerating growth, grains end up their migration at a finite radius, thus avoiding being accreted onto the central star.

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arXiv2013

Growing dust grains in protoplanetary discs - III. Vertical settling

We aim to derive a simple analytic model to understand the essential properties of vertically settling growing dust grains in laminar protoplanetary discs. Separating the vertical dynamics from the motion in the disc midplane, we integrate the equations of motion for both a linear and an exponential grain growth rate. Numerical integrations are performed for more complex growth models. We find that the settling efficiency depends on the value of the dimensionless parameter gamma, which characterises the relative efficiency of grain growth with respect to the gas drag. Since gamma is expected to be of order as the initial dust-to-gas ratio in the disc (of order 10^-2), grain growth enhances the energy dissipation of the dust particles and improve the settling efficiency in protoplanetary discs. This behaviour is mostly independent of the growth model considered as well as of the radial drift of the particles.

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arXiv2013

Planet gaps in the dust layer of 3D proto-planetary disks: Observability with ALMA

Among the numerous known extrasolar planets, only a handful have been imaged directly so far, at large orbital radii and in rather evolved systems. The Atacama Large Millimeter/submillimeter Array (ALMA) will have the capacity to observe these wide planetary systems at a younger age, thus bringing a better understanding of the planet formation process. Here we explore the ability of ALMA to detect the gaps carved by planets on wide orbits.

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arXiv2013

Characterisation of global flow and local fluctuations in 3D SPH simulations of protoplanetary discs

A complete and detailed knowledge of the structure of the gaseous component in protoplanetary discs is essential to the study of dust evolution during the early phases of pre-planetesimal formation. The aim of this paper is to determine if three-dimensional accretion discs simulated by the Smoothed Particle Hydrodynamics (SPH) method can reproduce the observational data now available and the expected turbulent nature of protoplanetary discs. The investigation is carried out by setting up a suite of diagnostic tools specifically designed to characterise both the global flow and the fluctuations of the gaseous disc. The main result concerns the role of the artificial viscosity implementation in the SPH method: in addition to the already known ability of SPH artificial viscosity to mimic a physical-like viscosity under specific conditions, we show how the same artificial viscosity prescription behaves like an implicit turbulence model. In fact, we identify a threshold for the parameters in the standard artificial viscosity above which SPH disc models present a cascade in the power spectrum of velocity fluctuations, turbulent diffusion and a mass accretion rate of the same order of magnitude as measured in observations. Furthermore, the turbulence properties observed locally in SPH disc models are accompanied by meridional circulation in the global flow of the gas, proving that the two mechanisms can coexist.

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arXiv2011

Revisiting the "radial-drift barrier" of planet formation and its relevance in observed protoplanetary discs

Context. To form metre-sized pre-planetesimals in protoplanetary discs, growing grains have to decouple from the gas before they are accreted onto the central star during their phase of fast radial migration and thus overcome the so-called "radial-drift barrier" (often inaccurately referred to as the "metre-size barrier"). Aims. To predict the outcome of the radial motion of dust grains in protoplanetary discs whose surface density and temperature follow power-law profiles, with exponent p and q respectively. We investigate both the Epstein and the Stokes drag regimes which govern the motion of the dust. Methods. We analytically integrate the equations of motion obtained from perturbation analysis. We compare these results with those from direct numerical integration of the equations of motion. Then, using data from observed discs, we predict the fate of dust grains in real discs. Results. When a dust grain reaches the inner regions of the disc, the acceleration due to the increase of the pressure gradient is counterbalanced by the increase of the gas drag. We find that most grains in the Epstein (resp. the Stokes) regime survive their radial migration if-p+q+1/2 \leq0 (resp. if q\leq 2/3). The majority of observed discs satisfies both-p+q+ 1/2 \leq0 and q\leq 2/3: a large fraction of both their small and large grains remain in the disc, for them the radial drift barrier does not exist.

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arXiv2010

Planet gaps in the dust layer of 3D protoplanetary disks: I. Hydrodynamical simulations of T Tauri disks

Context: While sub-micron- and micron-sized dust grains are generally well mixed with the gas phase in protoplanetary disks, larger grains will be partially decoupled and as a consequence have a different distribution from that of the gas. This has ramifications for predictions of the observability of protoplanetary disks, for which gas-only studies will provide an inaccurate picture. Specifically, criteria for gap opening in the presence of a planet have generally been studied for the gas phase, whereas the situation can be quite different in the dust layer once grains reach mm sizes, which is what will be observed by ALMA. Aims: We aim to investigate the formation and structure of a planetary gap in the dust layer of a protoplanetary disk with an embedded planet. Methods: We perform 3D, gas+dust SPH simulations of a protoplanetary disk with a planet on a fixed circular orbit at 40 AU to study the evolution of both the gas and dust distributions and densities in the disk. We run a series of simulations in which the planet mass and the dust grain size varies. Results: We show that the gap in the dust layer is more striking than in the gas phase and that it is deeper and wider for more massive planets as well as for larger grains. For a massive enough planet, we note that cm-sized grains remain inside the gap in corotation and that their population in the outer disk shows an asymmetric structure, a signature of disk-planet interactions even for a circular planetary orbit, which should be observable with ALMA.

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arXiv2008

SPH simulations of grain growth in protoplanetary disks

Aims: In order to understand the first stages of planet formation, when tiny grains aggregate to form planetesimals, one needs to simultaneously model grain growth, vertical settling and radial migration of dust in protoplanetary disks. In this study, we implement an analytical prescription for grain growth into a 3D two-phase hydrodynamics code to understand its effects on the dust distribution in disks. Methods: Following the analytic derivation of Stepinski & Valageas (1997), which assumes that grains stick perfectly upon collision, we implement a convenient and fast method of following grain growth in our 3D, two-phase (gas+dust) SPH code. We then follow the evolution of the size and spatial distribution of a dust population in a classical T Tauri star disk. Results: We find that the grains go through various stages of growth due to the complex interplay between gas drag, dust dynamics, and growth. Grains initially grow rapidly as they settle to the mid-plane, then experience a fast radial migration with little growth through the bulk of the disk, and finally pile-up in the inner disk where they grow more efficiently. This results in a bimodal distribution of grain sizes. Using this simple prescription of grain growth, we find that grains reach decimetric sizes in 10^5 years in the inner disk and survive the fast migration phase.

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arXiv2008

Dust evolution in protoplanetary disks

We investigate the behaviour of dust in protoplanetary disks under the action of gas drag using our 3D, two-fluid (gas+dust) SPH code. We present the evolution of the dust spatial distribution in global simulations of planetless disks as well as of disks containing an already formed planet. The resulting dust structures vary strongly with particle size and planetary gaps are much sharper than in the gas phase, making them easier to detect with ALMA than anticipated. We also find that there is a range of masses where a planet can open a gap in the dust layer whereas it doesn't in the gas disk. Our dust distributions are fed to the radiative transfer code MCFOST to compute synthetic images, in order to derive constraints on the settling and growth of dust grains in observed disks.

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