arXiv subjects

R. O. Chametla

Publications and source records attributed to R. O. Chametla.

15 recordsLinked to original sources

arXiv2026

Turbulence destroys thermal lobes around Mars-sized planetary embryos

The release of heat by a planetary embryo modifies the local density perturbations, forming thermal lobes in its vicinity, and thereby altering the torque exerted by the disk on the embryo. In laminar disks, these thermal torques can dominate the disk-embryo interaction, rendering the classical Lindblad and corotation torques largely subdominant. The aim of this work is to investigate how turbulence driven by the MRI instability affects the thermal lobes formed around a planetary embryo, and to analyze the resulting torque acting on the embryo. We evaluate the thermal torques exerted on a planetary embryo of mass $M_p=0.33M_{Mars}$ and on a planetary core with mass $M_{p}=1M_{\oplus}$, each embedded in a turbulent gaseous protoplanetary disk, by means of high-resolution 3D magnetohydrodynamics simulations that include thermal diffusion and an initially toroidal magnetic field. The magnetic field strength is characterized by the $β$-plasma parameter with $β\in\{50,1000\}$. We consider two values for the luminosity of the planetary embryo: $L=0$ (cold thermal lobes) and $L=L_c$ (hot thermal lobes), where $L_c$ represents the critical luminosity. We find that, even in the presence of a weak magnetic field and irrespective of the luminosity, for both planetary masses, the development of turbulence in the disk (which takes between 1.5 to 3 orbital periods) completely disrupts the thermal lobes. As a result, the torque acting on both the planetary embryo and the Earth-mass core displays a strongly oscillatory behavior. This suggests that planets with masses in the range $0.03M_{\oplus}\lesssim M_{p}\lesssim 1M_{\oplus}$ experience stochastic migration, as expected in turbulent disks. Thermal torques become inefficient in turbulent regions of protoplanetary disks, such as outside the dead zone, in both the inner and outer disk regions where the magnetorotational instability operates.

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arXiv2025

Multipolar magnetic configuration: a trace of post-mergers events in circumstellar disks around FS CMa Stars

Context: Observations suggest that magnetic fields of disk-bearing stars may have non-dipolar configurations. However, the influence of these configurations on magnetospheric accretion remains poorly understood. Aims: We aim to simulate magnetospheric accretion incorporating non-dipolar and strong magnetic field. Our model is informed by observations of IRAS 17449+2320, a post-merger belonging to the group of FS CMa stars, which indicate a dominant dipolar magnetic field with an additional quadrupole component. Methods: Using the PLUTO code, we conduct 2.5-D non-ideal viscous-resistive magnetohydrodynamical (MHD) simulations of star-disk magnetospheric interactions. We consider a thin accretion disk and strong stellar magnetic field ($B_\star= 6.2\mathrm{kG}$) under four configurations: pure dipole, pure quadrupole, dipole plus quadrupole, and dipole plus octupole. In the latter two cases, different magnetic polar strength ratios are explored. Results: For asymmetric magnetic field configurations, we find that accretion exhibits funnel streams below the midplane, indicating the dominance of the quadrupolar and octupolar components. In contrast, in dipolar configurations, we observe the formation of two symmetrical funnels with respect to the midplane. However, in the quadrupolar configuration, accretion is entirely confined to the disk midplane forming a cone-like pattern that leads to disk widening. Remarkably, the presence of a quadrupolar component gives rise to highly asymmetric substructures in the corona region. Conclusions: Multipolar stellar magnetic fields drive non-uniform accretion and lead to asymmetric density distributions in both the disk and corona. These results resemble observed features of some FS CMa post-mergers and Herbig Ae/Be stars, highlighting the critical role of magnetic field complexity in shaping circumstellar environments.

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arXiv2025

Resurgence of CO in a warm bubble around accreting protoplanets and its observability

The cold outer regions of protoplanetary disks are expected to contain a midplane-centered layer where gas-phase CO molecules freeze out and their overall abundance is low. The layer then manifests itself as a void in the channel maps of CO rotational emission lines. We explore whether the frozen-out layer can expose the circumplanetary environment of embedded accreting protoplanets to observations. To this end, we performed 3D radiative gas-dust hydrodynamic simulations with opacities determined by the redistribution of submicron- and millimeter-sized dust grains. A Jupiter-mass planet with an accretion luminosity of $\sim$$10^{-3}\,L_{\odot}$ was considered as the nominal case. The accretion heating sustains a warm bubble around the planet, which locally increases the abundance of gas-phase CO molecules. Radiative transfer predictions of the emergent sky images show that the bubble becomes a conspicuous CO emission source in channel maps. It appears as a low-intensity optically thick spot located in between the so-called dragonfly wings that trace the fore- and backside line-forming surfaces. The emission intensity of the bubble is nearly independent of the tracing isotopolog, suggesting a very rich observable chemistry, as long as its signal can be deblended from the extended disk emission. This can be achieved with isotopologs that are optically thin or weakly thermally stratified across the planet-induced gap, such as C$^{18}$O. For these, the bubble stands out as the brightest residual in synthetic ALMA observations after subtraction of axially averaged channel maps inferred from the disk kinematics, enabling new automatic detections of forming protoplanets. By contrast, the horseshoe flow steadily depletes large dust grains from the circumplanetary environment, which becomes unobservable in the submillimeter continuum, in accordance with the scarcity of ALMA detections.

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arXiv2025

Dust-void evolution driven by turbulent dust flux can induce runaway migration of Earth-mass planets

Torques from asymmetric dust structures (so-called dust-void and filamentary structures) formed around low-mass planets embedded in a non turbulent dust-gas disk can exceed the torques produced by the gas disk component, then governing the planet's orbital dynamics. Here, we investigate how these structures (hence the dust torque) change when the effect of turbulent dust diffusion and dust feedback are included, and the direct implications on the migration of Earth-like planets. Using the \textsc{Fargo3D} code, we perform 2D and 3D multifluid hydrodynamic simulations, focusing on a non-migrating planet with the mass $M_p=1.5\,M_\oplus$ in 2D and on migrating planets with $M_p\in[1.5,12]\,M_\oplus$ in 3D. We vary the $δ$-dimensionless diffusivity parameter in the range $[0,3\times10^{-3}]$ and consider three different Stokes numbers $\mathrm{St}=\{0.04,0.26,0.55\}$, which are representative of the gas, transitional and gravity-dominated regimes, respectively. In our 2D models, we find that turbulent diffusion of dust prevents the formation of the dust-void and filamentary structures when $δ>3\times10^{-4}$. Otherwise, dust structures survive turbulent diffusion flow. However, dust and total torques become positive only in transitional and gravity-dominated regimes. In our 3D models, we find that the dust-void is drastically modified and the high-density ring-shaped barrier delineating the dust-void disappears if $δ\gtrsim10^{-4} $, due to the effect of dust turbulent diffusion along with the back-reaction of the dust. For all values of $δ$, the filament in front of the planet is replaced by a low-density trench. Remarkably, as we allow the planets to migrate, the evolving dust-void can drive either runaway migration or outward (inward) oscillatory-torque migration. Our study thus suggests that low-mass Earth-like planets can undergo runaway migration in dusty disks.

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arXiv2024

Pebble-driven migration of low-mass planets in the 2D regime of pebble accretion

Pebbles drifting past a disk-embedded low-mass planet develop asymmetries in their distribution and exert a substantial gravitational torque on the planet, thus modifying its migration rate. Our aim is to assess how the distribution of pebbles and the resulting torque change in the presence of pebble accretion, focusing on its 2D regime. First, we performed 2D high-resolution multi-fluid simulations with Fargo3D but found that they are impractical for resolving pebble accretion due to the smoothing of the planetary gravitational potential. To remove the smoothing and directly trace pebbles accreted by the planet, we developed a new code, Deneb, which evolves an ensemble of pebbles, represented by Lagrangian superparticles, in a steady-state gaseous background. For small and moderate Stokes numbers, St $\lesssim 0.1$, pebble accretion creates two underdense regions with a front-rear asymmetry with respect to the planet. The underdensity trailing the planet is more extended. The resulting excess of pebble mass in front of the planet then makes the pebble torque positive and capable of outperforming the negative gas torque. Pebble accretion thus enables outward migration (previously thought to occur mainly for St $\gtrsim 0.1$) in a larger portion of the parameter space. It occurs for the planet mass $M_{pl}\lesssim3\,M_{\oplus}$ and for all the Stokes numbers considered in our study, St $\in$ [$10^{-2}, 0.785$], assuming a pebble-to-gas mass ratio of $Z = 0.01$. If some of the observed planets underwent outward pebble-driven migration during their accretion, the formation sites of their progenitor embryos could have differed greatly from the usual predictions of planet formation models. To enable an update of the respective models, we provide a scaling law for the pebble torque that can be readily incorporated in N-body simulations.

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arXiv2024

2.5-MHD models of circumstellar discs around FS~CMa post-mergers : II. Stationary accretion stage

We study the star-disc interaction in the presence of the strong magnetic field ($B_\star=6.2kG$) of a slowly rotating star. This situation describes a post-merger of the spectral type B and has not been previously investigated. We perform a set of resistive and viscosity $2.5D$-magnetohydrodynamical simulations using the PLUTO code. Based on our previous work, we consider the initial gas disc density $ρ_{d0}=10^{-13}\mathrm{gcm}^{-3}$ since it describes the conditions around IRAS 17449+2320 well. We find that the fall of gas towards the star occurs in the mid-plane, and remarkably, intermittent backflow takes place in the mid-plane in all of our models for $R\geq10R_\star$. However, we do not rule out that the funnel effect may occur and cause the accretion closer to the poles. Also, when larger values of viscosity ($α_ν=1$) and stellar rotation rate ($δ_\star=0.2$) are considered, we find that the disc exhibits a thickening which is characteristic of FS~CMa-type stellar objects. Additionally, we find that the poloidal magnetic field lines twist over short periods of time, leading to magnetic reconnection causing coronal heating that could explain the presence of the Raman lines found observationally in several FS~CMa stars. Lastly, we find the formation of several knots in the magnetic field lines near and in the mid-plane of the disc which produce perturbations in the density and velocity components, as well as the formation of shallow gaps whose position depends on the inflation of the magnetic field lines.

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arXiv2023

Accreting luminous low-mass planets escape from migration traps at pressure bumps

We investigate the migration of Mars- to super-Earth-sized planets in the vicinity of a pressure bump in a 3D radiative protoplanetary disc while accounting for the effect of accretion heat release. Pressure bumps have often been assumed to act as efficient migration traps, but we show that the situation changes when the thermal forces are taken into account. Our simulations reveal that for planetary masses $\lesssim$$2\,M_{\oplus}$, once their luminosity exceeds the critical value predicted by linear theory, thermal driving causes their orbits to become eccentric, quenching the positive corotation torque responsible for the migration trap. As a result, planets continue migrating inwards past the pressure bump. Additionally, we find that planets that remain circular and evolve in the super-Keplerian region of the bump exhibit a reversed asymmetry of their thermal lobes, with the heating torque having an opposite (negative) sign compared to the standard circular case, thus leading to inward migration as well. We also demonstrate that the super-critical luminosities of planets in question can be reached through the accretion of pebbles accumulating in the bump. Our findings have implications for planet formation scenarios that rely on the existence of migration traps at pressure bumps, as the bumps may repeatedly spawn inward-migrating low-mass embryos rather than harbouring newborn planets until they become massive.

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arXiv2023

On wave interference in planet migration: dead zone torques modified by active zone forcing

We investigate planetary migration in the dead zone of a protoplanetary disk where there are a set of spiral waves propagating inward due to the turbulence in the active zone and the Rossby wave instability (RWI), which occurs at the transition between the dead and active zones. We perform global 3D unstratified magnetohydrodynamical (MHD) simulations of a gaseous disk with the FARGO3D code, using weak gradients in the static resistivity profiles that trigger the formation of a vortex at the outer edge of the dead zone. We find that once the Rossby vortex develops, spiral waves in the dead zone emerge and interact with embedded migrating planets by wave interference, which notably changes their migration. The inward migration becomes faster depending on the mass of the planet, due mostly to the constructive (destructive) interference between the outer (inner) spiral arm of the planet and, the destruction of the dynamics of the horseshoe region by means of the set of background spiral waves propagating inward. The constructive wave interference produces a more negative Lindblad differential torque which inevitably leads to an inward migration. Lastly, for massive planets embedded in the dead zone, we find that the spiral waves can create an asymmetric wider and depeer gap than in the case of $α$-disks, and can prevent the formation of vortices at the outer edge of the gap. The latter could generate a faster or slower migration compared to the standard type-II migration.

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arXiv2023

2.5-MHD models of circumstellar discs around FS~CMa post-mergers : I. Non-stationary accretion stage

We investigate the dynamic evolution of gaseous region around FS~CMa post-mergers. Due to the slow rotation of a central B-type star, the dynamics is driven mainly by the magnetic field of the central star. Recent observations have allowed us to set a realistic initial conditions such as, the magnetic field value ($B_\star\approx6\times10^{3}G$), the mass of the central star ($M_\star=6M_\odot$), and the initial disc density $ρ_{d0}\in[10^{-13}\mathrm{g\,cm^{-3}},10^{-11}\mathrm{g \, cm^{-3}}] $. We use the PLUTO code to perform 2.5D-MHD simulations of thin and thick discs models. Especially relevant for the interpretation of the observed properties of FS~CMa post-mergers are the results for low-density discs, in which we find formation of a jet emerging from inner edge of the disc, as well as the formation of the so called "hot plasmoid" in the corona region. Jets are probably detected as discrete absorption components in the resonance lines of FS~CMa stars. Moreover, the magnetic field configuration in the low-density plasma region, favors the appearance of magnetocentrifugal winds from the disc. The currents toward the star created by the magnetic field may explain accidentally observed material infall. The disc structure is significantly changed due to the presence of the magnetic field. The magnetic field is also responsible for the formation of a hot corona as observed in several FS~CMa stars through the Raman lines. Our results are valid for all magnetic stars surrounded by a low density plasma, i.e., some of stars showing the B[e] phenomenon.

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arXiv2023

Evolution of the eccentricity and inclination of low-mass planets subjected to thermal forces: a numerical study

By means of three dimensional, high resolution hydrodynamical simulations we study the orbital evolution of weakly eccentric or inclined low-mass protoplanets embedded in gaseous discs subject to thermal diffusion. We consider both non-luminous planets, and planets that also experience the radiative feedback from their own luminosity. We compare our results to previous analytical work, and find that thermal forces (the contribution to the disc's force arising from thermal effects) match those predicted by linear theory within $\sim 20$%. When the planet's luminosity exceeds a threshold found to be within $10$% of that predicted by linear theory, its eccentricity and inclination grow exponentially, whereas these quantities undergo a strong damping below this threshold. In this regime of low luminosity indeed, thermal diffusion cools the surroundings of the planet and allows gas to accumulate in its vicinity. It is the dynamics of this gas excess that contributes to damp eccentricity and inclination. The damping rates obtained can be up to $h^{-1}$ times larger than those due to the resonant interaction with the disc, where $h$ is the disc's aspect ratio. This suggests that models that incorporate planet-disc interactions using well-known formulae based on resonant wave-launching to describe the evolution of eccentricity and inclination underestimate the damping action of the disc on the eccentricity and inclination of low-mass planets by an order of magnitude.

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arXiv2022

Estimating the depth of gaps opened by planets in eccentric orbit

Planets can carve gaps in the surface density of protoplanetary discs. The formation of these gaps can reduce the corotation torques acting on the planets. In addition, gaps can halt the accretion of solids onto the planets as dust and pebbles can be trapped at the edge of the gap. This accumulation of dust could explain the origin of the ring-like dust structures observed using high-resolution interferometry. In this work we provide an empirical scaling relation for the depth of the gap cleared by a planet on an eccentric orbit as a function of the planet-to-star mass ratio $q$, the disc aspect ratio $h$, Shakura-Sunyaev viscosity parameter $α$, and planetary eccentricity $e$. We construct the scaling relation using a heuristic approach: we calibrate a toy model based on the impulse approximation with 2D hydrodynamical simulations. The scaling reproduces the gap depth for moderate eccentricities ($e\leq 4h$) and when the surface density contrast outside and inside the gap is $\leq 10^{2}$. Our framework can be used as the basis of more sophisticated models aiming to predict the radial gap profile for eccentric planets.

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arXiv2022

Spreading pressure bumps in gas-dust discs can stall planet migration via planet-vortex interactions

We investigate the gravitational interaction between low- to intermediate-mass planets ($M_p \in[0.06-210]\,M_{\oplus}$) and two previously formed pressure bumps in a gas-dust protoplanetary disc. We explore how the disc structure changes due to planet-induced perturbations and also how the appearance of vortices affects planet migration. We use multifluid 2D hydrodynamical simulations and the dust is treated in the pressureless-fluid approximation, assuming a single grain size of $5\,μ{\mathrm{m}}$. The initial surface density profiles containing two bumps are motivated by recent observations of the protoplanetary disc HD163296. When planets are allowed to migrate, either a single planet from the outer pressure maximum or two planets from each pressure maximum, the initial pressure bumps quickly spread and merge into a single bump which is radially wide and has a very low amplitude. The redistribution of the disc material is accompanied by the Rossby Wave Instability (RWI) and an appearance of mini-vortices that merge in a short period of time to form a large vortex. The large vortex induces perturbations with a spiral wave pattern that propagate away from the vortex as density waves. We found that these vortex-induced spiral waves strongly interact with the spiral waves generated by the planet and we called this mechanism the "\textit{Faraway Interaction}". It facilitates much slower and/or stagnant migration of the planets and it excites their orbital eccentricities in some cases. Our study provides a new explanation for how rocky planets can come to have a slow migration in protoplanetary discs where vortex formation occurs.

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arXiv2018

Gaseous wakes and dynamical friction: mass-losing and mass-gaining perturbers

An extended gravitational object embedded in a parent system comprised of gas and collisionless particles may undergo both dynamical friction (DF) and mass loss by tidal forces. If the object is compact enough, it can increase its mass through accretion of material from the surrounding medium. We extend the classical linear analysis of DF on a constant-mass body in a gaseous medium to the case where its mass changes with time. We show that the structure of the wake may differ significantly from the constant-mass case. For instance, the front-back symmetry of density about subsonic constant-mass perturbers is broken down for variable-mass perturbers. The density wake keeps a memory of the past mass history of the perturber. For dissolving perturbers, the density field is more dense than expected using the instantaneous mass of the perturber in the classical formula. As a consequence, the instantaneous-mass approximation underestimates the drag force for mass-losing perturbers and overestimates it for mass-gaining perturbers. We present cases in which the percentage error in the drag force using the instantaneous-mass approximation is greater than 50%.

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arXiv2018

Torques on low-mass bodies in retrograde orbit in gaseous disks

We evaluate the torque acting on a gravitational perturber on a retrograde circular orbit in the midplane of a gaseous disk. We assume that the mass of this satellite is so low it weakly disturbs the disk (type I migration). The perturber may represent the companion of a binary system with a small mass ratio. We compare the results of hydrodynamical simulations with analytic predictions. Our two-dimensional (2D) simulations indicate that the torque acting on a perturber with softening radius $R_{\rm soft}$ can be accounted for by a scattering approach if $R_{\rm soft}<0.3H$, where $H$ is defined as the ratio between the sound speed and the angular velocity at the orbital radius of the perturber. For $R_{\rm soft}>0.3H$, the torque may present large and persistent oscillations, but the resultant time-averaged torque decreases rapidly with increasing $R_{\rm soft}/H$, in agreement with previous analytical studies. We then focus on the torque acting on small-size perturbers embedded in full three-dimensional (3D) disks and argue that the density waves propagating at distances $\lesssim H$ from the perturber contribute significantly to the torque because they transport angular momentum. We find a good agreement between the torque found in 3D simulations and analytical estimates based on ballistic orbits. We compare the radial migration timescales of prograde versus retrograde perturbers. For a certain range of the perturber's mass and aspect ratio of the disk, the radial migration timescale in the retrograde case may be appreciably shorter than in the prograde case. We also provide the smoothing length required in 2D simulations in order to account for 3D effects.

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arXiv2017

Gap formation by inclined massive planets in locally isothermal three-dimensional discs

We study gap formation in gaseous protoplanetary discs by a Jupiter mass planet. The planet's orbit is circular and inclined relative to the midplane of the disc. We use the impulse approximation to estimate the gravitational tidal torque between the planet and the disc, and infer the gap profile. For low-mass discs, we provide a criterion for gap opening when the orbital inclination is $\leq 30^{\circ}$. Using the FARGO3D code, we simulate the disc response to an inclined massive planet. The dependence of the depth and width of the gap obtained in the simulations on the inclination of the planet is broadly consistent with the scaling laws derived in the impulse approximation. Although we mainly focus on planets kept on fixed orbits, the formalism permits to infer the temporal evolution of the gap profile in cases where the inclination of the planet changes with time. This study may be useful to understand the migration of massive planets on inclined orbit, because the strength of the interaction with the disc depends on whether a gap is opened or not.

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