Context. Debris discs are traditionally studied using two distinct types of numerical models: statistical particle-in-a-box codes to study their collisional and size distribution evolution, and ...dynamical N-body models to study their spatial structure. The absence of collisions in N-body codes is in particular a major shortcoming, as collisional processes are expected to significantly alter the results obtained from pure N-body runs. Aims. We present a new numerical model, to study the spatial structure of perturbed debris discs in both a dynamical and collisional steady-state. We focus on the competing effects of gravitational perturbations by a massive body (planet or star), the collisional production of small grains, and the radiation pressure placing these grains in possibly dynamically unstable regions. Methods. We consider a disc of parent bodies in a dynamical steady-state, from which small radiation-pressure-affected grains are released in a series of runs, each corresponding to a different orbital position of the perturber, where particles are assigned a collisional destruction probability. These collisional runs produce successive position maps that are then recombined, following a complex procedure, to generate surface density profiles for each orbital position of the perturbing body. Results. We apply our code to the case of a circumprimary disc in a binary. We find pronounced structures inside and outside the dynamical stability regions. For low eB, the disc’s structure is time varying, with spiral arms in the dynamically “forbidden” region precessing with the companion star. For high eB, the disc is strongly asymmetric but time invariant, with a pronounced density drop in the binary’s periastron direction.
Context. Recent observations of the edge-on debris disk of AU Mic have revealed asymmetric, fast outward-moving arch-like structures above the disk midplane. Although asymmetries are frequent in ...debris disks, no model can readily explain the characteristics of these features. Aims. We present a model aiming to reproduce the dynamics of these structures, more specifically their high projected speeds and their apparent position. We test the hypothesis of dust emitted by a point source and then expelled from the system by the strong stellar wind of this young M-type star. In this model we make the assumption that the dust grains follow the same dynamics as the structures, i.e., they are not local density enhancements. Methods. We perform numerical simulations of test particle trajectories to explore the available parameter space, in particular the radial location R0 of the dust producing parent body and the size of the dust grains as parameterized by the value of β (ratio of stellar wind and radiation pressure forces over gravitation). We consider the cases of a static and of an orbiting parent body. Results. We find that for all considered scenarios (static or moving parent body), there is always a set of (R0,β) parameters able to fit the observed features. The common characteristics of these solutions is that they all require a high value of β, of around 6. This means that the star is probably very active, and the grains composing the structures are submicronic in order for observable grains to reach such high β values. We find that the location of the hypothetical parent body is closer in than the planetesimal belt, around 8 ± 2 au (orbiting case) or 28 ± 7 au (static case). A nearly periodic process of dust emission appears, of 2 yr in the orbiting scenarios and 7 yr in the static case. Conclusions. We show that the scenario of sequential dust releases by an unseen point-source parent body is able to explain the radial behavior of the observed structures. We predict the evolution of the structures to help future observations discriminate between the different parent body configurations that have been considered. In the orbiting parent body scenario, we expect new structures to appear on the northwest side of the disk in the coming years.
Context. The vertical thickness of debris discs is often used as a measure of these systems' dynamical excitation, and as clues to the presence of hidden massive perturbers such as planetary embryos. ...However, this argument might be flawed because the observed dust should be naturally placed on inclined orbits by the combined effect of radiation pressure and mutual collisions. Aims. We critically reinvestigate this issue and numerically estimate the “natural” vertical thickness of a collisionally evolving disc, in the absence of any additional perturbing body. Methods. We use a deterministic collisional code, to follow the dynamical evolution of a population of indestructible test grains suffering mutual inelastic impacts. Grain differential sizes as well as the effect of radiation pressure are taken into account. Results. We find that, under the coupled effect of radiation pressure and collisions, grains naturally acquire inclinations of a few degrees. The disc is stratified with respect to grain sizes, the smallest grains having the largest vertical dispersion and the largest being clustered closer to the midplane. Conclusions. Debris discs should have a minimum “natural” observed aspect ratio $h_{\mathrm{min}}$ ~ 0.04±0.02 from visible to mid-IR wavelengths, where the flux is dominated by the smallest bound grains. These values are comparable to the estimated thicknesses of several vertically resolved debris discs, as illustrated by the specific example of AU Mic. For all systems with h ~ $h_{\mathrm{min}}$, the presence (or absence) of embedded perturbing bodies cannot be inferred from the vertical dispersion of the disc.
Context. In most current debris disc models, the dynamical and the collisional evolutions are studied separately with N-body and statistical codes, respectively, because of stringent computational ...constraints. In particular, incorporating collisional effects (especially destructive collisions) into an N-body scheme has proven a very arduous task because of the exponential increase of particles it would imply. Aims. We present here LIDT-DD, the first code able to mix both approaches in a fully self-consistent way. Our aim is for it to be generic enough to be applied to any astrophysical case where we expect dynamics and collisions to be deeply interlocked with one another: planets in discs, violent massive breakups, destabilized planetesimal belts, bright exozodiacal discs, etc. Methods. The code takes its basic architecture from the LIDT3D algorithm for protoplanetary discs, but has been strongly modified and updated to handle the very constraining specificities of debris disc physics: high-velocity fragmenting collisions, radiation-pressure affected orbits, absence of gas that never relaxes initial conditions, etc. It has a 3D Lagrangian-Eulerian structure, where grains of a given size at a given location in a disc are grouped into super-particles or tracers whose orbits are evolved with an N-body code and whose mutual collisions are individually tracked and treated using a particle-in-a-box prescription designed to handle fragmenting impacts. To cope with the wide range of possible dynamics for same-sized particles at any given location in the disc, and in order not to lose important dynamical information, tracers are sorted and regrouped into dynamical families depending on their orbits. A complex reassignment routine that searches for redundant tracers in each family and reassignes them where they are needed, prevents the number of tracers from diverging. Results. The LIDT-DD code has been successfully tested on simplified cases for which robust results have been obtained in past studies: we retrieve the classical features of particle size distributions in unperturbed discs and the outer radial density profiles in ~r-1.5 outside narrow collisionally active rings as well as the depletion of small grains in dynamically cold discs. The potential of the new code is illustrated with the test case of the violent breakup of a massive planetesimal within a debris disc. Preliminary results show that we are able for the first time to quantify the timescale over which the signature of such massive break-ups can be detected. In addition to studying such violent transient events, the main potential future applications of the code are planet and disc interactions, and more generally, any configurations where dynamics and collisions are expected to be intricately connected.
Context. Near- and mid-infrared interferometric observations have revealed populations of hot and warm dust grains populating the inner regions of extrasolar planetary systems. These are known as ...exozodiacal dust clouds, or exozodis, reflecting the similarity with the solar system’s zodiacal cloud. Radiative transfer models have constrained the dust to be dominated by tiny submicron-sized, carbon-rich grains that are accumulated very close to the sublimation radius. The origin of this dust is an unsolved issue. Aims. We explore two exozodiacal dust production mechanisms, first re-investigating the Poynting-Robertson drag pile-up scenario, and then elaborating on the less explored but promising exocometary dust delivery scenario. Methods. We developed a new, versatile numerical model that calculates the dust dynamics, with non-orbit-averaged equations for the grains close to the star. The model includes dust sublimation and incorporates a radiative transfer code for direct comparison to the observations. We consider in this study four stellar types, three dust compositions, and we assume a parent belt at 50 au. Results. In the case of the Poynting-Robertson drag pile-up scenario, we find that it is impossible to produce long-lived submicron-sized grains close to the star. The inward drifting grains fill in the region between the parent belt and the sublimation distance, producing an unrealistically strong mid-infrared excess compared to the near-infrared excess. The dust pile-up at the sublimation radius is by far insufficient to boost the near-IR flux of the exozodi to the point where it dominates over the mid-infrared excess. In the case of the exocometary dust delivery scenario, we find that a narrow ring can form close to the sublimation zone, populated with large grains from several tens to several hundreds of micrometers in radius. Although not perfect, this scenario provides a better match to the observations, especially if the grains are carbon-rich. We also find that the number of active exocomets required to sustain the observed dust level is reasonable. Conclusions. We conclude that the hot exozodiacal dust detected by near-infrared interferometry is unlikely to result from inward grain migration by Poynting-Robertson drag from a distant parent belt, but could instead have an exocometary origin.
High levels of exozodiacal dust are observed around a growing number of main sequence stars. The origin of such dust is not clear, given that it has a short lifetime against both collisions and ...radiative forces. Even a collisional cascade with km-sized parent bodies, as suggested to explain outer debris discs, cannot survive sufficiently long. In this work we investigate whether the observed exozodiacal dust could originate from an outer planetesimal belt. We investigate the scattering processes in stable planetary systems to determine whether sufficient material could be scattered inwards in order to retain the exozodiacal dust at its currently observed levels. We use N-body simulations to investigate the efficiency of this scattering and its dependence on the architecture of the planetary system. The results of these simulations can be used to assess the ability of hypothetical chains of planets to produce exozodi in observed systems. We find that for older (>100 Myr) stars with exozodiacal dust, a massive, large radii (>20 AU) outer belt and a chain of tightly packed, low-mass planets would be required to retain the dust at its currently observed levels. This brings into question how many, if any, real systems possess such a contrived architecture and are therefore capable of scattering at sufficiently high rates to retain exozodi dust on long timescales.
Recent studies have shown that α Centauri B might be, from an observational point of view, an ideal candidate for the detection of an Earth-like planet in or near its habitable zone (0.5–0.9 au). We ...study here if such habitable planets can form, by numerically investigating the planet-formation stage which is probably the most sensitive to binarity effects: the mutual accretion of km-sized planetesimals. Using a state-of-the-art algorithm for computing the impact velocities within a test planetesimal population, we find that planetesimal growth is only possible, although marginally, in the innermost part of the habitable zone (HZ) around 0.5 au. Beyond this point, the combination of secular perturbations by the binary companion and gas drag drives the mutual velocities beyond the erosion limit. Impact velocities might later decrease during the gas removal phase, but this probably happens too late for preventing most km-sized objects to be removed by inward drift, thus preventing accretion from starting anew. A more promising hypothesis is that the binary formed in a crowded cluster, where it might have been wider in its initial stages, when planetary formation was ongoing. We explore this scenario and find that a starting separation roughly 15 au wider, or an eccentricity 2.5 times lower than the present ones, is required to have an accretion-friendly environment in the whole HZ.
We numerically explore planet formation around α Centauri A by focusing on the crucial planetesimals-to-embryos phase. Our approach is significantly improved with respect to the earlier work of ...Marzari & Scholl, since our deterministic N-body code computing the relative velocities between test planetesimals handles bodies with different size. Due to this step-up, we can derive the accretion versus fragmentation trend of a planetesimal population having any given size distribution. This is a critical aspect of planet formation in binaries since the pericenter alignment of planetesimal orbits due to the gravitational perturbations of the companion star and to gas friction strongly depends on size. Contrary to Marzari & Scholl, we find that, for the nominal case of a Minimum-Mass Solar Nebula gas disc, the region beyond ∼0.5 au from the primary is strongly hostile to planetesimal accretion. In this area, impact velocities between different-sized bodies are increased, by the differential orbital phasing, to values too high to allow mutual accretion. For any realistic size distribution for the planetesimal population, this accretion-inhibiting effect is the dominant collision outcome and the accretion process is halted. Results are relatively robust with respect to the profile and density of the gas disc. Except for an unrealistic almost gas-free case, the inner ‘accretion-safe’ area never extends beyond 0.75 au. We conclude that planet formation is very difficult in the terrestrial region around α Centauri A, unless it started from fast-formed very large (>30 km) planetesimals. Notwithstanding these unlikely initial conditions, the only possible explanation for the presence of planets around 1 au from the star would be the hypothetical outward migration of planets formed closer to the star or a different orbital configuration in the binary's early history. Our conclusions differ from those of several studies focusing on the later embryos-to-planets stage, confirming that the planetesimals-to-embryos phase is more affected by binary perturbations.
Context.
The optical properties of the second generation dust that we observe in debris disks remain quite elusive, whether it is the absorption efficiencies at millimeter wavelengths or the ...(un)polarized phase function at near-infrared wavelengths. Thankfully, the same particles are experiencing forces that are size dependent (e.g., radiation pressure) and, with high angular resolution observations, we can take advantage of this natural spatial segregation.
Aims.
Observations at different wavelengths probe different ranges of sizes; millimeter observations trace the larger grains, while near-infrared observations are sensitive to the other extreme of the size distribution. Consequently, there is a great synergy in combining both observational techniques to better constrain the optical properties of the particles.
Methods.
We present a new approach to simultaneously model observations from“Spectro-Polarimetric High Contrast Exoplanet REsearch” (SPHERE) and the“Atacama Large Millimeter Array” (ALMA) and apply it to the debris disk around HD 32297, putting the emphasis on the spatial distribution of the grains with different
β
values. This modeling approach requires few assumptions on the actual sizes of the particles and the interpretation can therefore be done a posteriori.
Results.
We find that the ALMA observations are best reproduced with a combination of small and large
β
values (0.03 and 0.42) while the SPHERE observations require several intervals of
β
values. We discuss the nature of the halo previously reported in ALMA observations, and hypothesize it could be caused by over-abundant μm-sized particles (the over-abundance being the consequence of their extended lifetime). We modeled the polarized phase function at near-infrared wavelengths, and fluffy aggregates larger than a few μm provide the best solution.
Conclusions.
Comparing our results with comets of the Solar System, we postulate that the particles released in the disk originate from rather pristine cometary bodies (to avoid compaction of the fluffy aggregates) and they are then set on highly eccentric orbits, which could explain the halo detected at long wavelengths.
Context.
Gas has been successfully detected in many extrasolar systems around mature stars aged between 10 Myr and ∼1 Gyr that include planetesimal belts. Gas in these mature disks is thought to be ...released from planetesimals and has been modeled using a viscous disk approach where the gas expands inwards and outwards from the belt where it is produced. Therefore, the gas has so far been assumed to make up the circumstellar disk orbiting the star; however, at low densities, this may not be an adequate assumption, as the gas could be blown out by the stellar wind instead.
Aims.
In this paper, we aim to explore the timeframe in which a gas disk transitions to such a gas wind and whether this information can be used to determine the stellar wind properties around main sequence stars, which are otherwise difficult to obtain.
Methods.
We developed an analytical model for A to M stars that can follow the evolution of gas outflows and target the moment of transition between a disk or a wind in order to make a comparison with current observations. The crucial criterion here is the gas density for which gas particles are no longer protected from the impact of stellar wind protons at high velocities and on radial trajectories.
Results.
We find that: (1) belts with a radial width, Δ
R
, with gas densities <7 (ΔR/50 au)
−
1 cm
−3
, would create a wind rather than a disk, which would explain the recent outflowing gas detection in NO Lup; (2) the properties of this belt wind can be used to measure stellar wind properties such as their densities and velocities; (3) very early-type stars can also form gas winds due to the star’s radiation pressure, instead of a stellar wind; (4) debris disks with low fractional luminosities,
f
, are more likely to create gas winds, which could be observed with current facilities.
Conclusions.
Systems containing low gas masses, such as Fomalhaut or TWA 7, or more generally, debris disks with fractional luminosities of
f
≲ 10
−5
(
L
⋆
/
L
⊙
)−0.37 or stellar luminosity ≳20 L⊙ (A0V or earlier) are more likely to create gas outflows (or belt winds) than gas disks. Gas that is observed to be outflowing at high velocity in the young system NO Lup could be an example of such belt winds. Future observing predictions in this wind region should account for the stellar wind in the attempt to detect the gas. The detection of these gas winds is possible with ALMA (CO and CO
+
could serve as good wind tracers). This would allow us to constrain the stellar wind properties of main-sequence stars, as these properties are otherwise difficult to measure, since, for example, there are no successful measures around A stars at present.
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