125 research outputs found
Inside-Out Planet Formation: VI. Oligarchic Coagulation of Planetesimals from a Pebble Ring?
Inside-Out Planet Formation (IOPF) is a theory addressing the origin of
Systems of Tightly-Packed Inner Planets (STIPs) via {\it in situ} formation and
growth of the planets. It predicts that a pebble ring is established at the
pressure maximum associated with the dead zone inner boundary (DZIB) with an
inner disk magnetorotational instability (MRI)-active region. Using direct
-body simulations, we study the collisional evolution of planetesimals
formed from such a pebble ring, in particular examining whether a single
dominant planet emerges. We consider a variety of models, including some in
which the planetesimals are continuing to grow via pebble accretion. We find
that the planetesimal ring undergoes oligarchic evolution, and typically turns
into 2 or 3 surviving oligarchs on nearly coplanar and circular orbits,
independent of the explored initial conditions or form of pebble accretion. The
most massive oligarchs typically consist of about of the total mass,
with the building-up process typically finishing within years.
However, a relatively massive secondary planet always remains with
of the mass of the primary. Such secondary planets have
properties that are inconsistent with the observed properties of the innermost
pairs of planets in STIPs. Thus, for IOPF to be a viable theory for STIP
formation, it needs to be shown how oligarchic growth of a relatively massive
secondary from the initial pebble ring can be avoided. We discuss some
potential additional physical processes that should be included in the modeling
and explored as next steps.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Stability of exomoons around the Kepler transiting circumbinary planets
The Kepler mission has detected a number of transiting circumbinary planets
(CBPs). Although currently not detected, exomoons could be orbiting some of
these CBPs, and they might be suitable for harboring life. A necessary
condition for the existence of such exomoons is their long-term dynamical
stability. Here, we investigate the stability of exomoons around the Kepler
CBPs using numerical -body integrations. We determine regions of stability
and obtain stability maps in the (a_m,i_pm) plane, where a_m is the initial
exolunar semimajor axis with respect to the CBP, and i_pm is the initial
inclination of the orbit of the exomoon around the planet with respect to the
orbit of the planet around the stellar binary. Ignoring any dependence on i_pm,
for most Kepler CBPs the stability regions are well described by the location
of the 1:1 mean motion commensurability of the binary orbit with the orbit of
the moon around the CBP. This is related to a destabilizing effect of the
binary compared to the case if the binary were replaced by a single body, and
which is borne out by corresponding 3-body integrations. For high inclinations,
the evolution is dominated by Lidov-Kozai oscillations, which can bring moons
in dynamically stable orbits to close proximity within the CBP, triggering
strong interactions such as tidal evolution, tidal disruption, or direct
collisions. This suggests that there is a dearth of highly-inclined exomoons
around the Kepler CBPs, whereas coplanar exomoons are dynamically allowed.Comment: Accepted for publication by MNRAS. 12 pages, 7 figure
Galactic Tide and Local Stellar Perturbations on the Oort Cloud: Creation of Interstellar Comets
Comets in the Oort cloud evolve under the influence of internal and external
perturbations, such as giant planets, stellar passages, and the galactic tidal
field. We aim to study the dynamical evolution of the comets in the Oort cloud,
accounting for external perturbations (passing stars and the galactic tide). We
first construct an analytical model of stellar encounters. We find that
individual perturbations do not modify the dynamics of the comets in the cloud
unless very close (< 0.5pc) encounters occur. Using proper motions, parallaxes,
and radial velocities from Gaia DR2, we construct an astrometric catalogue of
14,659 stars that are within 50pc from the Sun. For all these stars we
calculate the time and the closest distance to the Sun. We find that the
cumulative effect of relatively distant ( pc) passing stars can perturb
the comets in the Oort cloud. Finally, we study the dynamical evolution of the
comets in the Oort cloud under the influence of multiple stellar encounters
within 2.5pc from the Sun and the galactic tidal field over Myr. We
considered two models for the Oort cloud, compact (a 0.25 pc) and
extended (a pc). We find that the cumulative effect of stellar
encounters is the major perturber of the Oort cloud for a compact configuration
while for the extended, the galactic tide is the major perturber. In both
cases, the effect of passing stars and the galactic tide raises the semi-major
axis of \% of the comets at the edge of the cloud up to interstellar
regions (pc). This leads to the creation of transitional interstellar
comets, which might become interstellar objects due to external perturbations.
This raises the question about the existence of a cloud of objects in the
interstellar space which might overlap with our Oort cloud if we consider that
other planetary systems face similar processes for the ejection of comets.Comment: 13 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
A hybrid approach for solving the gravitational N-body problem with Artificial Neural Networks
Simulating the evolution of the gravitational N-body problem becomes
extremely computationally expensive as N increases since the problem complexity
scales quadratically with the number of bodies. We study the use of Artificial
Neural Networks (ANNs) to replace expensive parts of the integration of
planetary systems. Neural networks that include physical knowledge have grown
in popularity in the last few years, although few attempts have been made to
use them to speed up the simulation of the motion of celestial bodies. We study
the advantages and limitations of using Hamiltonian Neural Networks to replace
computationally expensive parts of the numerical simulation. We compare the
results of the numerical integration of a planetary system with asteroids with
those obtained by a Hamiltonian Neural Network and a conventional Deep Neural
Network, with special attention to understanding the challenges of this
problem. Due to the non-linear nature of the gravitational equations of motion,
errors in the integration propagate. To increase the robustness of a method
that uses neural networks, we propose a hybrid integrator that evaluates the
prediction of the network and replaces it with the numerical solution if
considered inaccurate. Hamiltonian Neural Networks can make predictions that
resemble the behavior of symplectic integrators but are challenging to train
and in our case fail when the inputs differ ~7 orders of magnitude. In
contrast, Deep Neural Networks are easy to train but fail to conserve energy,
leading to fast divergence from the reference solution. The hybrid integrator
designed to include the neural networks increases the reliability of the method
and prevents large energy errors without increasing the computing cost
significantly. For this problem, the use of neural networks results in faster
simulations when the number of asteroids is >70.Comment: Accepted for publication in the Journal of Computational Physic
Linking the formation and fate of exo-Kuiper belts within solar system analogues
Abstract Escalating observations of exo-minor planets and their destroyed remnants both passing through the solar system and within white dwarf planetary systems motivate an understanding of the orbital history and fate of exo-Kuiper belts and planetesimal discs. Here we explore how the structure of a 40 − 1000 au annulus of planetesimals orbiting inside of a solar system analogue that is itself initially embedded within a stellar cluster environment varies as the star evolves through all of its stellar phases. We attempt this computationally challenging link in four parts: (1) by performing stellar cluster simulations lasting 100 Myr, (2) by making assumptions about the subsequent quiescent 11 Gyr main-sequence evolution, (3) by performing simulations throughout the giant branch phases of evolution, and (4) by making assumptions about the belt’s evolution during the white dwarf phase. Throughout these stages, we estimate the planetesimals’ gravitational responses to analogues of the four solar system giant planets, as well as to collisional grinding, Galactic tides, stellar flybys, and stellar radiation. We find that the imprint of stellar cluster dynamics on the architecture of ≳ 100 km-sized exo-Kuiper belt planetesimals is retained throughout all phases of stellar evolution unless violent gravitational instabilities are triggered either (1) amongst the giant planets, or (2) due to a close (≪103 au) stellar flyby. In the absence of these instabilities, these minor planets simply double their semimajor axis while retaining their primordial post-cluster eccentricity and inclination distributions, with implications for the free-floating planetesimal population and metal-polluted white dwarfs
Fast and Credible Likelihood-Free Cosmology with Truncated Marginal Neural Ratio Estimation
Sampling-based inference techniques are central to modern cosmological data
analysis; these methods, however, scale poorly with dimensionality and
typically require approximate or intractable likelihoods. In this paper we
describe how Truncated Marginal Neural Ratio Estimation (TMNRE) (a new approach
in so-called simulation-based inference) naturally evades these issues,
improving the efficiency, scalability, and trustworthiness
of the inferred posteriors. Using measurements of the Cosmic Microwave
Background (CMB), we show that TMNRE can achieve converged posteriors using
orders of magnitude fewer simulator calls than conventional Markov Chain Monte
Carlo (MCMC) methods. Remarkably, the required number of samples is effectively
independent of the number of nuisance parameters. In addition, a property
called \emph{local amortization} allows the performance of rigorous statistical
consistency checks that are not accessible to sampling-based methods. TMNRE
promises to become a powerful tool for cosmological data analysis, particularly
in the context of extended cosmologies, where the timescale required for
conventional sampling-based inference methods to converge can greatly exceed
that of simple cosmological models such as CDM. To perform these
computations, we use an implementation of TMNRE via the open-source code
\texttt{swyft}.Comment: v2: accepted journal version. v1: 37 pages, 13 figures.
\texttt{swyft} is available at https://github.com/undark-lab/swyft, and
demonstration code for cosmological examples is available at
https://github.com/acole1221/swyft-CM
Birth cluster simulations of planetary systems with multiple super-Earths : initial conditions for white dwarf pollution drivers
Previous investigations have revealed that eccentric super-Earths represent a class of planets that are particularly effective at transporting minor bodies towards white dwarfs and subsequently polluting their atmospheres with observable chemical signatures. However, the lack of discoveries of these planets beyond a few astronomical units from their host stars prompts a better understanding of their orbital architectures from their nascent birth cluster. Here, we perform stellar cluster simulations of three-planet and seven-planet systems containing super-Earths on initially circular, coplanar orbits. We adopt the typical stellar masses of main-sequence progenitors of white dwarfs (1.5M⊙–2.5M⊙) as host stars and include 8000 main-sequence stars following a Kroupa initial mass function in our clusters. Our results reveal that about 30 per cent of the simulated planets generate eccentricities of at least 0.1 by the time of cluster dissolution, which would aid white dwarf pollution. We provide our output parameters to the community for potential use as initial conditions for subsequent evolution simulations
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