arXiv subjects

John Wimarsson

Publications and source records attributed to John Wimarsson.

6 recordsLinked to original sources

arXiv2026

Catastrophic tidal disruption of heterogeneous rubble piles: a tale of two regimes

The way a rubble-pile body deforms or disrupts under the influence of tidal forces can be directly tied to its internal strength and configuration. Computational modelling of such tidal disruption events provides an indispensable numerical laboratory for constraining the origin and evolution of small bodies in the Solar System. A majority of previous investigations into tidal disruption of rubble piles have mainly considered progenitors consisting of same-sized, spherical elements. Our study attempts to fill the existing gap in studies analysing the effect of aggregate heterogeneity on tidal disruption outcomes by varying element shape and size frequency distribution. Such heterogeneities have been shown to strongly influence rubble pile dynamics for impacts and rotational failure. We performed numerical simulations of parabolic and hyperbolic tidal encounters between six unique rubble-pile progenitors and the Earth using the N-body code GRAINS. The resulting mass distributions of generated fragments and tidal chain morphologies for the different progenitors were further tied to the internal strength of rubble piles. Two regimes of tidal disruption are identified. In the first regime, closest to the planet, the dynamic evolution is dominated by tidal forces. Here, particle shape, size distribution and resolution appear to have little importance for the resulting distribution of fragment masses. In the second, shear-controlled regime, the internal structure of the progenitor begins to strongly influence the resulting tidal chain morphology and properties of the surviving fragments. Heterogeneity originating from the shape and size frequency distribution of elements in rubble pile models has a substantial effect on the outcomes of tidal disruption events. These parameters must be carefully taken into account when future studies attempt to tie results from numerical models to observations.

astro-ph.EP↗

arXiv2025

The diverse shapes of binary asteroid satellites born from sub-escape-velocity moonlet mergers

Recent direct observations of atypically shaped rubble-pile satellites of sub-km asteroids in form of the spherically oblate Dimorphos and bilobate Selam challenge classical binary asteroid formation theories, which only explain the predominantly elongated population. This study further explores a rubble-pile satellite formation scenario for binary asteroid systems involving debris disks by investigating how mergers between moonlets with impact velocities below the mutual escape speed (sub-escape-velocity mergers) and tidal disruptions can create atypically shaped moons. We simulated sub-escape-velocity mergers between moonlets and studied the resulting structural evolution of the formed moon in a tidal environment using the polyhedral discrete elements method N-body code GRAINS. Firstly, we find that the shapes of rubble-pile moons formed by mergers in this regime are highly dependent on the shape and initial orientation of the involved moonlets. This can be explained by the moonlets largely retaining their individual structures during the impact. Secondly, we observe that mass-loss via tidal disruption for a bilobate object occurs in discrete regimes of distance to the primary. Closer to the primary, the innermost lobe is completely stripped off, while only a small piece of it is lost further out. Due to moonlets largely retaining their shape after undergoing a sub-escape-velocity merger, it is necessary to account for their non-sphericity to accurately model satellite formation in circumasteroidal debris disks. Moreover, the reshaping of merged objects via tidal disruption and distortion can produce oblate spheroid moons such as Dimorphos and highly elongated bilobate satellites with distinct necks such as Selam.

astro-ph.EP↗

arXiv2025

The Dynamical State of the Didymos System Before and After the DART Impact

NASA's Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the natural satellite of (65803) Didymos, on 2022 September 26, as a first successful test of kinetic impactor technology for deflecting a potentially hazardous object in space. The experiment resulted in a small change to the dynamical state of the Didymos system consistent with expectations and Level 1 mission requirements. In the pre-encounter paper Richardson (2022), predictions were put forward regarding the pre- and post-impact dynamical state of the Didymos system. Here we assess these predictions, update preliminary findings published after the impact, report on new findings related to dynamics, and provide implications for ESA's Hera mission to Didymos, scheduled for launch in 2024 with arrival in late December 2026. Pre-encounter predictions tested to date are largely in line with observations, despite the unexpected, flattened appearance of Didymos compared to the radar model and the apparent pre-impact oblate shape of Dimorphos (with implications for the origin of the system that remain under investigation). New findings include that Dimorphos likely became prolate due to the impact and may have entered a tumbling rotation state. A possible detection of a post-impact transient secular decrease in the binary orbital period suggests possible dynamical coupling with persistent ejecta. Timescales for damping of any tumbling and clearing of any debris are uncertain. The largest uncertainty in the momentum transfer enhancement factor of the DART impact remains the mass of Dimorphos, which will be resolved by the Hera mission.

astro-ph.EP↗

arXiv2024

Rapid formation of binary asteroid systems post rotational failure: a recipe for making atypically shaped satellites

Binary asteroid formation is a highly complex process, which has been highlighted with recent observations of satellites with unexpected shapes, such as the oblate Dimorphos by the NASA DART mission and the contact binary Selam by NASA's Lucy mission. There is no clear consensus on which dynamical mechanisms determine the final shape of these objects. In turn, we explore a formation pathway where spin-up and rotational failure of a rubble pile asteroid lead to mass-shedding and a wide circumasteroidal debris disk in which the satellite forms. Using a combination of smooth-particle hydrodynamical and N-body simulations, we study the dynamical evolution in detail. We find that a debris disk containing matter corresponding to a few percent of the primary asteroid mass extending beyond the fluid Roche limit can consistently form both oblate and bilobate satellites via a series of tidal encounters with the primary body and mergers with other gravitational aggregates. Principally, satellites end up prolate (elongated) and on synchronous orbits, accreting mainly in a radial direction while tides from the primary asteroid keep the shape intact. However, close encounters and mergers can break the orbital state, leading to orbital migration and deformation. Satellite-satellite impacts occurring in this regime have lower impact velocities than merger-driven moon formation in e.g. planetary rings, leading to soft impacts between differently sized, non-spherical bodies. The resulting post-merger shape of the satellite is highly dependent on the impact geometry. Only moons having experienced a prior mild or catastrophic tidal disruption during a close encounter with the primary asteroid can become oblate spheroids, which is consistent with the predominantly prolate observed population of binary asteroid satellites.

astro-ph.EP↗

arXiv2022

The velocity distribution of white dwarfs in Gaia EDR3

Using a penalised maximum likelihood we estimate, for the first time, the velocity distribution of white dwarfs in the Solar neighbourhood. Our sample consists of 129 675 white dwarfs within 500 pc in Gaia Early Data Release 3 The white dwarf velocity distributions reveal a similar structure to the rest of the Solar neighbourhood stars, reflecting that white dwarfs are subjected to the same dynamical processes. In the velocity distribution for three magnitude-binned subsamples we however find a novel structure at $(U, V) = (7, -19)$ km s$^{-1}$ in fainter samples, potentially related to the Coma Berenices stream. We also see a double-peaked feature in $U$-$W$ at $U \approx -30$ km s$^{-1}$ and in $V$-$W$ at $V \approx -20$ km s$^{-1}$ for fainter samples. We determine the velocity distribution and velocity moments as a function of absolute magnitude for two samples based on the bifurcation identified in Gaia Data Release 2 in the colour-magnitude diagram. The brighter, redder sequence has a larger velocity dispersion than the fainter, bluer sequence across all magnitudes. It is hard to reconcile this kinematic difference with a bifurcation caused purely by atmospheric composition, while it fits neatly with a significant age difference between the two sequences. Our results provide novel insights into the kinematic properties of white dwarfs and demonstrate the power of analytical techniques that work for the large fraction of stars that do not have measured radial velocities in the current era of large-scale astrometric surveys.

astro-ph.SR↗

arXiv2020

Promoted Mass Growth of Multiple, Distant Giant Planets through Pebble Accretion and Planet-Planet Collision

We propose a pebble-driven planet formation scenario to form giant planets with high multiplicity and large orbital distances in the early gas disk phase. We perform N-body simulations to investigate the growth and migration of low-mass protoplanets in the disk with inner viscously heated and outer stellar irradiated regions. The key feature of this model is that the giant planet cores grow rapidly by a combination of pebble accretion and planet-planet collisions. This consequently speeds up their gas accretion. Because of efficient growth, the planet transitions from rapid type I migration to slow type II migration early, reducing the inward migration substantially. Multiple giant planets can sequentially form in this way with increasing semimajor axes. Both mass growth and orbital retention are more pronounced when a large number of protoplanets are taken into account compared to the case of single planet growth. Eventually, a few numbers of giant planets form with orbital distances of a few to a few tens of AUs within $1.5{-}3$ Myr after the birth of the protoplanets. The resulting simulated planet populations could be linked to the substructures exhibited in disk observations as well as large orbital distance exoplanets observed in radial velocity and microlensing surveys.

astro-ph.EP↗