arXiv subjects

Uri Malamud

Publications and source records attributed to Uri Malamud.

26 records · Page 2Linked to original sources

arXiv2020

Tidal disruption of planetary bodies by white dwarfs I: A hybrid SPH-analytical approach

We introduce a new hybrid method to perform high-resolution tidal disruption simulations, at arbitrary orbits. An SPH code is used to simulate tidal disruptions only in the immediate spatial domain of the star, namely, where the tidal forces dominate over gravity, and then during the fragmentation phase in which the emerging tidal stream may collapse under its own gravity to form fragments. Following each hydrodynamical simulation, an analytical treatment is then applied to instantaneously transfer each fragment back to the tidal sphere for its subsequent disruption, in an iterative process. We validate the hybrid model by comparing it to both an analytical impulse approximation model of single tidal disruptions, as well as full-scale SPH simulations spanning the entire disc formation. The hybrid simulations are essentially indistinguishable from the full-scale SPH simulations, while computationally outperforming their counterparts by orders of magnitude. Thereby our new hybrid approach uniquely enables us to follow the long-term formation and continuous tidal disruption of the planet/planetesimal debris, without the resolution and orbital configuration limitation of previous studies. In addition, we describe a variety of future directions and applications for our hybrid model, which is in principle applicable to any star, not merely white dwarfs

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arXiv2020

Tidal disruption of planetary bodies by white dwarfs II: Debris disc structure and ejected interstellar asteroids

We make use of a new hybrid method to simulate the long-term, multiple-orbit disc formation through tidal disruptions of rocky bodies by white dwarfs, at high-resolution and realistic semi-major axis. We perform the largest-yet suite of simulations for dwarf and terrestrial planets, spanning four orders of magnitude in mass, various pericentre distances and semi-major axes between 3 AU and 150 AU. This large phase space of tidal disruption conditions has not been accessible through the use of previous codes. We analyse the statistical and structural properties of the emerging debris discs, as well as the ejected unbound debris contributing to the population of interstellar asteroids. Unlike previous tidal disruption studies of small asteroids which form ring-like structures on the original orbit, we find that the tidal disruption of larger bodies usually forms dispersed structures of interlaced elliptic eccentric annuli on tighter orbits. We characterize the (typically power-law) size-distribution of the ejected interstellar bodies as well as their composition, rotation velocities and ejection velocities. We find them to be sensitive to the depth (impact parameter) of the tidal disruption. Finally, we briefly discuss possible implications of our results in explaining the peculiar variability of Tabby's star, the origin of the transit events of ZTF J0139+5245 and the formation of a planetary core around SDSS J1228+1040.

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arXiv2018

Moonfalls: Collisions between the Earth and its past moons

During the last stages of the terrestrial planet formation, planets grow mainly through giant-impacts with large planetary embryos. The Earth's Moon was suggested to form through one of these impacts. However, since the proto-Earth has experienced many giant-impacts, several moons are naturally expected to form through a sequence of multiple (including smaller scale) impacts. Each impact potentially forms a sub-Lunar mass moonlet that interacts gravitationally with the proto-Earth and possibly with previously-formed moonlets. Such interactions result in either moonlet-moonlet mergers, moonlet ejections or infall of moonlets on the Earth. The latter possibility, leading to low-velocity moonlet-Earth collisions is explored here for the first time. We make use of SPH simulations and consider a range of moonlet masses, collision impact-angles and initial proto-Earth rotation rates. We find that grazing/tidal-collisions are the most frequent and produce comparable fractions of accreted-material and debris. The latter typically clump in smaller moonlets that can potentially later interact with other moonlets. Other collision geometries are more rare. Head-on collisions do not produce much debris and are effectively perfect mergers. Intermediate impact angles result in debris mass-fractions in the range of 2-25% where most of the material is unbound. Retrograde collisions produce more debris than prograde collisions, whose fractions depend on the proto-Earth initial rotation rate. Moonfalls can slightly change the rotation-rate of the proto-Earth. Accreted moonfall material is highly localized, potentially explaining the isotopic heterogeneities in highly siderophile elements in terrestrial rocks, and possibly forming primordial super-continent topographic features. Our results can be used for simple scaling laws and applied to n-body studies of the formation of the Earth and Moon.

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arXiv2017

Post-main-sequence evolution of icy minor planets. III. water retention in dwarf planets and exo-moons and implications for white dwarf pollution

Studies suggest that the pollution of white dwarf (WD) atmospheres arises from the accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases are not yet understood. Previous works studied the water retention in minor planets around main-sequence and evolving host stars, in order to evaluate the possibility that water survives inside minor planets around WDs. However, all of these studies focused on small, comet-sized to moonlet-sized minor planets, when the inferred mass inside the convection zones of He-dominated WDs could actually also be compatible with much more massive minor planets. In this study we therefore explore for the first time, the water retention inside exo-planetary dwarf planets, or moderate-sized moons, with radii of the order of hundreds of kilometres. We now cover nearly the entire potential mass range of minor planets. The rest of the parameter space considered in this study is identical to that of our previous study, and also includes multiple WD progenitor star masses. We find that water retention in more massive minor planets is still affected by the mass of the WD progenitor, however not as much as when small minor planets were considered. We also find that water retention is now almost always greater than zero. On average, the detected water fraction in He-dominated WD atmospheres should be at least 5\%, irrespective of the assumed initial water composition, if it came from a single accretion event of an icy dwarf planet or moon. This finding also strengthens the possibility of WD habitability. To finalize our previous and current findings, we provide a code which may be freely used as a service to the community. The code calculates ice and water retention by interpolation, spanning the full mass range of both minor planets and their host stars.

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arXiv2017

The Contraction/Expansion History of Charon with implication for its Planetary Scale Tectonic Belt

The New Horizons mission to the Kuiper Belt has recently revealed intriguing features on the surface of Charon, including a network of chasmata, cutting across or around a series of high topography features, conjoining to form a belt. It is proposed that this tectonic belt is a consequence of contraction/expansion episodes in the moon's evolution associated particularly with compaction, differentiation and geochemical reactions of the interior. The proposed scenario involves no need for solidification of a vast subsurface ocean and/or a warm initial state. This scenario is based on a new, detailed thermo-physical evolution model of Charon that includes multiple processes. According to the model, Charon experiences two contraction/expansion episodes in its history that may provide the proper environment for the formation of the tectonic belt. This outcome remains qualitatively the same, for several different initial conditions and parameter variations. The precise orientation of Charon's tectonic belt, and the cryovolcanic features observed south of the tectonic belt may have involved a planetary-scale impact, that occurred only after the belt had already formed.

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arXiv2017

Post main sequence evolution of icy minor planets: Implications for water retention and white dwarf pollution

Most observations of polluted white dwarf atmospheres are consistent with accretion of water depleted planetary material. Among tens of known cases, merely two cases involve accretion of objects that contain a considerable mass fraction of water. The purpose of this study is to investigate the relative scarcity of these detections. Based on a new and highly detailed model, we evaluate the retention of water inside icy minor planets during the high luminosity stellar evolution that follows the main sequence. Our model fully considers the thermal, physical, and chemical evolution of icy bodies, following their internal differentiation as well as water depletion, from the moment of their birth and through all stellar evolution phases preceding the formation of the white dwarf. We also account for different initial compositions and formation times. Our results show that previous studies have either underestimated or overestimated water retention. We also reaffirm that water can survive in a variety of circumstances and in great quantities, and therefore other possibilities are discussed in order to explain the infrequency of water detections. We find that the most likely explanation is that water does not survive the dissociation of tidally disrupted minor planets, and instead a direct impact onto the white dwarf is required. We show that this possibility is statistically plausible, and predict that if this hypothesis is correct, future observations of extremely water rich atmospheres will involve only helium dominated DB white dwarfs.

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arXiv2017

Post main sequence evolution of icy minor planets II: water retention and white dwarf pollution around massive progenitor stars

Most studies suggest the pollution of white dwarf (WD) atmospheres arises from accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases are not yet understood. Here we study the water retention of small icy bodies in exo-solar planetary systems, as their respective host stars evolve through and off the main sequence and eventually become WDs. We explore for the first time a wide range of star masses and metallicities. We find that the mass of the WD progenitor star is of crucial importance for the retention of water, while its metallicity is relatively unimportant. We predict that minor planets around lower-mass WD progenitors would retain more water in general, and would do so at closer distances from the WD, compared with high-mass progenitors. The dependence of water retention on progenitor mass and other parameters has direct implications for the origin of observed WD pollution, and we discuss how our results and predictions might be tested in the future as more observations of WDs with long cooling-ages become available.

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arXiv2015

A 1-D evolutionary model for icy satellites, applied to Enceladus

We develop a long-term 1-D evolution model for icy satellites that couples multiple processes: water migration and differentiation, geochemical reactions and silicate phase transitions, compaction by self-gravity, and ablation. The model further considers the following energy sources and sinks: tidal heating, radiogenic heating, geochemical energy released by serpentinization or absorbed by mineral dehydration, gravitational energy and insolation, and heat transport by conduction, convection, and advection. We apply the model to Enceladus, by guessing the initial conditions that would render a structure compatible with present-day observations, assuming the initial structure to have been homogeneous. Assuming the satellite has been losing water continually along its evolution, we postulate that it was formed as a more massive, more icy and more porous satellite, and gradually transformed into its present day state due to sustained long-term tidal heating. We consider several initial compositions and evolution scenarios and follow the evolution for the age of the Solar System, testing the present day model results against the available observational constraints. Our model shows the present configuration to be differentiated into a pure icy mantle, several tens of km thick, overlying a rocky core, composed of dehydrated rock at the center and hydrated rock in the outer part. For Enceladus, it predicts a higher rock/ice mass ratio than previously assumed and a thinner ice mantle, compatible with recent estimates based on gravity field measurements. Although, obviously, the model cannot be used to explain local phenomena, it sheds light on the internal structure invoked in explanations of localized features and activities.

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