arXiv subjects

Uri Malamud

Publications and source records attributed to Uri Malamud.

At least 19 recordsLinked to original sources

arXiv2026

Shock-heated Away: The Impact of Radiative Cooling on Gas-Phase Transitions in Supernova Remnants

Supernova (SN) feedback plays a central role in regulating the structure of the interstellar medium (ISM) through the injection of energy and momentum. The amount of hot gas produced by a supernova is a key quantity that determines how efficiently SN feedback heats the ISM and drives mass exchange between its different gas phases, here defined as cold ($T < 10^3\, \mathrm{K}$), warm ($10^3\, \mathrm{K} < T < 2\times10^4\, \mathrm{K}$), and hot ($T > 2\times10^4\, \mathrm{K}$) gas. However, previous studies have reported discrepant amounts of hot gas formed under otherwise similar ambient conditions. To resolve these disagreements, we quantify the amount of hot gas produced by individual SN explosions using a suite of controlled simulations spanning a broad range of ISM environments that include both uniform and turbulent, multiphase backgrounds. We show that radiative cooling is a key factor regulating hot-gas production, and that differences in cooling efficiency can account for some of the discrepancies reported in the literature. We derive a simple predictive relation for the peak hot-gas mass attained during the evolution of a supernova remnant in terms of the mean ambient density, the initial phase distribution, and the efficiency of gas cooling, which we parameterize as the cooling time over a key temperature range of $10^{4.5}\,\mathrm{K} \lesssim T \lesssim 10^{5.1}\,\mathrm{K}$. Finally, using tracer particles, we distinguish the evaporation of cold and warm gas into the hot phase and derive physically motivated expressions for the evaporation efficiency. Our results provide simple, predictive relations for hot-gas production and phase transitions that can be incorporated into subgrid models of SN feedback in galaxy formation simulations.

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arXiv2026

Multiphase turbulence as the origin of OH+, H2O+ and H3+ column density scatter in the local ISM

Observations of the reactive ions OH+, H2O+ and H3+ in the Galactic interstellar medium reveal large sight-line-to-sight-line scatter in their column densities, commonly interpreted as evidence for substantial variations in the cosmic-ray ionization rate (CRIR). We revisit this interpretation using high-resolution three-dimensional magneto-hydrodynamic simulations of the multiphase ISM with time-dependent chemistry for H, H2, H+ and electrons, building on the fiducial model of Godard et al. (2023). We find that a single CRIR of ~2 10^{-16} s^{-1}, together with standard Galactic-scale parameters, naturally produces broad column-density distributions for all three tracers in good agreement with the observed medians and percentile widths, with no fine tuning. Reaching this match requires that the post-processing of OH+, H2O+ and H3+ retain the time-dependent H2 field generated by the turbulent flow rather than assume chemical equilibrium: turbulence drives long-lived H2 enhancements in the unstable neutral medium where OH+ and H2O+ predominantly reside, and an equilibrium treatment under-predicts their columns substantially. H3+, which receives most of its column from denser CNM gas closer to equilibrium, is much less affected. Our results caution against interpreting sight-line-to-sight-line scatter as direct evidence for large CRIR fluctuations, and motivate a shift from independent 1D equilibrium analyses toward 3D dynamical frameworks when inferring ionization conditions in the ISM.

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arXiv2025

Momentum transfer coefficient constraints for the 2024 PDC25 Hypothetical Asteroid Impact Scenario

In Epoch 2 of the 2024 PDC25 Hypothetical Asteroid Impact Scenario, an asteroid is confirmed to be on a collision course with the Earth, and its size and surface composition have been well characterized via a flyby mission. A kinetic impactor deflection strategy is the most technologically mature path in order to mitigate this threat. Our goal is to constrain the possible range in momentum transfer coefficients, with implications for the number of impactors and the disruption risk. We conduct a series of numerical simulations, using a shock physics smoothed particle hydrodynamics code, in which we vary the impact velocity, cohesive properties and physical properties (mass / porosity) of the target asteroid. Given a judiciously chosen impactor mass, we show that the momentum transfer coefficient range is capable of a moderate-to-large enhancement of the asteroid deflection, yet keeps the disruption risk firmly at bay. These results are generally unique in having higher impact velocities compared to most previous studies.

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arXiv2024

Realistic outcomes of moon-moon collisions in Lunar formation theory

The multiple impact hypothesis proposes that the Moon formed through a series of smaller collisions, rather than a single giant impact. This study advances our understanding of this hypothesis, as well as moon collisions in other contexts, by exploring the implications of these smaller impacts, employing a novel methodological approach that combines self-consistent initial conditions, hybrid hydrodynamic/N-body simulations, and the incorporation of material strength. Our findings challenge the conventional assumption of perfect mergers in previous models, revealing a spectrum of collision outcomes including partial accretion and mass loss. These outcomes are sensitive to collision parameters and Earth's tidal influence, underscoring the complex dynamics of lunar accretion. Importantly, we demonstrate that incorporating material strength is important for accurately simulating moonlet-sized impacts. This inclusion significantly affects fragmentation, tidal disruption, and the amount of material ejected or accreted onto Earth, ultimately impacting the Moon's growth trajectory. By accurately modeling diverse collision outcomes, our hybrid approach provides a powerful new framework for understanding the Moon's formation. We show that most collisions (~90%) do not significantly erode the largest moonlet, supporting the feasibility of lunar growth through accretion. Moreover, we revise previous estimates of satellite disruption, suggesting a higher survival rate and further bolstering the multiple-impact scenario.

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arXiv2024

New versus past silica crush curve experiments: application to Dimorphos benchmarking impact simulations

Crush curves are of fundamental importance to numerical modeling of small and porous astrophysical bodies. The empirical literature often measures them for silica grains, and different studies have used various methods, sizes, textures, and pressure conditions. Here we review past studies and supplement further experiments in order to develop a full and overarching understanding of the silica crush curve behavior. We suggest a new power-law function that can be used in impact simulations of analog materials similar to micro-granular silica. We perform a benchmarking study to compare this new crush curve to the parametric quadratic crush curve often used in other studies, based on the study case of the DART impact onto the asteroid Dimorphos. We find that the typical quadratic crush curve parameters do not closely follow the silica crushing experiments, and as a consequence they under (over) estimate compression close (far) from the impact site. The new crush curve presented here, applicable to pressures between a few hundred Pa and up to 1.1 GPa, might therefore be more precise. Additionally, it is not calibrated by case-specific parameters, and can be used universally for comet- or asteroid-like bodies, given an assumed composition similar to micro-granular silica.

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arXiv2024

A link between the size and composition of comets

All cometary nuclei that formed in the early Solar System incorporated radionuclides and therefore were subject to internal radiogenic heating. Previous work predicts that if comets have a pebble-pile structure internal temperature build-up is enhanced due to very low thermal conductivity, leading to internal differentiation. An internal thermal gradient causes widespread sublimation and migration of either ice condensates, or gases released from amorphous ice hosts during their crystallisation. Overall, the models predict that the degree of differentiation and re-distribution of volatile species to a shallower near-surface layer depends primarily on nucleus size. Hence, we hypothesise that cometary activity should reveal a correlation between the abundance of volatile species and the size of the nucleus. To explore this hypothesis we have conducted a thorough literature search for measurements of the composition and size of cometary nuclei, compiling these into a unified database. We report a statistically significant correlation between the measured abundance of CO/H$_{2}$O and the size of cometary nuclei. We further recover the measured slope of abundance as a function of size, using a theoretical model based on our previous thermophysical models, invoking re-entrapment of outward migrating high volatility gases in the near-surface pristine amorphous ice layers. This model replicates the observed trend and supports the theory of internal differentiation of cometary nuclei by early radiogenic heating. We make our database available for future studies, and we advocate for collection of more measurements to allow more precise and statistically significant analyses to be conducted in the future.

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arXiv2024

Uranus and Neptune as methane planets: producing icy giants from refractory planetesimals

Uranus and Neptune are commonly considered ice giants, and it is often assumed that, in addition to a solar mix of hydrogen and helium, they contain roughly twice as much water as rock. This classical picture has led to successful models of their internal structure and has been understood to be compatible with the composition of the solar nebula during their formation (Reynolds and Summers 1965; Podolak and Cameron 1974; Podolak and Reynolds 1984; Podolak et al. 1995; Nettelmann et al. 2013). However, the dominance of water has been recently questioned (Teanby et al. 2020; Helled and Fortney 2020; Podolak et al. 2022). Planetesimals in the outer solar system are composed mainly of refractory materials, leading to an inconsistency between the icy composition of Uranus and Neptune and the ice-poor planetesimals they accreted during formation (Podolak et al. 2022). Here we elaborate on this problem, and propose a new potential solution. We show that chemical reactions between planetesimals dominated by organic-rich refractory materials and the hydrogen in gaseous atmospheres of protoplanets can form large amounts of methane 'ice'. Uranus and Neptune could thus be compatible with having accreted refractory-dominated planetesimals, while still remaining icy. Using random statistical computer models for a wide parameter space, we show that the resulting methane-rich internal composition could be a natural solution, giving a good match to the size, mass and moment of inertia of Uranus and Neptune, whereas rock-rich models appear to only work if a rocky interior is heavily mixed with hydrogen. Our model predicts a lower than solar hydrogen to helium ratio, which can be tested. We conclude that Uranus, Neptune and similar exoplanets could be methane-rich, and discuss why Jupiter and Saturn cannot.

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arXiv2024

White dwarf systems: exoplanets and debris disks

Although there is abundant and diverse observational evidence in support of white dwarf stars hosting planets or debris disks which form in the catastrophic destruction of various planetary bodies, the key processes that explain these observations are still being intensely investigated. The study of white dwarf planetary systems offers a unique perspective on exo-solar composition, that cannot be obtained by any other means. This chapter describes the various observational techniques that are used in order to find and characterize exo-planets and debris disks around white dwarfs. In turn, it discusses how to theoretically interpret these observations by surveying an array of various research tools and models currently employed in this field.

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arXiv2023

An Equation of State of CO for use in Planetary Modeling

Although carbon monoxide (CO) is an abundant molecule and may have great importance for planetary interiors, measurements of its properties are difficult due to its extreme volatility. We calculate the equation of state for CO over a range of temperature and density that is applicable to the conditions in planetary interiors. Previous experimental and theoretical studies cover only a limited temperature-density range. Our calculations match these early results well, but now cover the full range of relevance. The method of calculation is based on the general-purpose quotidian equation of state described by More et al. (1988), which is here used in order to generate a freely downloadable look-up table to be used by the community.

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arXiv2022

Asynchronous accretion can mimic diverse white dwarf pollutants I: core and mantle fragments

Polluted white dwarfs serve as astrophysical mass spectrometers - their photospheric abundances are used to infer the composition of planetary objects that accrete onto them. We show that due to asymmetries in the accretion process, the composition of the material falling onto a star may vary with time during the accretion of a single planetary body. Consequently, the instantaneous photospheric abundances of white dwarfs do not necessarily reflect the bulk composition of their pollutants, especially when their diffusion timescales are short. In particular, we predict that when an asteroid with an iron core tidally disrupts around a white dwarf, a larger share of its mantle is ejected, and that the core/mantle fraction of the accreting material varies with time during the event. Crucially, this implies that the core fraction of differentiated pollutants cannot be determined for white dwarfs with short diffusion timescales, which sample only brief episodes of longer accretion processes. The observed population of polluted white dwarfs backs up the proposed theory. More white dwarfs have accreted material with high Fe/Ca than low Fe/Ca relative to stellar abundance ratios, indicating the ejection of mantle material. Additionally, we find tentative evidence that the accretion rate of iron decreases more rapidly than that of magnesium or calcium, hinting at variability of the accreted composition. Further corroboration of the proposed theory will come from the up-coming analysis of large samples of young white dwarfs.

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arXiv2022

Asynchronous accretion can mimic diverse white dwarf pollutants II: water content

Volatiles, notably water, are key to the habitability of rocky planets. The presence of water in planetary material can be inferred from the atmospheric oxygen abundances of polluted white dwarfs, but this interpretation is often complex. We study the accretion process, and find that ices may sublimate and accrete before more refractory minerals reach the star. As a result, a white dwarf's relative photospheric abundances may vary with time during a single accretion event, and do not necessarily reflect the bulk composition of a pollutant. We offer two testable predictions for this hypothesis: 1. cooler stars will more often be inferred to have accreted wet pollutants, and 2. there will be rare occurrences of accretion events with inferred volatile levels far exceeding those of pristine comets. To observationally test these predictions, we statistically constrain the water content of white dwarf pollutants. We find that in the current sample, only three stars show statistically significant evidence of water at the 2$σ$ level, due to large typical uncertainties in atmospheric abundances and accretion states. In the future, an expanded sample of polluted white dwarfs with hydrogen-dominated atmospheres will allow for the corroboration of our theoretical predictions. Our work also shows the importance of interpreting pollutant compositions statistically, and emphasizes the requirement to reduce uncertainties on measured abundances to allow for statistically significant constraints on their water content.

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arXiv2022

Are there any pristine comets? Constraints from pebble structure

We show that if comets (or any small icy planetesimals such as Kuiper belt objects) are composed of pebble piles, their internal radiogenic as well as geochemical heating results in considerably different evolutionary outcomes compared to similar past studies. We utilize a 1D thermo-physical evolution code, modified to include state-of-the-art empirical measurements of pebble thermal conductivity and compression, the latter obtained through a new laboratory experiment presented here for the first time. Results indicate that due to the low pebble thermal conductivity, the peak temperatures attained during evolution are much higher than in any previous study given the same formation time. Assuming meteoritic radiogenic abundances, we find that only extremely small, sub-kilometre comets have the potential to retain the primordial, uniform and thermally unprocessed composition from which they formed. Comets with radii in excess of about 20 km are typically swept by rapid and energetically powerful aqueous hydration reactions. Across the full range of comet sizes and formation times, evolutions result in the processing and differentiation of various volatile species, and a radially heterogeneous nucleus stucture. Our computations however also indicate that the assumed fraction of radionuclides is a pivotal free parameter, because isotopic analyses of the only available cometary samples suggest that no 26Al was ever present in comet 81P/Wild 2. We show that if comets formed early in the protoplanetary disc (within 1-3 Myr), the radionuclide abundances indeed must be much smaller than those typically assumed based on meteoritic samples. We discuss the importance of our findings for the formation, present-day attributes and future research of comets.

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arXiv2022

Random Models for Exploring Planet Compositions I: Uranus as an Example

Modeling the interior of a planet is difficult because the small number of measured parameters is insufficient to constrain the many variables involved in describing the interior structure and composition. One solution is to invoke additional constraints based on arguments about how the planet formed. However, a planet's actual structure and composition may hold clues to its formation which would be lost if this structure were not allowed by the initial assumptions. It is therefore interesting to explore the space of allowable compositions and structures in order to better understand which cosmogonic constraints are absolutely necessary. To this end, we describe a code for generating random, monotonic, density distributions, rho(r), that fit a given mass, radius, and moment of inertia. Integrating the equation of hydrostatic equilibrium gives the pressure, P(r), at each point in the body. We then provide three algorithms for generating a monotonic temperature distribution, T(r), and an associated composition that is consistent with the rho-P relation, and realistic equations of state. We apply this code to Uranus as a proof of concept, and show that the ratio of rock to water cannot be much larger than 2.

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arXiv2021

A road-map to white dwarf pollution: Tidal disruption, eccentric grind-down, and dust accretion

A significant fraction of white dwarfs show metal lines indicative of pollution with planetary material but the accretion process remains poorly understood. The main aim of this paper is to produce a road-map illustrating several potential routes for white dwarf pollution and to link these paths to observational outcomes. Our proposed main road begins with the tidal disruption of a scattered asteroid and the formation of a highly eccentric tidal disc with a wide range of fragment sizes. Accretion of these fragments by Poynting-Robertson (PR) drag alone is too slow to explain the observed rates. Instead, in the second stage, several processes including differential apsidal precession cause high-velocity collisions between the eccentric fragments. Large asteroids produce more fragments when they disrupt, causing rapid grind-down and generating short and intense bursts of dust production, whereas smaller asteroids grind down over longer periods of time. In the final stage, the collisionally produced dust circularises and accretes onto the white dwarf via drag forces. We show that optically thin dust accretion by PR drag produces large infrared (IR) excesses when the accretion rate exceeds 10^7 g/s. We hypothesise that around white dwarfs accreting at a high rate, but with no detected infrared excess, dust circularisation requires enhanced drag - for instance due to the presence of gas near the disc's pericentre.

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arXiv2021

The entry geometry and velocity of planetary debris into the Roche sphere of a white dwarf

Our knowledge of white dwarf planetary systems predominately arises from the region within a few Solar radii of the white dwarfs, where minor planets break up, form rings and discs, and accrete onto the star. The entry location, angle and speed into this Roche sphere has rarely been explored but crucially determines the initial geometry of the debris, accretion rates onto the photosphere, and ultimately the composition of the minor planet. Here we evolve a total of over 10^5 asteroids with single-planet N-body simulations across the giant branch and white dwarf stellar evolution phases to quantify the geometry of asteroid injection into the white dwarf Roche sphere as a function of planetary mass and eccentricity. We find that lower planetary masses increase the extent of anisotropic injection and decrease the probability of head-on (normal to the Roche sphere) encounters. Our results suggest that one can use dynamical activity within the Roche sphere to make inferences about the hidden architectures of these planetary systems.

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arXiv2021

Circularization of tidal debris around white dwarfs: implications for gas production and dust variability

White dwarf (WD) pollution is thought to arise from the tidal disruption of planetary bodies. The initial fragment stream is extremely eccentric, while observational evidence suggest that discs are circular or nearly so. Here we propose a novel mechanism to bridge this gap and show that the fragments can rapidly circularise through dust or gas drag when they interact with a pre-existing compact disc. We assume that the tidal stream mainly consists of small cohesive fragments in the size range 10-1000 m, capable of resisting the WD tidal forces, whereas the compact discs span a wide mass range. We provide an analytical model, accompanied by N-body simulations, and find a large parameter space in fragment sizes and orbital separation that leads to full circularization. Partial circularization is possible for compact discs that are several orders of magnitudes less massive. We show that dust-induced circularization inherently produces gas as tidal fragments collisionally vaporize the pre-existing dust along their path. We show that ongoing gas production has a higher probability to occur during the early stages of tidal disruption events, resulting from the fact that smaller fragments are the first to circularize. Intermittent gas production however becomes more likely as the tidal stream matures. This could explain why only a small subset of systems with dusty compact discs also have an observed gaseous component. Additionally, the interaction yields fragment erosion by collisional shattering, sputtering, sublimation and possibly ram-pressure. Material scattered by the collisions might form a thin dusty halo that evolves through PR drag, in compatibility with observed infrared variability.

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arXiv2020

Origin of (2014) MU69-like Kuiper-belt contact binaries from wide binaries

Following its flyby and first imaging the Pluto-Charon binary, the New Horizons spacecraft visited the Kuiper-Belt-Object (KBO) (486958) 2014 MU69 (Arrokoth). Imaging showed MU69 to be a contact-binary, made of two individual lobes connected by a narrow neck, rotating at low spin period (15.92 h), and having high obliquity (~98 deg), similar to other KBO contact-binaries inferred through photometric observations. The origin of such peculiar configurations is puzzling, and all scenarios suggested for the origins of contact-binaries fail to reproduce such properties and their likely high frequency. Here we show that semi-secular perturbations operating only on ultra-wide (~0.1-0.4 Hill-radius) KBO-binaries can robustly lead to gentle, slow-speed binary mergers at arbitrarily high obliquities, but low rotational velocities, that can reproduce MU69's (and similar oblique contact binaries) characteristics. Using N-body simulations, we find that ~15% of all ultra-wide binaries with cosine-uniform inclination distribution are likely to merge through this process. Moreover, we find that such mergers are sufficiently gentle as to only slightly deform the KBO shape, and can produce the measured rotation speed of MU69. The semi-secular contact-binary formation channel not only explains the observed properties of MU69, but could also apply for other Kuiper/asteroid belt binaries, and for Solar/extra-solar moon systems.

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arXiv2020

Collisional formation of massive exomoons of super-terrestrial exoplanets

Exomoons orbiting terrestrial or super-terrestrial exoplanets have not yet been discovered; their possible existence and properties are therefore still an unresolved question. Here we explore the collisional formation of exomoons through giant planetary impacts. We make use of smooth particle hydrodynamical (SPH) collision simulations and survey a large phase-space of terrestrial/super-terrestrial planetary collisions. We characterize the properties of such collisions, finding one rare case in which an exomoon forms through a graze&capture scenario, in addition to a few graze&merge or hit&run scenarios. Typically however, our collisions form massive circumplanetary discs, for which we use follow-up N-body simulations in order to derive lower-limit mass estimates for the ensuing exomoons. We investigate the mass, long-term tidal-stability, composition and origin of material in both the discs and the exomoons. Our giant-impact models often generate relatively iron-rich moons, that form beyond the synchronous radius of the planet, and would thus tidally evolve outward with stable orbits, rather than be destroyed. Our results suggest that it is extremely difficult to collisionally form currently-detectable exomoons orbiting super-terrestrial planets, through single giant impacts. It might be possible to form massive, detectable exomoons through several mergers of smaller exomoons, formed by multiple impacts, however more studies are required in order to reach a conclusion. Given the current observational initiatives, the search should focus primarily on more massive planet categories. However, about a quarter of the exomoons predicted by our models are approximately Mercury-mass or more, and are much more likely to be detectable given a factor 2 improvement in the detection capability of future instruments, providing further motivation for their development.

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