arXiv subjects

J. A. Kegerreis

Publications and source records attributed to J. A. Kegerreis.

5 recordsLinked to original sources

arXiv2025

Euclid Quick Data Release (Q1): VIS processing and data products

This paper describes the VIS Processing Function (VIS PF) of the Euclid ground segment pipeline, which processes and calibrates raw data from the VIS camera. We present the algorithms used in each processing element, along with a description of the on-orbit performance of VIS PF, based on Performance Verification (PV) and Q1 data. We demonstrate that the principal performance metrics (image quality, astrometric accuracy, photometric calibration) are within pre-launch specifications. The image-to-image photometric scatter is less than $0.8\%$, and absolute astrometric accuracy compared to Gaia is $5$ mas Image quality is stable over all Q1 images with a full width at half maximum (FWHM) of $0.\!^{\prime\prime}16$. The stacked images (combining four nominal and two short exposures) reach $I_\mathrm{E} = 25.6$ ($10σ$, measured as the variance of $1.\!^{\prime\prime}3$ diameter apertures). We also describe quality control metrics provided with each image, and an appendix provides a detailed description of the provided data products. The excellent quality of these images demonstrates the immense potential of Euclid VIS data for weak lensing. VIS data, covering most of the extragalactic sky, will provide a lasting high-resolution atlas of the Universe.

astro-ph.IM↗

arXiv2025

No dilute core produced in simulations of giant impacts on to Jupiter

A giant impact has been proposed as a possible formation mechanism for Jupiter's dilute core -- the planet's inferred internal structure in which the transition between its core of heavy elements and its predominantly hydrogen-helium envelope is gradual rather than a discrete interface. A past simulation suggested that a head-on impact of a $10~M_\oplus$ planet into an almost fully formed, differentiated Jupiter could lead to a post-impact planet with a smooth compositional gradient and a central heavy-element fraction as low as $Z\approx0.5$. Here, we present simulations of giant impacts on to Jupiter using improved numerical methods to reassess the feasibility of this scenario. We use the REMIX smoothed particle hydrodynamics (SPH) formulation, which has been newly developed to improve the treatment of mixing in SPH simulations. We note that, as in previous works, chemical mixing is not included in these models and that incorporating such processes at sub-particle scales could improve numerical convergence. We perform giant impact simulations with varying speeds, angles, pre-impact planet structures, and equations of state. In all of our simulations, heavy elements re-settle over short time-scales to form a differentiated core, even in cases where the core is initially disrupted into a transiently mixed state. A dilute core is not produced in any of our simulations. These results, combined with recent observations that indicate Saturn also has a dilute core, suggest that such structures are produced as part of the extended formation and evolution of giant planets, rather than through extreme, low-likelihood giant impacts.

astro-ph.EP↗

arXiv2020

Atmospheric Erosion by Giant Impacts onto Terrestrial Planets

We examine the mechanisms by which atmosphere can be eroded by giant impacts onto Earth-like planets with thin atmospheres, using 3D smoothed particle hydrodynamics simulations with sufficient resolution to directly model the fate of low-mass atmospheres. We present a simple scaling law to estimate the fraction lost for any impact angle and speed in this regime. In the canonical Moon-forming impact, only around 10% of the atmosphere would have been lost from the immediate effects of the collision. There is a gradual transition from removing almost none to almost all of the atmosphere for a grazing impact as it becomes more head-on or increases in speed, including complex, non-monotonic behaviour at low impact angles. In contrast, for head-on impacts, a slightly greater speed can suddenly remove much more atmosphere. Our results broadly agree with the application of 1D models of local atmosphere loss to the ground speeds measured directly from our simulations. However, previous analytical models of shock-wave propagation from an idealised point-mass impact significantly underestimate the ground speeds and hence the total erosion. The strong dependence on impact angle and the interplay of multiple non-linear and asymmetrical loss mechanisms highlight the need for 3D simulations in order to make realistic predictions.

astro-ph.EP↗

arXiv2020

Planetary Giant Impacts: Convergence of High-Resolution Simulations using Efficient Spherical Initial Conditions and SWIFT

We perform simulations of giant impacts onto the young Uranus using smoothed particle hydrodynamics (SPH) with over 100 million particles. This 100--1000$\times$ improvement in particle number reveals that simulations with below 10^7 particles fail to converge on even bulk properties like the post-impact rotation period, or on the detailed erosion of the atmosphere. Higher resolutions appear to determine these large-scale results reliably, but even 10^8 particles may not be sufficient to study the detailed composition of the debris -- finding that almost an order of magnitude more rock is ejected beyond the Roche radius than with 10^5 particles. We present two software developments that enable this increase in the feasible number of particles. First, we present an algorithm to place any number of particles in a spherical shell such that they all have an SPH density within 1% of the desired value. Particles in model planets built from these nested shells have a root-mean-squared velocity below 1% of the escape speed, which avoids the need for long precursor simulations to produce relaxed initial conditions. Second, we develop the hydrodynamics code SWIFT for planetary simulations. SWIFT uses task-based parallelism and other modern algorithmic approaches to take full advantage of contemporary supercomputer architectures. Both the particle placement code and SWIFT are publicly released.

astro-ph.EP↗

arXiv2018

Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion

We perform a suite of smoothed particle hydrodynamics simulations to investigate in detail the results of a giant impact on the young Uranus. We study the internal structure, rotation rate, and atmospheric retention of the post-impact planet, as well as the composition of material ejected into orbit. Most of the material from the impactor's rocky core falls in to the core of the target. However, for higher angular momentum impacts, significant amounts become embedded anisotropically as lumps in the ice layer. Furthermore, most of the impactor's ice and energy is deposited in a hot, high-entropy shell at a radius of ~3 Earth radii. This could explain Uranus' observed lack of heat flow from the interior and be relevant for understanding its asymmetric magnetic field. We verify the results from the single previous study of lower resolution simulations that an impactor with a mass of at least 2 Earth masses can produce sufficiently rapid rotation in the post-impact Uranus for a range of angular momenta. At least 90% of the atmosphere remains bound to the final planet after the collision, but over half can be ejected beyond the Roche radius by a 2 or 3 Earth mass impactor. This atmospheric erosion peaks for intermediate impactor angular momenta (~3*10^36 kg m^2 s^-1). Rock is more efficiently placed into orbit and made available for satellite formation by 2 Earth mass impactors than 3 Earth mass ones, because it requires tidal disruption that is suppressed by the more massive impactors.

astro-ph.EP↗