SciPost Astro. 3, 001 (2024) ·
published 2 July 2024

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The recently observed population of 540 freefloating Jupitermass objects, including 40 dynamically soft pairs, and two triples, in the Trapezium cluster have raised interesting questions on their formation and evolution. We test various scenarios for the origin and survivability of these free floating Jupitermass objects and Jupitermass Binary Objects (JuMBOs) in the Trapezium cluster. The numerical calculations are performed by direct $N$body integration of the stars and planets in the Trapezium cluster starting with a wide variety of planets in various configurations. We discuss four models: $\mathcal{SPP}$, in which selected stars have two outer orbiting Jupitermass planets; $\mathcal{SPM}$, where selected stars are orbited by Jupitermass planetmoon pairs; $\mathcal{ISF}$ in which JuMBOs form in situ with the stars, and $\mathcal{FFC}$, where we introduce a population of freefloating single Jupitermass objects, but no initialised binaries. Models $\mathcal{FFC}$ and $\mathcal{SPP}$ fail to produce enough JuMBOs. Models $\mathcal{SPM}$ can produce sufficient freefloaters and JuMBOs, but requires unusually wide orbits for the planetmoon system around the star. The observed JuMBOs and freefloating Jupitermass objects in the Trapezium cluster are best reproduced if they formed in pairs and as freefloaters together with the other stars in a smooth (Plummer) density profile with a virial radius of $\sim 0.5$pc. A fractal (with fractal dimension 1.6) stellar density distribution also works, but requires relatively recent formations ($\raise.5ex\hbox{$\buildrel>\over\sim$} 0.2$Myr after the other stars formed) or a high ($\raise.5ex\hbox{$\buildrel>\over\sim$} 50$%) initial binary fraction. This would make the primordial binary fraction of JuMBOs even higher than the already large observation fraction of $\sim 8$% (42/540). The fraction of JuMBOs will continue to drop with time, and the lack of JuMBOs in Upper Scorpius could then result in its higher age, causing more JuMBOs to be ionized. We then also predict that the interstellar density of Jupitermass objects (mostly singles with some $\sim 2$% lucky surviving binaries) is $\sim 0.05$ per pc$^{3}$ (or around 0.24 per star).
SciPost Astro. 2, 002 (2022) ·
published 1 April 2022

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A planet hardly ever survives the supernova of the host star in a bound orbit, because mass loss in the supernova and the natal kick imparted to the newly formed compact object cause the planet to be ejected. A planet in orbit around a binary has a considerably higher probability to survive the supernova explosion of one of the inner binary stars. In those cases, the planet most likely remains bound to the companion of the exploding star, whereas the compact object is ejected. We estimate this to happen to $\sim 1/33$ the circumbinary planetary systems. These planetary orbits tend to be highly eccentric ($e \ {\raise.5ex\hbox{$\buildrel>\over\sim$}}\ 0.9$), and $\sim 20$% of these planets have retrograde orbits compared to their former binary. The probability that the planet as well as the binary (now with a compact object) remains bound is about ten times smaller ($\sim 3\cdot 10^{3}$). We then expect the Milky way Galaxy to host $\ {\raise.5ex\hbox{$\buildrel<\over\sim$}}\ 10$ xray binaries that are still orbited by a planet, and $\ {\raise.5ex\hbox{$\buildrel<\over\sim$}}\ 150$ planets that survived in orbit around the compact object's companion. These numbers should be convolved with the fraction of massive binaries that is orbited by a planet.
SciPost Astro. 2, 001 (2022) ·
published 17 January 2022

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Since the discovery of the accelerated cosmic expansion, one of the most important tasks in observational cosmology is to determine the nature of the dark energy. We should build our understanding on a minimum of assumptions in order to avoid biases from assumed cosmological models. The two most important functions describing the evolution of the universe and its structures are the expansion function E(a) and the linear growth factor D_+(a). The expansion function has been determined in previous papers in a modelindependent way using distance moduli to typeIa supernovae and assuming only a metric theory of gravity, spatial isotropy and homogeneity. Here, we extend this analysis in three ways: (1) We extend the data sample by combining the Pantheon measurements of typeIa supernovae with measurements of baryonic acoustic oscillations; (2) we substantially simplify and generalise our method for reconstructing the expansion function; and (3) we use the reconstructed expansion function to determine the linear growth factor of cosmic structures, equally independent of specific assumptions on an underlying cosmological model other than the usual spatial symmetries. We show that the result is quite insensitive to the initial conditions for solving the growth equation, leaving the presentday matterdensity parameter {\Omega}_m0 as the only relevant parameter for an otherwise purely empirical and accurate determination of the growth factor.
SciPost Astro. 1, 001 (2020) ·
published 14 May 2020

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We present the smoothedparticle hydrodynamics simulation code, BonsaiSPH, which is a continuation of our previously developed gravityonly hierarchical $N$body code (called Bonsai). The code is optimized for Graphics Processing Unit (GPU) accelerators which enables researchers to take advantage of these powerful computational resources. BonsaSPH produces simulation results comparable with stateoftheart, CPU based, codes, but using an order of magnitude less computation time. The code is freely available online and the details are described in this work.