arXiv subjects

Troels Haugboelle

Publications and source records attributed to Troels Haugboelle.

At least 19 recordsLinked to original sources

arXiv2025

The Formation of Protoplanetary Disks through Pre-Main Sequence Bondi-Hoyle Accretion

Protoplanetary disks are traditionally described as finite mass reservoirs left over by the gravitational collapse of the protostellar core, a view that strongly constrains both disk evolution and planet formation models. We propose a different scenario where protoplanetary disks of pre-main sequence stars are primarily assembled by Bondi-Hoyle accretion from the parent gas cloud. We demonstrate that Bondi-Hoyle accretion can supply not only the mass, but also the angular momentum necessary to explain the observed size of protoplanetary disks. Additionally, we predict how the specific angular momentum of protoplanetary disks scales with stellar mass. Our conclusions are based on a new analytical derivation of the scaling of the angular momentum in turbulent flows, which we confirm with a numerical simulation of supersonic turbulence. A key outcome of our analysis is the recognition that density fluctuations in supersonic turbulence--previously overlooked in studies of cloud and core rotation--lead to a significant increase in angular momentum at disk-forming scales. This revised understanding of disk formation and evolution alleviates several longstanding observational discrepancies and compels substantial revisions to current models of disk and planet formation.

astro-ph.GA↗

arXiv2021

Hybrid accretion of carbonaceous chondrites by radial transport across the Jupiter barrier

Understanding the origin of chondritic components and their accretion pathways is critical to unravel the magnitude of mass transport in the protoplanetary disk, the accretionary history of the terrestrial planet region and, by extension, its prebiotic inventory. Here, we trace the heritage of pristine components from the relatively unaltered CV chondrite Leoville through their mass-independent Cr and mass-dependent Zn isotope compositions. Investigating these chondritic fractions in such detail reveals an onion-shell structure of chondrules, which is characterized by 54Cr- and 66Zn-poor cores surrounded by increasingly 54Cr- and 66Zn-rich igneous rims and an outer coating of fine-grained dust. This is interpreted as a progressive addition of 54Cr- and 66Zn-rich, CI-like material to the accretion region of these carbonaceous chondrites. Our findings show that the observed Cr isotopic range in chondrules from more altered CV chondrites is the result of chemical equilibration between chondrules and matrix during secondary alteration. The 54Cr-poor nature of the cores of Leoville chondrules implies formation in the inner Solar System and subsequent massive outward chondrule transport past the Jupiter barrier. At the same time, CI-like dust is transferred inwards. We propose that the accreting Earth acquired CI-like dust through this mechanism within the lifetime of the disk. This radial mixing of chondrules and matrix shows the limited capacity of Jupiter to act as an efficient barrier and maintain the proposed non-carbonaceous and carbonaceous chondrite dichotomy over time. Finally, also considering current astrophysical models, we explore both inner and outer Solar System origins for the CV chondrite parent body.

astro-ph.EP↗

arXiv2017

Zoom-Simulations of Protoplanetary Disks starting from GMC scales

We investigate the formation of protoplanetary disks around nine solar mass stars formed in the context of a (40 pc)$^3$ Giant Molecular Cloud model, using RAMSES adaptive-mesh refinement simulations extending over a scale range of about 4 million, from an outer scale of 40 pc down to cell sizes of 2 AU. Our most important result is that the accretion process is heterogeneous in multiple ways; in time, in space, and among protostars of otherwise similar mass. Accretion is heterogeneous in time, in the sense that accretion rates vary during the evolution, with generally decreasing profiles, whose slopes vary over a wide range, and where accretion can increase again if a protostar enters a region with increased density and low speed. Accretion is heterogeneous in space, because of the mass distribution, with mass approaching the accreting star-disk system in filaments and sheets. Finally, accretion is heterogeneous among stars, since the detailed conditions and dynamics in the neighborhood of each star can vary widely. We also investigate the sensitivity of disk formation to physical conditions, and test their robustness by varying numerical parameters. We find that disk formation is robust even when choosing the least favorable sink particle parameters, and that turbulence cascading from larger scales is a decisive factor in disk formation. We also investigate the transport of angular momentum, finding that the net inward mechanical transport is compensated for mainly by an outward directed magnetic transport, with a contribution from gravitational torques usually subordinate to the magnetic transport.

astro-ph.SR↗

arXiv2016

Tracking the distribution of $^{26}$Al and $^{60}$Fe during the early phases of star and disk evolution

The short-lived $^{26}$Al and $^{60}$Fe radionuclides are synthesized and expelled in the interstellar medium by core-collapse supernova events. The solar system's first solids, calcium-aluminium refractory inclusions (CAIs), contain evidence for the former presence of the $^{26}$ Al nuclide defining the canonical $^{26}$Al/$^{27}$ Al ratio of $\sim5 \times10^{-5}$. A different class of objects temporally related to canonical CAIs are CAIs with fractionation and unidentified nuclear effects (FUN CAIs), which record a low initial $^{26}$Al/$^{27}$Al of $10^{-6}$. The contrasting level of $^{26}$Al between these objects is often interpreted as reflecting the admixing of the $^{26}$Al nuclide during the early formative phase of the Sun. We use giant molecular cloud (GMC) scale adaptive mesh-refinement numerical simulations to trace the abundance of $^{26}$Al and $^{60}$Fe in star-forming gas during the early stages of accretion of individual low mass protostars. We find that the $^{26}$Al/$^{27}$Al and $^{60}$Fe/$^{56}$Fe ratios of accreting gas within a vicinity of 1000 AU of the stars follow the predicted decay curves of the initial abundances at time of star formation without evidence of spatial or temporal heterogeneities for the first 100 kyr of star formation. Therefore, the observed differences in $^{26}$Al/$^{27}$Al ratios between FUN and canonical CAIs are likely not caused by admixing of supernova material during the early evolution of the proto-Sun. Selective thermal processing of dust grains is a more viable scenario to account for the heterogeneity in $^{26}$Al/$^{27}$Al ratios at the time of solar system formation.

astro-ph.SR↗

arXiv2016

Supernova Driving. I. The Origin of Molecular Cloud Turbulence

Turbulence is ubiquitous in molecular clouds (MCs), but its origin is still unclear because MCs are usually assumed to live longer than the turbulence dissipation time. Interstellar medium (ISM) turbulence is likely driven by SN explosions, but it has never been demonstrated that SN explosions can establish and maintain a turbulent cascade inside MCs consistent with the observations. In this work, we carry out a simulation of SN-driven turbulence in a volume of (250 pc)$^3$, specifically designed to test if SN driving alone can be responsible for the observed turbulence inside MCs. We find that SN driving establishes a velocity scaling consistent with the usual scaling laws of supersonic turbulence, suggesting that previous idealized simulations of MC turbulence, driven with a random, large-scale volume force, were correctly adopted as appropriate models for MC turbulence, despite the artificial driving. We also find that the same scaling laws extend to the interior of MCs, and that the velocity-size relation of the MCs selected from our simulation is consistent with that of MCs from the Outer-Galaxy Survey, the largest MC sample available. The mass-size relation and the mass and size probability distributions also compare successfully with those of the Outer Galaxy Survey. Finally, we show that MC turbulence is super-Alfvénic with respect to both the mean and rms magnetic-field strength. We conclude that MC structure and dynamics are the natural result of SN-driven turbulence.

astro-ph.GA↗

arXiv2012

Photon-Plasma: a modern high-order particle-in-cell code

We present the Photon-Plasma code, a modern high order charge conserving particle-in-cell code for simulating relativistic plasmas. The code is using a high order implicit field solver and a novel high order charge conserving interpolation scheme for particle-to-cell interpolation and charge deposition. It includes powerful diagnostics tools with on-the-fly particle tracking, synthetic spectra integration, 2D volume slicing, and a new method to correctly account for radiative cooling in the simulations. A robust technique for imposing (time-dependent) particle and field fluxes on the boundaries is also presented. Using a hybrid OpenMP and MPI approach the code scales efficiently from 8 to more than 250.000 cores with almost linear weak scaling on a range of architectures. The code is tested with the classical benchmarks particle heating, cold beam instability, and two-stream instability. We also present particle-in-cell simulations of the Kelvin-Helmholtz instability, and new results on radiative collisionless shocks.

astro-ph.SR↗

arXiv2012

A Simple Law of Star Formation

We show that supersonic MHD turbulence yields a star formation rate (SFR) as low as observed in molecular clouds (MCs), for characteristic values of the free-fall time divided by the dynamical time, $t_{\rm ff}/t_{\rm dyn}$, the alfvénic Mach number, ${\cal M}_{\rm a}$, and the sonic Mach number, ${\cal M}_{\rm s}$. Using a very large set of deep adaptive-mesh-refinement simulations, we quantify the dependence of the SFR per free-fall time, $ε_{\rm ff}$, on the above parameters. Our main results are: i) $ε_{\rm ff}$ decreases exponentially with increasing $t_{\rm ff}/t_{\rm dyn}$, but is insensitive to changes in ${\cal M}_{\rm s}$, for constant values of $t_{\rm ff}/t_{\rm dyn}$ and ${\cal M}_{\rm a}$. ii) Decreasing values of ${\cal M}_{\rm a}$ (stronger magnetic fields) reduce $ε_{\rm ff}$, but only to a point, beyond which $ε_{\rm ff}$ increases with a further decrease of ${\cal M}_{\rm a}$. iii) For values of ${\cal M}_{\rm a}$ characteristic of star-forming regions, $ε_{\rm ff}$ varies with ${\cal M}_{\rm a}$ by less than a factor of two. We propose a simple star-formation law, based on the empirical fit to the minimum $ε_{\rm ff}$, and depending only on $t_{\rm ff}/t_{\rm dyn}$: $ε_{\rm ff} \approx ε_{\rm wind} \exp(-1.6 \,t_{\rm ff}/t_{\rm dyn})$. Because it only depends on the mean gas density and rms velocity, this law is straightforward to implement in simulations and analytical models of galaxy formation and evolution.

astro-ph.GA↗

arXiv2012

Halo abundances and shear in void models

We study the non-linear gravitational collapse of dark matter into halos through numerical N-body simulations of Lemaitre-Tolman-Bondi void models. We extend the halo mass function formalism to these models in a consistent way. This extension not only compares well with the simulated data at all times and radii, but it also gives interesting clues about the impact of the background shear on the growth of perturbations. Our results give hints about the possibility of constraining the background shear via cluster number counts, which could then give rise to strong constraints on general inhomogeneous models, of any scale.

astro-ph.CO↗

arXiv2011

Radiation from sub-Larmor scale magnetic fields

Spontaneous rapid growth of strong magnetic fields is rather ubiquitous in high-energy density environments ranging from astrophysical sources (e.g., gamma-ray bursts and relativistic shocks), to reconnection, to laser-plasma interaction laboratory experiments, where they are produced by kinetic streaming instabilities of the Weibel type. Relativistic electrons propagating through these sub-Larmor-scale magnetic fields radiate in the jitter regime, in which the anisotropy of the magnetic fields and the particle distribution have a strong effect on the produced radiation. Here we develop the general theory of jitter radiation, which includes (i) anisotropic magnetic fields and electron velocity distributions, (ii) the effects of trapped electrons and (iii) extends the description to large deflection angles of radiating particles thus establishing a cross-over between the classical jitter and synchrotron regimes. Our results are in remarkable agreement with the radiation spectra obtained from particle-in-cell simulations of the classical Weibel instability. Particularly interesting is the onset of the field growth, when the transient hard synchrotron-violating spectra are common as a result of the dominant role of the trapped population. This effect can serve as a distinct observational signature of the violent field growth in astrophysical sources and lab experiments. It is also interesting that a system with small-scale fields tends to evolve toward the small-angle jitter regime, which can, under certain conditions, dominate the overall emission of a source.

astro-ph.HE↗

arXiv2011

Large scale structure simulations of inhomogeneous LTB void models

We perform numerical simulations of large scale structure evolution in an inhomogeneous Lemaitre-Tolman-Bondi (LTB) model of the Universe. We follow the gravitational collapse of a large underdense region (a void) in an otherwise flat matter-dominated Einstein-deSitter model. We observe how the (background) density contrast at the centre of the void grows to be of order one, and show that the density and velocity profiles follow the exact non-linear LTB solution to the full Einstein equations for all but the most extreme voids. This result seems to contradict previous claims that fully relativistic codes are needed to properly handle the non-linear evolution of large scale structures, and that local Newtonian dynamics with an explicit expansion term is not adequate. We also find that the (local) matter density contrast grows with the scale factor in a way analogous to that of an open universe with a value of the matter density OmegaM(r) corresponding to the appropriate location within the void.

astro-ph.CO↗

arXiv2010

Radiation Spectral Synthesis of Relativistic Filamentation

Radiation from many astrophysical sources, e.g. gamma-ray bursts and active galactic nuclei, is believed to arise from relativistically shocked collisionless plasmas. Such sources often exhibit highly transient spectra evolving rapidly, compared with source lifetimes. Radiation emitted from these sources is typically associated with non-linear plasma physics, complex field topologies and non-thermal particle distributions. In such circumstances a standard synchrotron paradigm may fail to produce accurate conclusions regarding the underlying physics. Simulating spectral emission and spectral evolution numerically in various relativistic shock scenarios is then the only viable method to determine the detailed physical origin of the emitted spectra. In this Letter we present synthetic radiation spectra representing the early stage development of the filamentation (streaming) instability of an initially unmagnetized plasma, which is relevant for both collisionless shock formation and reconnection dynamics in relativistic astrophysical outflows, as well as for laboratory astrophysics experiments. Results were obtained using a highly efficient "in situ" diagnostics method, based on detailed particle-in-cell modeling of collisionless plasmas. The synthetic spectra obtained here are compared with those predicted by a semi-analytical model for jitter radiation from the filamentation instability, the latter including self-consistent generated field topologies and particle distributions obtained from the simulations reported upon here. Spectra exhibit dependence on the presence - or absence - of an inert plasma constituent, when comparing baryonic plasmas (i.e. containing protons) with pair plasmas. The results also illustrate that considerable care should be taken when using lower-dimensional models to obtain information about the astrophysical phenomena generating observed spectra.

astro-ph.HE↗

arXiv2010

3D modeling of collisionless shocks

Two dimensional modeling of collisionless shocks has been of tremendous importance in understanding the physics of the non-linear evolution, momentum transfer and particle acceleration, but current computer capacities have now reached a point where three dimensional modeling is becoming feasible. We present the first three dimensional model of a fully developed and relaxed relativistic ion-electron shock, and analyze and compare it to similar 2D models. Quantitative and qualitative differences are found with respect to the two-dimensional models. The shock jump conditions are of course different, because of the extra degree of freedom, but in addition it is found that strong parallel electric fields develop at the shock interface, the level of magnetic field energy is lower, and the non-thermal particle distribution is shallower with a powerlaw index of ~2.2.

astro-ph.HE↗

arXiv2010

Neutrinos in Non-linear Structure Formation - The Effect on Halo Properties

We use N-body simulations to find the effect of neutrino masses on halo properties, and investigate how the density profiles of both the neutrino and the dark matter components change as a function of the neutrino mass. We compare our neutrino density profiles with results from the N-one-body method and find good agreement. We also show and explain why the Tremaine-Gunn bound for the neutrinos is not saturated. Finally we study how the halo mass function changes as a function of the neutrino mass and compare our results with the Sheth-Tormen semi-analytic formulae. Our results are important for surveys which aim at probing cosmological parameters using clusters, as well as future experiments aiming at measuring the cosmic neutrino background directly.

astro-ph.CO↗

arXiv2010

Non-Gaussianity from Axion Monodromy Inflation

We study the primordial non-Gaussinity predicted from simple models of inflation with a linear potential and superimposed oscillations. This generic form of the potential is predicted by the axion monodromy inflation model, that has recently been proposed as a possible realization of chaotic inflation in string theory, where the monodromy from wrapped branes extends the range of the closed string axions to beyond the Planck scale. The superimposed oscillations in the potential can lead to new signatures in the CMB spectrum and bispectrum. In particular the bispectrum will have a new distinct shape. We calculate the power spectrum and bispectrum of curvature perturbations in the model, as well as make analytic estimates in various limiting cases. From the numerical analysis we find that for a wide range of allowed parameters the model produces a feature in the bispectrum with fnl ~ 50 or larger while the power spectrum is almost featureless. This model is therefore an example of a string-inspired inflationary model which is testable mainly through its non-Gaussian features. Finally we provide a simple analytic fitting formula for the bispectrum which is accurate to approximately 5% in all cases, and easily implementable in codes designed to provide non-Gaussian templates for CMB analyses.

hep-ph↗

arXiv2008

The radial BAO scale and Cosmic Shear, a new observable for Inhomogeneous Cosmologies

As an alternative explanation of the dimming of distant supernovae it has recently been advocated that we live in a special place in the Universe near the centre of a large spherical void described by a Lemaitre-Tolman-Bondi (LTB) metric. In this scenario, the Universe is no longer homogeneous and isotropic, and the apparent late time acceleration is actually a consequence of spatial gradients. We propose in this paper a new observable, the normalized cosmic shear, written in terms of directly observable quantities, and calculable in arbitrary inhomogeneous cosmologies. This will allow future surveys to determine whether we live in a homogeneous universe or not. In this paper we also update our previous observational constraints from geometrical measures of the background cosmology. We include the Union Supernovae data set of 307 Type Ia supernovae, the CMB acoustic scale and the first measurement of the radial baryon acoustic oscillation scale. Even though the new data sets are significantly more constraining, LTB models -- albeit with slightly larger voids -- are still in excellent agreement with observations, at chi^2/d.o.f. = 307.7/(310-4)=1.005. Together with the paper we also publish the updated easyLTB code used for calculating the models and for comparing them to the observations.

astro-ph↗

arXiv2008

Confronting Lemaitre-Tolman-Bondi models with Observational Cosmology

The possibility that we live in a special place in the universe, close to the centre of a large void, seems an appealing alternative to the prevailing interpretation of the acceleration of the universe in terms of a LCDM model with a dominant dark energy component. In this paper we confront the asymptotically flat Lemaitre-Tolman-Bondi (LTB) models with a series of observations, from Type Ia Supernovae to Cosmic Microwave Background and Baryon Acoustic Oscillations data. We propose two concrete LTB models describing a local void in which the only arbitrary functions are the radial dependence of the matter density Omega_M and the Hubble expansion rate H. We find that all observations can be accommodated within 1 sigma, for our models with 4 or 5 independent parameters. The best fit models have a chi^2 very close to that of the LCDM model. We perform a simple Bayesian analysis and show that one cannot exclude the hypothesis that we live within a large local void of an otherwise Einstein-de Sitter model.

astro-ph↗

arXiv2008

The Effect of Thermal Neutrino Motion on the Non-linear Cosmological Matter Power Spectrum

We have performed detailed studies of non-linear structure formation in cosmological models with light neutrinos. For the first time the effect of neutrino thermal velocities has been included in a consistent way, and the effect on the matter power spectrum is found to be significant. The effect is large enough to be measured in future, high precision surveys. Additionally, we provide a simple but accurate analytic expression for the suppression of fluctuation power due to massive neutrinos. Finally, we describe a simple and fast method for including the effect of massive neutrinos in large-scale N-body simulations which is accurate at the 1% level for \sum m_nu < 0.15 eV.

astro-ph↗

arXiv2008

Looking the void in the eyes - the kSZ effect in LTB models

As an alternative explanation of the dimming of distant supernovae it has recently been advocated that we live in a special place in the Universe near the centre of a large void described by a Lemaitre-Tolman-Bondi (LTB) metric. The Universe is no longer homogeneous and isotropic and the apparent late time acceleration is actually a consequence of spatial gradients in the metric. If we did not live close to the centre of the void, we would have observed a Cosmic Microwave Background (CMB) dipole much larger than that allowed by observations. Hence, until now it has been argued, for the model to be consistent with observations, that by coincidence we happen to live very close to the centre of the void or we are moving towards it. However, even if we are at the centre of the void, we can observe distant galaxy clusters, which are off-centre. In their frame of reference there should be a large CMB dipole, which manifests itself observationally for us as a kinematic Sunyaev-Zeldovich (kSZ) effect. kSZ observations give far stronger constraints on the LTB model compared to other observational probes such as Type Ia Supernovae, the CMB, and baryon acoustic oscillations. We show that current observations of only 9 clusters with large error bars already rule out LTB models with void sizes greater than approximately 1.5 Gpc and a significant underdensity, and that near future kSZ surveys like the Atacama Cosmology Telescope, South Pole Telescope, APEX telescope, or the Planck satellite will be able to strongly rule out or confirm LTB models with giga parsec sized voids. On the other hand, if the LTB model is confirmed by observations, a kSZ survey gives a unique possibility of directly reconstructing the expansion rate and underdensity profile of the void.

astro-ph↗