arXiv subjects

Dominik Schleicher

Publications and source records attributed to Dominik Schleicher.

18 recordsLinked to original sources

arXiv2026

Global 3D simulations of convection and dynamos in Red Giants

Understanding the internal dynamics of red giants is essential for studying angular momentum transport and magnetic field generation during stellar evolution. We present three-dimensional magnetohydrodynamic simulations of a $1M_{\odot}$ red giant at the bottom of the red giant branch using a star-in-a-box setup with the Pencil Code, together with ongoing efforts to refine the numerical model. By varying the rotation rate, we explore different Rossby numbers and identify systematic changes in the convective dynamics and large-scale flows. As the rotation increases, convective motions become more anisotropic, showing clear signatures of rotational constraint and a shift in the dominant convective scales. These changes are accompanied by variations in the differential rotation profile, reflecting the redistribution of angular momentum within the convective envelope. The impact of rotation is also evident in the growth rate and saturation level of the magnetic field, indicating a strong dependence of dynamo efficiency on the underlying flow structure. Together, these results highlight the interplay between convection, rotation, and large-scale flows in shaping magnetic activity in red giant stars.

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arXiv2024

Demographics of black holes at $<$100 R$_{\rm g}$ scales: accretion flows, jets, and shadows

Using the Event Horizon Telescope (EHT), the gravitationally lensed rings around the supermassive black holes (SMBHs) in Messier 87 (M87) and Sagittarius A* (Sgr A*) have now been successfully imaged at a resolution under 10 gravitational radii (R$_{\rm g}$ $ = \rm{GM/c^2}$). To expand studies beyond M87 and Sgr A*, we have constructed the Event Horizon and Environs (ETHER) sample, a comprehensive database encompassing approximately 3.15 million SMBH mass estimates, $\sim$ 20,000 Very-Long Baseline Interferometry (VLBI) radio flux densities, and $\sim$ 36,000 hard X-ray flux densities. This database is designed to identify and optimize target selection for the EHT and its upgrades on the ground and in space. We have identified a Gold Sample (GS) of nearby low-luminosity Active Galactic Nuclei (AGNs) within it that are ideal for studying jet bases and potentially imaging black hole shadows. We observed 27 of these AGNs using the EHT from 2022 to 2024, providing an opportunity to resolve and image accretion flows and jets at resolutions of $\leq$ 100 R$_{\rm g}$. Only a few SMBHs have sufficiently high enough flux density to be imaged at scales of $\leq$ 50 R$_{\rm g}$ with the present EHT. Among these are M87, Sgr A*, NGC4594 (Sombrero/M104), NGC4261, and NGC4374 (Messier 84/M84). Of these, NGC4261, Sombrero, and M84 have been observed and/or are scheduled for deep imaging with EHT+ALMA from 2023 to 2025. Sombrero, NGC4261, M84, NGC4278, and NGC5232 are clearly detected in our EHT+ALMA observations in 2022, indicating that the 230 GHz flux density from the accretion flows is significantly high. Ongoing imaging of the ETHER GS will enable measurements of black hole mass and spin, help constrain General Relativity, and enrich our understanding of jet launching and accretion inflows across a broad multi-parameter space, including black hole mass, spin, accretion rate, and orientation.

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arXiv2023

Small-scale dynamo with finite correlation times

Fluctuation dynamos occur in most turbulent plasmas in astrophysics and are the prime candidates for amplifying and maintaining cosmic magnetic fields. A few analytical models exist to describe their behaviour but they are based on simplifying assumptions. For instance the well-known Kazantsev model assumes an incompressible flow that is delta-correlated in time. However, these assumptions can break down in the interstellar medium as it is highly compressible and the velocity field has a finite correlation time. Using the renewing flow method developed by Bhat and Subramanian (2014), we aim to extend Kazantsev's results to a more general class of turbulent flows. The cumulative effect of both compressibility and finite correlation time over the Kazantsev spectrum is studied analytically. We derive an equation for the longitudinal two-point magnetic correlation function in real space to first order in the correlation time $τ$ and for an arbitrary degree of compressibility (DOC). This generalised Kazantsev equation encapsulates the original Kazantsev equation. In the limit of small Strouhal numbers $St \propto τ$ we use the WKB approximation to derive the growth rate and scaling of the magnetic power spectrum. We find the result that the Kazantsev spectrum is preserved, i.e. $M_k(k)\sim k^{3/2}$. The growth rate is also negligibly affected by the finite correlation time; however, it is reduced by the finite magnetic diffusivity, and the DOC together.

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arXiv2023

Formation of supermassive stars in the first star clusters

The formation of supermassive stars is believed to be an essential intermediate step for the formation of the massive black hole seeds that become the supermassive black holes powering the quasars observed in the early Universe. Numerical simulations have shown that supermassive stars can form in atomic-cooling halos when protostars reach accretion rates higher than $\sim10^{-2}$ M$_\odot$ yr$^{-1}$ and fragmentation is suppressed on pc scales. It is however still uncertain if a supermassive star still emerges when fragmentation occurs at smaller scales and a cluster of stars is formed instead. In this work we explore the problem of massive object formation due to the interplay of collisions and accretion in star clusters at low metallicity. We model a small embedded cluster of accreting protostars following sub-parsec scale fragmentation during the collapse of a primordial gas cloud and follow its evolution by performing $N$-body plus hydrodynamical simulations. Our results show that supermassive stars with 10$^3$ and 10$^4$ M$_\odot$ are always formed due to the interplay of collisions and accretion, and in some cases these objects are part of a binary system. The resulting supermassive star is surrounded by tens of smaller stars with typical masses in the range $1$-$100$ M$_\odot$.

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arXiv2021

The mean free path approximation and stellar collisions in star clusters: Numerical exploration of the analytic rates and the role of perturbations on binary star mergers

In this paper we compute predictions for the number of stellar collisions derived from analytic models based on the mean free path (MFP) approximation and compare them to the results of $N$-body simulations. Our goal is to identify the cluster conditions under which the MFP approximation remains valid. Adopting a range of particle numbers ($100\leq N\leq5000$) and different combinations of particle masses and radii, we explore three different channels leading to stellar collisions, all of which are expected to occur in realistic stellar environments. At high densities, binaries form from isolated three-body interactions of single stars. Hence, we consider collisions between single stars and collisions involving binary stars, after they form in our simulations. For the latter, we consider two channels for mergers, namely direct stellar collisions during chaotic single-binary interactions and perturbation-driven mergers of binaries due to random walks in eccentricity approaching unity. In the densest systems considered here, a very massive object is formed at the cluster centre, causing local stellar orbits to become increasingly Keplerian and the assumptions going into our analytic model to break down. Before reaching this limit, we obtain excellent agreement between our theoretical predictions and the simulations: the analytic rates are typically accurate to within one standard deviation for the entire parameter space considered here, but the agreement is best for short integration times. Our results have direct implications for blue straggler formation in dense star clusters, and stellar mergers in galactic nuclei hosting massive black holes.

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arXiv2021

Radiation Hydrodynamical Simulations of the Birth of Intermediate-Mass Black Holes in the First Galaxies

The leading contenders for the seeds of $z > 6$ quasars are direct-collapse black holes (DCBHs) forming in atomically-cooled halos at $z \sim$ 20. However, the Lyman-Werner (LW) UV background required to form DCBHs of 10$^5$ \Ms\ are extreme, about 10$^4$ J$_{21}$, and may have been rare in the early universe. Here we investigate the formation of intermediate-mass black holes (IMBHs) under moderate LW backgrounds of 100 and 500 J$_{21}$ that were much more common at early times. These backgrounds allow halos to grow to a few 10$^6$ - 10$^7$ \Ms\ and virial temperatures of nearly 10$^4$ K before collapsing but do not completely sterilize them of H$_2$. Gas collapse then proceeds via Ly$α$ and rapid H$_2$ cooling at rates that are 10 - 50 times those in normal Pop III star-forming haloes but less than those in purely atomically-cooled haloes. Pop III stars accreting at such rates become blue and hot, and we find that their ionizing UV radiation limits their final masses to 1800 - 2800 \Ms\, at which they later collapse to IMBHs. Moderate LW backgrounds thus produced IMBHs in far greater numbers than DCBHs in the early universe.

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arXiv2020

Small-N Collisional Dynamics V: Beyond the Realm of Not-So-Small-N

Direct collisions between finite-sized particles occur commonly in many areas of astrophysics. Such collisions are typically mediated by chaotic, bound gravitational interactions involving small numbers of particles. An important application is stellar collisions, which occur commonly in dense star clusters, and their relevance for the formation of various types of stellar exotica. In this paper, we return to our study of the collision rates and probabilities during small-number chaotic gravitational interactions ($N$ $\lesssim$ 10), moving beyond the small-number particle limit and into the realm of larger particle numbers ($N$ $\gtrsim$ 10$^3$) to test the extent of validity of our analytic model as a function of the particle properties and the number of interacting particles. This is done using direct $N$-body simulations of stellar collisions in dense star clusters, by varying the relative numbers of particles with different particle masses and radii. We compute the predicted rate of collisions using the mean free path approximation, adopting the point-particle limit and using the sticky-star approximation as our collision criterion. We evaluate its efficacy in the regime where gravitational-focusing is important by comparing the theoretical rates to numerical simulations. Using the tools developed in previous papers in this series, in particular Collision Rate Diagrams, we illustrate that our predicted and simulated rates are in excellent agreement, typically consistent with each other to within one standard deviation.

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arXiv2020

Formation of Pop II star clusters in the aftermath of a pair instability supernova

Pop II stars formed a few hundred million years after the Big Bang were key drivers of cosmic reionization and building blocks of high redshift galaxies. How and when these stars formed is a subject of ongoing research. We conduct cosmological radiation hydrodynamical simulations to investigate the formation of Pop II star clusters in dark matter halos forming at $z=10-25$ in the aftermath of a pair instability supernova (PISN). Our simulations model the formation of Pop III and Pop II stars in a self-consistent manner along with their radiative, chemical and SN feedback in halos of $\rm 5 \times 10^5- 7 \times 10^7~M_{\odot}$. We find that a PISN evacuates the gas from halos $\rm \leq 3 \times 10^{6}~M_{\odot}$ and thereafter shuts off in situ star formation for at-least 30 Myr. Pop II stellar clusters of $\rm 923~M_{\odot}$ and $\rm 6800~M_{\odot}$ form in halos of $\rm 3.5 \times 10^7~M_{\odot}$ and $\rm 7.2 \times 10^7~M_{\odot}$, respectively. The mode of star formation is highly episodic and mainly regulated by Pop II SN feedback. The average star formation rates are $\rm 10^{-5}-10^{-4}~M_{\odot}/yr$ and the star formation efficiency is less than 1\%.

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arXiv2019

The role of gas fragmentation during the formation of supermassive black holes

We have performed cosmological hydrodynamic simulations to study the effect of fragmentation on the SMBH seed mass in the direct collapse formation scenario. We considered different background UV intensities, host halo spin, and halo merger histories. Our simulations in low-spin halos, in the presence of a strong UV background are consistent with the Direct Collapse Black Hole model, in which a single massive object $\sim10^5$ M$_{\odot}$ is formed in the center of a proto-galaxy. While in our simulations under the presence of a low UV background, we find fragmentation and the formation of various minor seeds. These fragments have masses of 10$^3$ - 10$^4$ M$_{\odot}$. These values are significant if we consider the potential mergers between them, and the fact that these minor objects are formed earlier in cosmic time compared to the massive single seeds. In one of our simulations, we observed gas fragmentation even in the presence of a strong UV intensity. Said structure arose in a dark matter halo that formed after various merger episodes, and the one with the highest spin value. The final mass obtained was $\sim 10^5$ M$_{\odot}$ in this run. From these results, we conclude that fragmentation in fact produces less massive objects, however, they are still prone to merge. In simulations that formed many fragments, they all approach the most massive one with time. We see no uniqueness in the strength of the UV intensity value required to achieve a DCBH, since it depends in other factors like the system dynamics in our cases.

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arXiv2018

The formation of protostellar binaries in primordial minihalos

The first stars are known to form in primordial gas, either in minihalos with about $10^6$~M$_\odot$ or so-called atomic cooling halos of about $10^8$~M$_\odot$. Simulations have shown that gravitational collapse and disk formation in primordial gas yield dense stellar clusters. In this paper, we focus particularly on the formation of protostellar binary systems, and aim to quantify their properties during the early stage of their evolution. For this purpose, we combine the smoothed particle hydrodynamics code GRADSPH with the astrochemistry package KROME. The GRADSPH-KROME framework is employed to investigate the collapse of primordial clouds in the high-density regime, exploring the fragmentation process and the formation of binary systems. We observe a strong dependence of fragmentation on the strength of the turbulent Mach number $\mathcal{M}$ and the rotational support parameter $β$. Rotating clouds show significant fragmentation, and have produced several Pop.~III proto-binary systems. We report maximum and minimum mass accretion rates of $2.31 \times 10^{-1}$~M$_{\odot}$ yr$^{-1}$ and $2.18\times 10^{-4}$~M$_{\odot}$ yr$^{-1}$. The mass spectrum of the individual Pop III proto-binary components ranges from $0.88$~M$_{\odot}$ to $31.96$~M$_{\odot}$ and has a sensitive dependence on the Mach number $\mathcal{M}$ as well as on the rotational parameter $β$. We also report a range from $\sim0.01$ to $\sim1$ for the mass ratio of our proto-binary systems.

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arXiv2015

SKA Deep Polarization and Cosmic Magnetism

Deep surveys with the SKA1-MID array offer for the first time the opportunity to systematically explore the polarization properties of the microJy source population. Our knowledge of the polarized sky approaching these levels is still very limited. In total intensity the population will be dominated by star-forming and normal galaxies to intermediate redshifts ($z \sim1-2$), and low-luminosity AGN to high redshift. The polarized emission from these objects is a powerful probe of their intrinsic magnetic fields and of their magnetic environments. For redshift of order 1 and above the broad bandwidth of the mid-bands span the Faraday thick and thin regimes allowing study of the intrinsic polarization properties of these objects as well as depolarization from embedded and foreground plasmas. The deep field polarization images will provide Rotation Measures data with very high solid angle density allowing a sensitive statistical analysis of the angular variation of RM on critical arc-minute scales from a magnetic component of Large Scale Structure of the Universe.

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arXiv2015

Planet formation in post-common-envelope binaries

To understand the evolution of planetary systems, it is important to investigate planets in highly evolved stellar systems, and to explore the implications of their observed properties with respect to potential formation scenarios. Observations suggest the presence of giant planets in post-common-envelope binaries (PCEBs). A particularly well-studied system with planetary masses of 1.7 M_J and 7.0 M_J is NN Ser. We show here that a pure first-generation scenario where the planets form before the common envelope (CE) phase and the orbits evolve due to the changes in the gravitational potential is inconsistent with the current data. We propose a second-generation scenario where the planets are formed from the material that is ejected during the CE, which may naturally explain the observed planetary masses. In addition, hybrid scenarios where the planets form before the CE and evolve due to the accretion of the ejected gas appear as a realistic possibility.

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arXiv2015

Studies of Relativistic Jets in Active Galactic Nuclei with SKA

Relativistic jets in active galactic nuclei (AGN) are among the most powerful astrophysical objects discovered to date. Indeed, jetted AGN studies have been considered a prominent science case for SKA, and were included in several different chapters of the previous SKA Science Book (Carilli & Rawlings 2004). Most of the fundamental questions about the physics of relativistic jets still remain unanswered, and await high-sensitivity radio instruments such as SKA to solve them. These questions will be addressed specially through analysis of the massive data sets arising from the deep, all-sky surveys (both total and polarimetric flux) from SKA1. Wide-field very-long-baseline-interferometric survey observations involving SKA1 will serve as a unique tool for distinguishing between extragalactic relativistic jets and star forming galaxies via brightness temperature measurements. Subsequent SKA1 studies of relativistic jets at different resolutions will allow for unprecedented cosmological studies of AGN jets up to the epoch of re-ionization, enabling detailed characterization of the jet composition, magnetic field, particle populations, and plasma properties on all scales. SKA will enable us to study the dependence of jet power and star formation on other properties of the AGN system. SKA1 will enable such studies for large samples of jets, while VLBI observations involving SKA1 will provide the sensitivity for pc-scale imaging, and SKA2 (with its extraordinary sensitivity and dynamic range) will allow us for the first time to resolve and model the weakest radio structures in the most powerful radio-loud AGN.

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arXiv2014

Magnetic field amplification by the small-scale dynamo in the early Universe

In this paper we show that the Universe is already strongly magnetized at very early epochs during cosmic evolution. Our calculations are based on the efficient amplification of weak magnetic seed fields, which are unavoidably present in the early Universe, by the turbulent small-scale dynamo. We identify two mechanisms for the generation of turbulence in the radiation dominated epoch where velocity fluctuations are produced by the primordial density perturbation and by possible first-order phase transitions at the electroweak or QCD scales. We show that all the necessities for the small-scale dynamo to work are fulfilled. Hence, this mechanism, operating due to primordial density perturbations, guarantees fields with comoving field strength $B_0\sim10^{-6}\varepsilon^{1/2}$ nG on scales up to $λ_c\sim0.1$ pc, where $\varepsilon$ is the saturation efficiency. The amplification of magnetic seed fields could be even larger if there are first-order phase transitions in the early Universe. Where, on scales up to $λ_c\sim100$ pc, the comoving field strength due to this mechanism will be $B_0\sim10^{-3}\varepsilon^{1/2}$ nG at the present time. Such fields, albeit on small scales, can play an important role in structure formation and could provide an explanation to the apparently observed magnetic fields in the voids of the large-scale structure.

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arXiv2012

The Small-Scale Dynamo at Low Magnetic Prandtl Numbers

The present-day Universe is highly magnetized, even though the first magnetic seed fields were most probably extremely weak. To explain the growth of the magnetic field strength over many orders of magnitude fast amplification processes need to operate. The most efficient mechanism known today is the small-scale dynamo, which converts turbulent kinetic energy into magnetic energy leading to an exponential growth of the magnetic field. The efficiency of the dynamo depends on the type of turbulence indicated by the slope of the turbulence spectrum v(l) \propto l^{theta}, where v(l) is the eddy velocity at a scale l. We explore turbulent spectra ranging from incompressible Kolmogorov turbulence with theta = 1/3 to highly compressible Burgers turbulence with theta = 1/2. In this work we analyze the properties of the small-scale dynamo for low magnetic Prandtl numbers Pm, which denotes the ratio of the magnetic Reynolds number, Rm, to the hydrodynamical one, Re. We solve the Kazantsev equation, which describes the evolution of the small-scale magnetic field, using the WKB approximation. In the limit of low magnetic Prandtl numbers the growth rate is proportional to Rm^{(1-theta)/(1+theta)}. We furthermore discuss the critical magnetic Reynolds number Rm_crit, which is required for small-scale dynamo action. The value of Rm_crit is roughly 100 for Kolmogorov turbulence and 2700 for Burgers. Furthermore, we discuss that Rm_crit provides a stronger constraint in the limit of low Pm than it does for large Pm. We conclude that the small-scale dynamo can operate in the regime of low magnetic Prandtl numbers, if the magnetic Reynolds number is large enough. Thus, the magnetic field amplification on small scales can take place in a broad range of physical environments and amplify week magnetic seed fields on short timescales.

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arXiv2012

The Small-Scale Dynamo and Non-Ideal MHD in Primordial Star Formation

We study the amplification of magnetic fields during the formation of primordial halos. The turbulence generated by gravitational infall motions during the formation of the first stars and galaxies can amplify magnetic fields very efficiently and on short timescales up to dynamically significant values. Using the Kazantsev theory, which describes the so-called small-scale dynamo - a magnetohydrodynamical process converting kinetic energy from turbulence into magnetic energy - we can then calculate the growth rate of the small-scale magnetic field. Our calculations are based on a detailed chemical network and we include non-ideal magnetohydrodynamical effects such as ambipolar diffusion and Ohmic dissipation. We follow the evolution of the magnetic field up to larger scales until saturation occurs on the Jeans scale. Assuming a weak magnetic seed field generated by the Biermann battery process, both Burgers and Kolmogorov turbulence lead to saturation within a rather small density range. Such fields are likely to become relevant after the formation of a protostellar disk and, thus, could influence the formation of the first stars and galaxies in the Universe.

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arXiv2012

Magnetic Field Amplification by Small-Scale Dynamo Action: Dependence on Turbulence Models and Reynolds and Prandtl Numbers

The small-scale dynamo is a process by which turbulent kinetic energy is converted into magnetic energy, and thus is expected to depend crucially on the nature of turbulence. In this work, we present a model for the small-scale dynamo that takes into account the slope of the turbulent velocity spectrum v(l) ~ l^theta, where l and v(l) are the size of a turbulent fluctuation and the typical velocity on that scale. The time evolution of the fluctuation component of the magnetic field, i.e., the small-scale field, is described by the Kazantsev equation. We solve this linear differential equation for its eigenvalues with the quantum-mechanical WKB-approximation. The validity of this method is estimated as a function of the magnetic Prandtl number Pm. We calculate the minimal magnetic Reynolds number for dynamo action, Rm_crit, using our model of the turbulent velocity correlation function. For Kolmogorov turbulence (theta=1/3), we find that the critical magnetic Reynolds number is approximately 110 and for Burgers turbulence (theta=1/2) approximately 2700. Furthermore, we derive that the growth rate of the small-scale magnetic field for a general type of turbulence is Gamma ~ Re^((1-theta)/(1+theta)) in the limit of infinite magnetic Prandtl numbers. For decreasing magnetic Prandtl number (down to Pm approximately larger than 10), the growth rate of the small-scale dynamo decreases. The details of this drop depend on the WKB-approximation, which becomes invalid for a magnetic Prandtl number of about unity.

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arXiv2011

The First Magnetic Fields

We review current ideas on the origin of galactic and extragalactic magnetic fields. We begin by summarizing observations of magnetic fields at cosmological redshifts and on cosmological scales. These observations translate into constraints on the strength and scale magnetic fields must have during the early stages of galaxy formation in order to seed the galactic dynamo. We examine mechanisms for the generation of magnetic fields that operate prior during inflation and during subsequent phase transitions such as electroweak symmetry breaking and the quark-hadron phase transition. The implications of strong primordial magnetic fields for the reionization epoch as well as the first generation of stars is discussed in detail. The exotic, early-Universe mechanisms are contrasted with astrophysical processes that generate fields after recombination. For example, a Biermann-type battery can operate in a proto-galaxy during the early stages of structure formation. Moreover, magnetic fields in either an early generation of stars or active galactic nuclei can be dispersed into the intergalactic medium.

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