arXiv subjects

Ian A. Bonnell

Publications and source records attributed to Ian A. Bonnell.

At least 19 recordsLinked to original sources

arXiv2026

Supernova feedback in porous photoionized Giant Molecular Clouds

We present a new suite of numerical simulations of Type II supernovae (SNe) detonating in Giant Molecular Clouds with a variety of density structures shaped by photoionization feedback. Ionizing radiation sculpts cavities and channels that guide SN energy to emerge from the cloud as shock-driven blowouts, rather than as a coherent spherically expanding shell as assumed in most sub-grid SN models adopted in galaxy or cosmological simulations. We investigate how such outflows differ to the 1-D descriptions, and whether or not the perturbations induced by the blowouts are sensitive to the host cloud's structure. A channelling parameter $P_\mathrm{chnl}$ is introduced to characterise the cloud's porosity and boundness using the morphology of the ionized channels. Our results reveal that the outflow velocities, whilst consistently higher than that of the spherical blasts, are in fact rather independent of the porosity of its local environment. The total kinetic energy and momentum deposited also appear similar across all runs. What is most sensitive to $P_\mathrm{chnl}$ is the mass of the materials carried in the outflows and their migration distances. It implies that SNe exploding in compact clouds with distinctive channel structures may have more confined metal injection radii and shortened turbulent driving scales, which consequently lead to a clumpier interstellar medium with higher density and metallicity fluctuations. We argue that molecular cloud structures play an equally important role to SN rates and energy budgets in stellar feedback sub-grid modelling.

astro-ph.GA↗

arXiv2025

Semi-confined supernova feedback in HII region bubbles

Galactic-scale simulations rely on sub-grid models to provide prescriptions for the coupling between supernova (SN) feedback and the interstellar medium (ISM). Many of these models are computed in 1-D to allow for an efficient way to account for the variability of properties of their local environment. However, small-scale simulations revealed that the release of energy from SNe within molecular clouds can be highly asymmetrical. This is largely due to the presence of pre-SN feedback, such as ionizing radiation, that are able to carve cavities and channels around the progenitors prior to their detonation. Being partially confined, the SN energy escapes into the outer ISM preferentially through these channels, departing from the spherically symmetric 1-D descriptions. To understand by how much the feedback output could differ, we present a theoretical model for a semi-confined SN. The problem concerns a SN expanding into an evolved HII region, bounded by a molecular cloud with pre-existing vents. With the aid of simple 3-D hydrodynamical simulations, we show that this mode of energy release increases the local dynamical impact of the outflows, and extends the timescales over which the SN is energetically coupled to the surrounding matter. We also show that the amount of small-scale solenoidal turbulence driven by semi-confined SNe may be amplified.

astro-ph.GA↗

arXiv2025

The Role of Magnetic Fields in the Formation of High-Mass Star-Forming Cores

Magnetic fields are often invoked as playing a primary role in star formation and in the formation of high-mass stars. We investigate the effect of magnetic fields on the formation of high-mass cores using the 3-dimensional smoothed particle magnetohydrodynamics (SPMHD) code PHANTOM. We follow the collapse of six molecular clouds of mass 1000 M$_{\odot}$, four of which are initially magnetized with mass-to-flux ratios 3, 5, 10 and 100, respectively, and two purely hydrodynamic clouds with varying initial strengths of turbulence. We then apply an in-house clump-finding algorithm to the 3D SPH data in order to quantify the differences in mass and properties of the cores across the degrees of magnetic and turbulent support. We find that although the magnetic fields cause differences in the global cloud evolution, the masses and properties of the cores which form are broadly similar across the different initial conditions. Cores initially form with masses comparable to the initial thermal Jeans mass of the clouds, and then slowly increase in mass with time. We find that regardless of initial magnetization, the fields become dynamically relevant at densities of $ρ> 1\times10^{-17}$ g cm$^{-3}$ - comparable to core densities - and channel material along the field lines, decreasing the stable magnetic Jeans mass, such that the limiting factor for fragmentation is the thermal Jeans mass. We conclude that magnetic fields are not capable of forming and supporting initially high-mass cores against fragmentation.

astro-ph.GA↗

arXiv2025

Competitive Accretion and the Baryonic Fraction in Galaxies

The baryonic fraction of galaxies is observed to vary with the mass of its dark matter (DM) halo. Low-mass galaxies have low baryonic fractions which increase to a maximum for masses near $10^{12}\ M_{\odot}$, and decreases thereafter with increasing galaxy mass. This trend is generally attributed to the action of feedback from star formation at the low end and of active galactic nuclei at the high-mass end. An alternative is that the baryonic fraction is at least partially due to the ability of galaxies to competitively accrete gas in a group or clustered environment. Most galaxies in a group including those of lower masses, orbit the cluster centre at significant speeds and hence their accretion is limited by a Bondi-Hoyle type process, $\dot{M}_{acc} \propto M_{DM}^2$. In contrast, the few high-mass galaxies reside in the core of the cluster and accrete in a tidal accretion process, $\dot{M}_{acc} \propto M_{DM}^{2/3}$. These two mechanisms result in a baryonic mass fraction that increases as $M_{DM}$ at low masses and decreases as $M_{DM}^{-1/3}$ at high masses. This model predicts that lower-mass halos in small-N groups should have higher baryonic fractions relative to those in large clusters.

astro-ph.CO↗

arXiv2025

Hybrid radiation hydrodynamics scheme with adaptive gravity-tree-based pseudo-particles

HII regions powered by ionizing radiation from massive stars drive the dynamical evolution of the interstellar medium. Fast radiative transfer methods for incorporating photoionization effects are thus essential in astrophysical simulations. Previous work by Petkova et al. established a hybrid radiation hydrodynamics (RHD) scheme that couples Smoothed Particle Hydrodynamics (SPH) to grid-based Monte Carlo Radiative Transfer (MCRT) code. This particle-mesh scheme employs the Exact mapping method for transferring fluid properties between SPH particles and Voronoi grids on which the MCRT simulation is carried out. The mapping, however, can become computationally infeasible with large numbers of particles or grid cells. We present a novel optimization method that adaptively converts gravity tree nodes into pseudo-SPH particles. These pseudo-particles act in place of the SPH particles when being passed to the MCRT code, allowing fluid resolutions to be temporarily reduced in regions which are less dynamically affected by radiation. A smoothing length solver and a neighbour-finding scheme dedicated to tree nodes have been developed. We also describe the new heating and cooling routines implemented for improved thermodynamic treatment. We show that this tree-based RHD scheme produces results in strong agreement with benchmarks, and achieves a speed-up that scales with the reduction in the number of particle-cell pairs being mapped.

astro-ph.IM↗

arXiv2024

Hybrid Radiation Hydrodynamics scheme with gravity tree-based adaptive optimization algorithm

Modelling the interaction between ionizing photons emitted from massive stars and their environment is essential to further our understanding of galactic ecosystems. We present a hybrid Radiation-Hydrodynamics (RHD) scheme that couples an SPH code to a grid-based Monte Carlo Radiative Transfer code. The coupling is achieved by using the particle positions as generating sites for a Voronoi grid, and applying a precise mapping of particle-interpolated densities onto the grid cells that ensures mass conservation. The mapping, however, can be computationally infeasible for large numbers of particles. We introduce our tree-based algorithm for optimizing coupled RHD codes. Astrophysical SPH codes typically utilize tree-building procedures to sort particles into hierarchical groups (referred to as nodes) for evaluating self-gravity. Our algorithm adaptively walks the gravity tree and transforms the extracted nodes into pseudo-SPH particles, which we use for the grid construction and mapping. This method allows for the temporary reduction of fluid resolution in regions that are less affected by the radiation. A neighbour-finding scheme is implemented to aid our smoothing length solver for nodes. We show that the use of pseudo-particles produces equally accurate results that agree with benchmarks, and achieves a speed-up that scales with the reduction in the final number of particle-cell pairs being mapped.

astro-ph.IM↗

arXiv2023

Gas and star kinematics in cloud-cloud collisions

We model the collision of molecular clouds to investigate the role of the initial properties on the remnants. Our clouds collide and evolve in a background medium that is approximately ten times less dense than the clouds, and we show that this relatively dense background is dynamically important for the evolution of the collision remnants. Given the motion of the clouds and the remnants through the background, we develop, implement, and introduce dynamic boundary conditions. We investigate the effect of the initial cloud mass, velocity, internal turbulence, and impact angle. The initial velocity and its velocity components have the largest affect on the remnant. This affects the spatial extent of the remnant, which affects the number of resulting star clusters and the distribution of their masses. The less extended remnants tend to have fewer, but more massive, clusters. Unlike the clusters, the gas distributions are relatively insensitive to the initial conditions, both the distribution of the bulk gas properties and the gas clumps. In general, cloud collisions are relatively insensitive to their initial conditions when modelled hydrodynamically in a dynamically important background medium.

astro-ph.GA↗

arXiv2022

On the origin of magnetic fields in stars II: The effect of numerical resolution

Are the kG-strength magnetic fields observed in young stars a fossil field left over from their formation or are they generated by a dynamo? Our previous numerical study concluded that magnetic fields must originate by a dynamo process. Here, we continue that investigation by performing even higher numerical resolution calculations of the gravitational collapse of a 1~M$_\odot$ rotating, magnetised molecular cloud core through the first and second collapse phases until stellar densities are reached. Each model includes Ohmic resistivity, ambipolar diffusion, and the Hall effect. We test six numerical resolutions, using between $10^5$ and $3\times10^7$ particles to model the cloud. At all but the lowest resolutions, magnetic walls form in the outer parts of the first hydrostatic core, with the maximum magnetic field strength located within the wall rather than at the centre of the core. At high resolution, this magnetic wall is disrupted by the Hall effect, producing a magnetic field with a spiral-shaped distribution of intensity. As the second collapse occurs, this field is dragged inward and grows in strength, with the maximum field strength increasing with resolution. As the second core forms, the maximum field strength exceeds 1~kG in our highest resolution simulations, and the stellar core field strength exceeds this threshold at the highest resolution. Our resolution study suggests that kG-strength magnetic fields may be implanted in low-mass stars during their formation, and may persist over long timescales given that the diffusion timescale for the magnetic field exceeds the age of the Universe.

astro-ph.SR↗

arXiv2021

ALMA observations of the Extended Green Object G19.01$-$0.03: I. A Keplerian disc in a massive protostellar system

Using the Atacama Large Millimetre/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), we observed the Extended Green Object (EGO) G19.01$-$0.03 with sub-arcsecond resolution from 1.05 mm to 5.01 cm wavelengths. Our $\sim0.4''\sim1600$ AU angular resolution ALMA observations reveal a velocity gradient across the millimetre core MM1, oriented perpendicular to the previously known bipolar molecular outflow, that is consistently traced by 20 lines of 8 molecular species with a range of excitation temperatures, including complex organic molecules (COMs). Kinematic modelling shows the data are well described by models that include a disc in Keplerian rotation and infall, with an enclosed mass of $40-70 \mathrm{M}_{\odot}$ (within a 2000 AU outer radius) for a disc inclination angle of $i=40^{\circ}$, of which $5.4-7.2 \mathrm{M}_{\odot}$ is attributed to the disc. Our new VLA observations show that the 6.7 GHz Class II methanol masers associated with MM1 form a partial ellipse, consistent with an inclined ring, with a velocity gradient consistent with that of the thermal gas. The disc-to-star mass ratio suggests the disc is likely to be unstable and may be fragmenting into as-yet-undetected low mass stellar companions. Modelling the centimetre--millimetre spectral energy distribution of MM1 shows the ALMA 1.05 mm continuum emission is dominated by dust, whilst a free-free component, interpreted as a hypercompact HII region, is required to explain the VLA $\sim$5 cm emission. The high enclosed mass derived for a source with a moderate bolometric luminosity ($\sim$10$^{4} \mathrm{L}_{\odot}$) suggests that the MM1 disc may feed an unresolved high-mass binary system.

astro-ph.GA↗

arXiv2021

Modelling of ionising feedback with Smoothed Particle Hydrodynamics and Monte Carlo Radiative Transfer on a Voronoi grid

The ionising feedback of young massive stars is well known to influence the dynamics of the birth environment and hence plays an important role in regulating the star formation process in molecular clouds. For this reason, modern hydrodynamics codes adopt a variety of techniques accounting for these radiative effects. A key problem hampering these efforts is that the hydrodynamics are often solved using smoothed particle hydrodynamics (SPH), whereas radiative transfer is typically solved on a grid. Here we present a radiation-hydrodynamics (RHD) scheme combining the SPH code Phantom and the Monte Carlo Radiative Transfer (MCRT) code CMacIonize, using the particle distribution to construct a Voronoi grid on which the MCRT is performed. We demonstrate that the scheme successfully reproduces the well-studied problem of D-type H II region expansion in a uniform density medium. Furthermore, we use this simulation setup to study the robustness of the RHD code with varying choice of grid structure, density mapping method, and mass and temporal resolution. To test the scheme under more realistic conditions, we apply it to a simulated star-forming cloud reminiscing those in the Central Molecular Zone of our galaxy, in order to estimate the amount of ionised material that a single source could create. We find that a stellar population of several $10^3~\rm{M_{\odot}}$ is needed to noticeably ionise the cloud. Based on our results, we formulate a set of recommendations to guide the numerical setup of future and more complex simulations of star forming clouds.

astro-ph.GA↗

arXiv2021

The impact of non-ideal magnetohydrodynamic processes on discs, outflows, counter-rotation and magnetic walls during the early stages of star formation

Non-ideal magnetohydrodynamic (MHD) processes -- namely Ohmic resistivity, ambipolar diffusion and the Hall effect -- modify the early stages of the star formation process and the surrounding environment. Collectively, they have been shown to promote disc formation and promote or hinder outflows. But which non-ideal process has the greatest impact? Using three-dimensional smoothed particle radiation non-ideal MHD simulations, we model the gravitational collapse of a rotating, magnetised cloud through the first hydrostatic core phase to shortly after the formation of the stellar core. We investigate the impact of each process individually and collectively. Including any non-ideal process decreases the maximum magnetic field strength by at least an order of magnitude during the first core phase compared to using ideal MHD, and promotes the formation of a magnetic wall. When the magnetic field and rotation vectors are anti-aligned and the Hall effect is included, rotationally supported discs of $r \gtrsim 20$~au form; when only the Hall effect is included and the vectors are aligned, a counter-rotating pseudo-disc forms that is not rotationally supported. Rotationally supported discs of $r \lesssim 4$~au form if only Ohmic resistivity or ambipolar diffusion are included. The Hall effect suppresses first core outflows when the vectors are anti-aligned and suppresses stellar core outflows independent of alignment. Ohmic resistivity and ambipolar diffusion each promote first core outflows and delay the launching of stellar core outflows. Although each non-ideal process influences star formation, these results suggest that the Hall effect has the greatest influence.

astro-ph.GA↗

arXiv2020

Supernova feedback and the energy deposition in molecular clouds

Feedback from supernovae is often invoked as an important process in limiting star formation, removing gas from galaxies and hence as a determining process in galaxy formation. Here we report on numerical simulations investigating the interaction between supernova explosions and the natal molecular cloud. We also consider the cases with and without previous feedback from the high-mass star in the form of ionising radiation and stellar winds. The supernova is able to find weak points in the cloud and create channels through which it can escape, leaving much of the well shielded cloud largely unaffected. This effect is increased when the channels are pre-existing due to the effects of previous stellar feedback. The expanding supernova deposits its energy in the gas that is in these exposed channels, and hence sweeps up less mass when feedback has already occurred, resulting in faster outflows with less radiative losses. The full impact of the supernova explosion is then able to impact the larger scale of the galaxy in which it abides. We conclude that supernova explosions only have moderate effects on their dense natal environments but that with pre-existing feedback, the energetic effects of the supernova are able to escape and affect the wider scale medium of the galaxy.

astro-ph.GA↗

arXiv2018

Clumpy shocks as the driver of velocity dispersion in molecular clouds: the effects of self-gravity and magnetic fields

We revisit an alternate explanation for the turbulent nature of molecular clouds - namely, that velocity dispersions matching classical predictions of driven turbulence can be generated by the passage of clumpy material through a shock. While previous work suggested this mechanism can reproduce the observed Larson relation between velocity dispersion and size scale ($σ\propto L^Γ$ with $Γ\approx 0.5$), the effects of self-gravity and magnetic fields were not considered. We run a series of smoothed particle magnetohydrodynamics experiments, passing clumpy gas through a shock in the presence of a combination of self-gravity and magnetic fields. We find powerlaw relations between $σ$ and $L$ throughout, with indices ranging from $Γ=0.3-1.2$. These results are relatively insensitive to the strength and geometry of magnetic fields, provided that the shock is relatively strong. $Γ$ is strongly sensitive to the angle between the gas' bulk velocity and the shock front, and the shock strength (compared to the gravitational boundness of the pre-shock gas). If the origin of the $σ-L$ relation is in clumpy shocks, deviations from the standard Larson relation constrain the strength and behaviour of shocks in spiral galaxies.

astro-ph.GA↗

arXiv2017

A clustered origin for isolated massive stars

High-mass stars are commonly found in stellar clusters promoting the idea that their formation occurs due to the physical processes linked with a young stellar cluster. It has recently been reported that isolated high-mass stars are present in the Large Magellanic Cloud. Due to their low velocities it has been argued that these are high-mass stars which formed without a surrounding stellar cluster. In this paper we present an alternative explanation for the origin of these stars in which they formed in a cluster environment but are subsequently dispersed into the field as their natal cluster is tidally disrupted in a merger with a higher-mass cluster. They escape the merged cluster with relatively low velocities typical of the cluster interaction and thus of the larger scale velocity dispersion, similarly to the observed stars. $N$-body simulations of cluster mergers predict a sizeable population of low velocity ($\le$ 20 km s$^{-1}$), high-mass stars at distances of > 20 pc from the cluster. High-mass clusters in which gas poor mergers are frequent would be expected to commonly have halos of young stars, including high-mass stars, that were actually formed in a cluster environment.

astro-ph.GA↗

arXiv2017

Streaming Motions and Kinematic Distances to Molecular Clouds

We present high-resolution smoothed particle hydrodynamics simulations of a region of gas flowing in a spiral arm and identify dense gas clouds to investigate their kinematics with respect to a Milky Way model. We find that, on average, the gas in the arms can have a net radial streaming motion of $v_R \approx -9 \,\mathrm{km/s}$ and rotate $\approx 6 \,\mathrm{km/s}$ slower than the circular velocity. This translates to average peculiar motions towards the Galaxy centre and opposite to Galactic rotation. These results may be sensitive to the assumed spiral arm perturbation, which is $\approx 3\%$ of the disc potential in our model. We compare the actual distance and the kinematic estimate and we find that streaming motions introduce systematic offsets of $\approx 1$ kpc. We find that the distance error can be as large as $\pm 2$ kpc and the recovered cloud positions have distributions that can extend significantly into the inter-arm regions. We conclude that this poses a difficulty in tracing spiral arm structure in molecular cloud surveys.

astro-ph.GA↗

arXiv2017

Fast and accurate Voronoi density gridding from Lagrangian hydrodynamics data

Voronoi grids have been successfully used to represent density structures of gas in astronomical hydrodynamics simulations. While some codes are explicitly built around using a Voronoi grid, others, such as Smoothed Particle Hydrodynamics (SPH), use particle-based representations and can benefit from constructing a Voronoi grid for post-processing their output. So far, calculating the density of each Voronoi cell from SPH data has been done numerically, which is both slow and potentially inaccurate. This paper proposes an alternative analytic method, which is fast and accurate. We derive an expression for the integral of a cubic spline kernel over the volume of a Voronoi cell and link it to the density of the cell. Mass conservation is ensured rigorously by the procedure. The method can be applied more broadly to integrate a spherically symmetric polynomial function over the volume of a random polyhedron.

astro-ph.IM↗

arXiv2013

Line Profiles of Cores within Clusters: II Signatures of Dynamical Collapse during High Mass Star Formation

Observations of atomic or molecular lines can provide important information about the physical state of star forming regions. In order to investigate the line profiles from dynamical collapsing massive star forming regions (MSFRs), we model the emission from hydrodynamic simulations of a collapsing cloud in the absence of outflows. By performing radiative transfer calculations, we compute the optically thick HCO+ and optically thin N2H+ line profiles from two collapsing regions at different epochs. Due to large-scale collapse, the MSFRs have large velocity gradients, reaching up to 20 km/s/pc across the central core. The optically thin lines typically contain multiple velocity components resulting from the superposition of numerous density peaks along the line-of-sight. The optically thick lines are only marginally shifted to the blue side of the optically thin line profiles, and frequently do not have a central depression in their profiles due to self-absorption. As the regions evolve the lines become brighter and the optically thick lines become broader. The lower order HCO+ (1-0) transitions are better indicators of collapse than the higher order (4-3) transitions. We also investigate how the beam sizes affect profile shapes. Smaller beams lead to brighter and narrower lines that are more skewed to the blue in HCO+ relative to the true core velocity, but show multiple components in N2H+. High resolution observations (e.g. with ALMA) can test these predictions and provide insights into the nature of MSFRs.

astro-ph.GA↗

arXiv2013

Misaligned streamers around a galactic centre black hole from a single cloud's infall

We follow the near radial infall of a prolate cloud onto a 4 x 10^6 Msun supermassive black hole in the Galactic Centre using smoothed particle hydrodynamics (SPH). We show that a prolate cloud oriented perpendicular to its orbital plane naturally produces a spread in angular momenta in the gas which can translate into misaligned discs as is seen in the young stars orbiting Sagittarius A*. A turbulent or otherwise highly structured cloud is necessary to avoid cancelling too much angular momentum through shocks at closest approach. Our standard model of a 2 x 10^4 Msun gas cloud brought about the formation of a disc within 0.3 pc from the black hole and a larger, misaligned streamer at 0.5 pc. A total of 1.5 x 10^4 Msun of gas formed these structures. Our exploration of the simulation parameter space showed that when star formation occurred, it resulted in top-heavy IMFs with stars on eccentric orbits with semi-major axes 0.02 to 0.3 pc and inclinations following the gas discs and streamers. We suggest that the single event of an infalling prolate cloud can explain the occurrence of multiple misaligned discs of young stars.

astro-ph.GA↗