ORCID Profile
0000-0003-0688-5332
Current Organisation
University of St Andrews
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Publisher: Oxford University Press (OUP)
Date: 03-2013
DOI: 10.1093/MNRAS/STT182
Publisher: Oxford University Press (OUP)
Date: 28-10-2016
Publisher: Cambridge University Press (CUP)
Date: 2016
DOI: 10.1017/PASA.2016.34
Abstract: In this paper, we introduce Nicil : Non-Ideal magnetohydrodynamics Coefficients and Ionisation Library. Nicil is a stand-alone Fortran90 module that calculates the ionisation values and the coefficients of the non-ideal magnetohydrodynamics terms of Ohmic resistivity, the Hall effect, and ambipolar diffusion. The module is fully parameterised such that the user can decide which processes to include and decide upon the values of the free parameters, making this a versatile and customisable code. The module includes both cosmic ray and thermal ionisation the former includes two ion species and three species of dust grains (positively charged, negatively charged, and neutral), and the latter includes five elements which can be doubly ionised. We demonstrate tests of the module, and then describe how to implement it into an existing numerical code.
Publisher: Oxford University Press (OUP)
Date: 28-01-2016
DOI: 10.1093/MNRAS/STW013
Publisher: Oxford University Press (OUP)
Date: 19-09-2016
Publisher: Oxford University Press (OUP)
Date: 28-05-2020
Abstract: Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of circumstellar discs that form nearly simultaneously with the formation of the protostar. This is done by modelling the gravitational collapse of a 1 M⊙ gas cloud that is threaded with a magnetic field and imposed with both rotational and turbulent velocities. We investigate magnetic fields that are parallel/antiparallel and perpendicular to the rotation axis, two rotation rates, and four Mach numbers. Disc formation occurs preferentially in the models that include non-ideal MHD where the magnetic field is antiparallel or perpendicular to the rotation axis. This is independent of the initial rotation rate and level of turbulence, suggesting that subsonic turbulence plays a minimal role in influencing the formation of discs. Aside from first core outflows that are influenced by the initial level of turbulence, non-ideal MHD processes are more important than turbulent processes during the formation of discs around low-mass stars.
Publisher: Elsevier BV
Date: 07-2009
Publisher: Oxford University Press (OUP)
Date: 23-01-2021
Abstract: We have performed Smoothed Particle Magneto-Hydrodynamics (SPMHD) calculations of colliding clouds to investigate the formation of massive stellar clusters, adopting a timestep criterion to prevent large ergence errors. We find that magnetic fields do not impede the formation of young massive clusters (YMCs), and the development of high star formation rates, although we do see a strong dependence of our results on the direction of the magnetic field. If the field is initially perpendicular to the collision, and sufficiently strong, we find that star formation is delayed, and the morphology of the resulting clusters is significantly altered. We relate this to the large lification of the field with this initial orientation. We also see that filaments formed with this configuration are less dense. When the field is parallel to the collision, there is much less lification of the field, dense filaments form, and the formation of clusters is similar to the purely hydrodynamical case. Our simulations reproduce the observed tendency for magnetic fields to be aligned perpendicularly to dense filaments, and parallel to low density filaments. Overall our results are in broad agreement with past work in this area using grid codes.
Publisher: Oxford University Press (OUP)
Date: 14-02-2018
DOI: 10.1093/MNRAS/STY392
Publisher: Frontiers Media SA
Date: 13-12-2018
Publisher: Oxford University Press (OUP)
Date: 18-01-2022
Abstract: 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⊙ rotating, magnetized 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 105 and 3 × 107 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 time-scales given that the diffusion time-scale for the magnetic field exceeds the age of the Universe.
Publisher: Oxford University Press (OUP)
Date: 05-09-2018
Publisher: Elsevier BV
Date: 04-2009
Publisher: Oxford University Press (OUP)
Date: 26-11-2021
Abstract: We present Ekster, a new method for simulating star clusters from birth in a live galaxy simulation that combines the smoothed-particle hydrodynamics (SPH) method Phantom with the N-body method PeTar. With Ekster, it becomes possible to simulate in idual stars in a simulation with only moderately high resolution for the gas, allowing us to study whole sections of a galaxy rather than be restricted to in idual clouds. We use this method to simulate star and star cluster formation in spiral arms, investigating massive giant molecular clouds (GMCs) and spiral arm regions with lower mass clouds, from two galaxy models with different spiral potentials. After selecting these regions from pre-run galaxy simulations, we re-s le the particles to obtain a higher resolution. We then re-simulate these regions for 3 Myr to study where and how star clusters form. We analyse the early evolution of the embedded star clusters in these regions. We find that the massive GMC regions, which are more common with stronger spiral arms, form more massive clusters than the sections of spiral arms containing lower mass clouds. Clusters form both by accreting gas and by merging with other proto-clusters, the latter happening more frequently in the denser GMC regions.
Publisher: Oxford University Press (OUP)
Date: 26-03-2021
Publisher: Oxford University Press (OUP)
Date: 21-07-2014
Publisher: Oxford University Press (OUP)
Date: 17-08-2018
Publisher: Oxford University Press (OUP)
Date: 10-08-2021
Abstract: 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, magnetized cloud through the first hydrostatic core phase to shortly after the formation of the stellar core. We investigate the impact of each process in idually 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 ≳ 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 ≲ 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.
Publisher: Oxford University Press (OUP)
Date: 21-03-2013
DOI: 10.1093/MNRAS/STT346
Publisher: Oxford University Press (OUP)
Date: 09-08-2019
Abstract: We present results from the first radiation non-ideal magnetohydrodynamics (MHD) simulations of low-mass star cluster formation that resolve the fragmentation process down to the opacity limit. We model 50 M⊙ turbulent clouds initially threaded by a uniform magnetic field with strengths of 3, 5 10, and 20 times the critical mass-to-magnetic flux ratio, and at each strength, we model both an ideal and non-ideal (including Ohmic resistivity, ambipolar diffusion, and the Hall effect) MHD cloud. Turbulence and magnetic fields shape the large-scale structure of the cloud, and similar structures form regardless of whether ideal or non-ideal MHD is employed. At high densities (106 ≲ nH ≲ 1011 cm−3), all models have a similar magnetic field strength versus density relation, suggesting that the field strength in dense cores is independent of the large-scale environment. Albeit with limited statistics, we find no evidence for the dependence of the initial mass function on the initial magnetic field strength, however, the star formation rate decreases for models with increasing initial field strengths the exception is the strongest field case where collapse occurs primarily along field lines. Protostellar discs with radii ≳ 20 au form in all models, suggesting that disc formation is dependent on the gas turbulence rather than on magnetic field strength. We find no evidence for the magnetic braking catastrophe, and find that magnetic fields do not hinder the formation of protostellar discs.
Publisher: Oxford University Press (OUP)
Date: 23-12-2020
Abstract: We investigate and discuss protostellar discs in terms of where the various non-ideal magnetohydrodynamics (MHD) processes are important. We find that the traditional picture of a magnetized disc (where Ohmic resistivity is dominant near the mid-plane, surrounded by a region dominated by the Hall effect, with the remainder of the disc dominated by ambipolar diffusion) is a great oversimplification. In simple parametrized discs, we find that the Hall effect is typically the dominant term throughout the majority of the disc. More importantly, we find that in much of our parametrized discs, at least two non-ideal processes have coefficients within a factor of 10 of one another, indicating that both are important and that naming a dominant term underplays the importance of the other terms. Discs that were self-consistently formed in our previous studies are also dominated by the Hall effect, and the ratio of ambipolar diffusion and Hall coefficients is typically less than 10, suggesting that both terms are equally important and listing a dominant term is misleading. These conclusions become more robust once the magnetic field geometry is taken into account. In agreement with the literature we review, we conclude that non-ideal MHD processes are important for the formation and evolution of protostellar discs. Ignoring any of the non-ideal processes, especially ambipolar diffusion and the Hall effect, yields an incorrect description of disc evolution.
Publisher: Oxford University Press (OUP)
Date: 03-07-2014
Publisher: Cambridge University Press (CUP)
Date: 2018
DOI: 10.1017/PASA.2018.25
Abstract: We present Phantom , a fast, parallel, modular, and low-memory smoothed particle hydrodynamics and magnetohydrodynamics code developed over the last decade for astrophysical applications in three dimensions. The code has been developed with a focus on stellar, galactic, planetary, and high energy astrophysics, and has already been used widely for studies of accretion discs and turbulence, from the birth of planets to how black holes accrete. Here we describe and test the core algorithms as well as modules for magnetohydrodynamics, self-gravity, sink particles, dust–gas mixtures, H 2 chemistry, physical viscosity, external forces including numerous galactic potentials, Lense–Thirring precession, Poynting–Robertson drag, and stochastic turbulent driving. Phantom is hereby made publicly available.
Publisher: Oxford University Press (OUP)
Date: 26-08-2014
Publisher: Oxford University Press (OUP)
Date: 05-01-2018
Publisher: Oxford University Press (OUP)
Date: 28-07-2022
Abstract: Direct observational measurements of the magnetic field strength in pre-stellar cores typically find supercritical mass-to-flux ratios, suggesting that the magnetic field is insufficient to prevent gravitational collapse. These measurements suffer from significant uncertainties an alternative approach is to utilize the sensitivity of pre-stellar chemistry to the evolutionary history, and indirectly constrain the degree of magnetic support. We combine non-ideal magnetohydrodynamic simulations of pre-stellar cores with time-dependent chemistry and radiative transfer modelling, producing synthetic observations of the model cores in several commonly observed molecular lines. We find that molecules strongly affected by freeze-out, such as CS and HCN, typically have much lower line intensities in magnetically subcritical models compared to supercritical ones, due to the longer collapse time-scales. Subcritical models also produce much narrower lines for all species investigated. Accounting for a range of core properties, ages, and viewing angles, we find that supercritical models are unable to reproduce the distribution of CS and N2H+ line strengths and widths seen in an observational s le, whereas subcritical models are in good agreement with the available data. This suggests that despite presently having supercritical mass-to-flux ratios, pre-stellar cores form as magnetically subcritical objects.
Publisher: Oxford University Press (OUP)
Date: 28-05-2020
Abstract: Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of protostars by following the gravitational collapse of 1 M⊙ gas clouds through the first hydrostatic core to stellar densities. The clouds are imposed with both rotational and turbulent velocities, and are threaded with a magnetic field that is parallel/antiparallel or perpendicular to the rotation axis we investigate two rotation rates and four Mach numbers. The initial radius and mass of the stellar core are only weakly dependent on the initial parameters. In the models that include ideal MHD, the magnetic field strength implanted in the protostar at birth is much higher than observed, independent of the initial level of turbulence only non-ideal MHD can reduce this strength to near or below the observed levels. This suggests that not only is ideal MHD an incomplete picture of star formation, but that the magnetic fields in low mass stars are implanted later in life by a dynamo process. Non-ideal MHD suppresses magnetically launched stellar core outflows, but turbulence permits thermally launched outflows to form a few years after stellar core formation.
Publisher: Oxford University Press (OUP)
Date: 12-04-2019
Publisher: American Astronomical Society
Date: 05-07-2016
Publisher: IOP Publishing
Date: 10-2022
Abstract: Free-floating (or rogue) planets are planets that are liberated (or ejected) from their host systems. Although simulations predict their existence in substantial numbers, direct observational evidence for free-floating planets with masses below ∼5 M Jup is still lacking. Several cycle-1 observing programs with JWST aim to hunt for them in four different star-forming clusters. These surveys are designed to be sensitive to masses of 1–15 M Jup (assuming a hot-start formation), which corresponds to spectral types of early L to late T for the ages of these clusters. If the existing simulations are not wide off the mark, we show here that the planned programs are likely to find up to 10–20 giant rogue planets in moderate density clusters like NGC1333 or IC348, and several dozen to ∼100 in high-density regions like NGC2024 and the Orion Nebula Cluster. These numbers correspond to 1%–5% of the total cluster population they could be substantially higher if stars form multiple giant planets at birth. In contrast, the number of free-floating brown dwarfs, formed from core collapse (like stars) is expected to be significantly lower, only about 0.25% of the number of stars, or 1–7 for the clusters considered here. Below 10 M Jup that number drops further by an order of magnitude. We also show that the planned surveys are not at risk of being significantly contaminated by field brown dwarfs in the foreground or background, after spectroscopic confirmation. Taken together, our results imply that if a population of L and T dwarfs were to be found in these JWST surveys, it is expected to be predominantly made up of rogue planets.
Publisher: Oxford University Press (OUP)
Date: 09-12-2016
Publisher: Oxford University Press (OUP)
Date: 10-07-2019
Abstract: We investigate differences in the molecular abundances between magnetically super- and subcritical pre-stellar cores, performing three-dimensional non-ideal magnetohydrodynamical (MHD) simulations with varying densities and magnetic field strengths, and post-processing the results with a time-dependent gas–grain chemical code. Most molecular species show significantly more central depletion in subcritical models, due to the longer duration of collapse. However, the directly observable quantities – the molecule to hydrogen column density ratios – are generally too similar for observational data to discriminate between models. The profiles of N2H+ and HCO+ show qualitative differences between supercritical and subcritical models on scales of $0.01 \\, {\\rm pc}$, which may allow the two cases to be distinguished. However, this requires knowledge of the hydrogen column density, which is not directly measureable, and predicted line intensity profiles from radiative transfer modelling are similar for these molecules. Other commonly observed species, such as HCN and CH3OH, have line intensity profiles that differ more strongly between models, and so are more promising as tracers of the mechanism of cloud collapse.
Publisher: Oxford University Press (OUP)
Date: 20-04-2021
Abstract: Determining the importance of magnetic fields in star-forming environments is h ered by the difficulty of accurately measuring both field strength and gas properties in molecular clouds. We post-process three-dimensional non-ideal magnetohydrodynamic simulations of pre-stellar cores with a time-dependent chemical network, and use radiative transfer modelling to calculate self-consistent molecular line profiles. Varying the initial mass-to-flux ratio from subcritical to supercritical results in significant changes to both the intensity and shape of several observationally important molecular lines. We identify the peak intensity ratio of N2H+ to CS lines, and the CS J = 2–1 blue-to-red peak intensity ratio, as promising diagnostics of the initial mass-to-flux ratio, with N2H+/CS values of & .6 (& .2) and CS blue/red values of & (& ) indicating subcritical (supercritical) collapse. These criteria suggest that, despite presently being magnetically supercritical, L1498 formed from subcritical initial conditions.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for James Wurster.