Anthony P. Whitworth - Cardiff University, UK

Anthony P. Whitworth
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Name
Anthony P. Whitworth
Affiliation
Cardiff University, UK
City
Cardiff
Country
United Kingdom

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Astrophysics (16)
 
Astrophysics of Galaxies (15)
 
Solar and Stellar Astrophysics (7)
 
Earth and Planetary Astrophysics (1)

Publications Authored By Anthony P. Whitworth

2017Apr
Authors: Derek Ward-Thompson, Kate Pattle, Pierre Bastien, Ray S. Furuya, Woojin Kwon, Shih-Ping Lai, Keping Qiu, David Berry, Minho Choi, Simon Coudé, James Di Francesco, Thiem Hoang, Erica Franzmann, Per Friberg, Sarah F. Graves, Jane S. Greaves, Martin Houde, Doug Johnstone, Jason M. Kirk, Patrick M. Koch, Jungmi Kwon, Chang Won Lee, Di Li, Brenda C. Matthews, Joseph C. Mottram, Harriet Parsons, Andy Pon, Ramprasad Rao, Mark Rawlings, Hiroko Shinnaga, Sarah Sadavoy, Sven van Loo, Yusuke Aso, Do-Young Byun, Eswariah Chakali, Huei-Ru Chen, Mike C. -Y. Chen, Wen Ping Chen, Tao-Chung Ching, Jungyeon Cho, Antonio Chrysostomou, Eun Jung Chung, Yasuo Doi, Emily Drabek-Maunder, Stewart P. S. Eyres, Jason Fiege, Rachel K. Friesen, Gary Fuller, Tim Gledhill, Matt J. Griffin, Qilao Gu, Tetsuo Hasegawa, Jennifer Hatchell, Saeko S. Hayashi, Wayne Holland, Tsuyoshi Inoue, Shu-ichiro Inutsuka, Kazunari Iwasaki, Il-Gyo Jeong, Ji-hyun Kang, Miju Kang, Sung-ju Kang, Koji S. Kawabata, Francisca Kemper, Gwanjeong Kim, Jongsoo Kim, Kee-Tae Kim, Kyoung Hee Kim, Mi-Ryang Kim, Shinyoung Kim, Kevin M. Lacaille, Jeong-Eun Lee, Sang-Sung Lee, Dalei Li, Hua-bai Li, Hong-Li Liu, Junhao Liu, Sheng-Yuan Liu, Tie Liu, A-Ran Lyo, Steve Mairs, Masafumi Matsumura, Gerald H. Moriarty-Schieven, Fumitaka Nakamura, Hiroyuki Nakanishi, Nagayoshi Ohashi, Takashi Onaka, Nicolas Peretto, Tae-Soo Pyo, Lei Qian, Brendan Retter, John Richer, Andrew Rigby, Jean-François Robitaille, Giorgio Savini, Anna M. M. Scaife, Archana Soam, Motohide Tamura, Ya-Wen Tang, Kohji Tomisaka, Hongchi Wang, Jia-Wei Wang, Anthony P. Whitworth, Hsi-Wei Yen, Hyunju Yoo, Jinghua Yuan, Chuan-Peng Zhang, Guoyin Zhang, Jianjun Zhou, Lei Zhu, Philippe André, C. Darren Dowell, Sam Falle, Yusuke Tsukamoto

We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2) camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions which the survey will aim to answer. Read More

We investigate, using 3D hydrodynamic simulations, the fragmentation of pressure-confined, vertically stratified, self-gravitating gaseous layers. The confining pressure is either thermal pressure acting on both surfaces, or thermal pressure acting on one surface and ram-pressure on the other. In the linear regime of fragmentation, the dispersion relation we obtain agrees well with that derived by Elmegreen & Elmegreen (1978), and consequently deviates from the dispersion relations based on the thin shell approximation (Vishniac 1983) or pressure assisted gravitational instability (W\"unsch et al. Read More

We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long, initially sub-critical but accreting filaments. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length scale which is roughly 4 times the filament's diameter. Read More

We use Smoothed Particle Hydrodynamic simulations of cold, uniform density, self-gravitating filaments, to investigate their longitudinal collapse timescales; these timescales are important because they determine the time available for a filament to fragment into cores. A filament is initially characterised by its line-mass, $\mu$, its radius, $R$ (or equivalently its density $\rho\!=\!\mu/\pi R^2$), and its aspect ratio, $A\;\,(\equiv Z/R$, where $Z$ is its half-length). The gas is only allowed to contract longitudinally, i. Read More

We review current theories for the origin of the Stellar Initial Mass Function (IMF) with particular focus on the extent to which the IMF can be considered universal across various environments. To place the issue in an observational context, we summarize the techniques used to determine the IMF for different stellar populations, the uncertainties affecting the results, and the evidence for systematic departures from universality under extreme circumstances. We next consider theories for the formation of prestellar cores by turbulent fragmentation and the possible impact of various thermal, hydrodynamic and magneto-hydrodynamic instabilities. Read More

The time evolution of the probability density function (PDF) of the mass density is formulated and solved for systems in free-fall using a simple appoximate function for the collapse of a sphere. We demonstrate that a pressure-free collapse results in a power-law tail on the high-density side of the PDF. The slope quickly asymptotes to the functional form $\mathrm{P}_v(\rho)\propto\rho^{-1. Read More

Using observations of cores to infer their intrinsic properties requires the solution of several poorly constrained inverse problems. Here we address one of these problems, namely to deduce from the projected aspect ratios of the cores in Ophiuchus their intrinsic three-dimensional shapes. Four models are proposed, all based on the standard assumption that cores are randomly orientated ellipsoids, and on the further assumption that a core's shape is not correlated with its absolute size. Read More

Protostars grow in mass by accreting material through their discs, and this accretion is initially their main source of luminosity. The resulting radiative feedback heats the environments of young protostars, and may thereby suppress further fragmentation and star formation. There is growing evidence that the accretion of material onto protostars is episodic rather than continuous; most of it happens in short bursts that last up to a few hundred years, whereas the intervals between these outbursts of accretion could be thousands of years. Read More

2011Sep
Affiliations: 1Cardiff University, UK, 2Sheffield University, UK, 3Cardiff University, UK

It is speculated that the accretion of material onto young protostars is episodic. We present a computational method to include the effects of episodic accretion in radiation hydrodynamic simulations of star formation. We find that during accretion events protostars are "switched on", heating and stabilising the discs around them. Read More

Recent discoveries of strongly misaligned transiting exoplanets pose a challenge to the established planet formation theory which assumes planetary systems to form and evolve in isolation. However, the fact that the majority of stars actually do form in star clusters raises the question how isolated forming planetary systems really are. Besides radiative and tidal forces the presence of dense gas aggregates in star-forming regions are potential sources for perturbations to protoplanetary discs or systems. Read More

We present simulations of initially stable isothermal clouds exposed to ionising radiation from a discrete external source, and identify the conditions that lead to radiatively driven implosion and star formation. We use the Smoothed Particle Hydrodynamics code SEREN and an HEALPix-based photo-ionisation algorithm to simulate the propagation of the ionising radiation and the resulting dynamical evolution of the cloud. We find that the incident ionising flux, $\Phi_{_{\rm LyC}}$, is the critical parameter determining the cloud evolution. Read More

2011Mar
Affiliations: 1Cardiff University, 2Cardiff University, 3Sheffield University

A star acquires much of its mass by accreting material from a disc. Accretion is probably not continuous but episodic. We have developed a method to include the effects of episodic accretion in simulations of star formation. Read More

2010Dec
Affiliations: 1Cardiff University, 2ESO Germany, 3Cardiff University, 4Service d' Astrophysique, C.E. Saclay

A large fraction of brown dwarfs and low-mass H-burning stars may form by gravitational fragmentation of protostellar discs. We explore the conditions for disc fragmentation and we find that they are satisfied when a disc is large enough (>100 AU) so that its outer regions can cool efficiently, and it has enough mass to be gravitationally unstable, at such radii. We perform radiative hydrodynamic simulations and show that even a disc with mass 0. Read More

We use SPH simulations to investigate the gravitational fragmentation of expanding shells through the linear and non--linear regimes. The results are analysed using spherical harmonic decomposition to capture the initiation of structure during the linear regime; the potential-based method of Smith et al. (2009) to follow the development of clumps in the mildly non-linear regime; and sink particles to capture the properties of the final bound objects during the highly non-linear regime. Read More

2010Sep
Affiliations: 1School of Physics and Astronomy, Cardiff University, UK, 2School of Physics and Astronomy, Cardiff University, UK

A large fraction of brown dwarfs and low-mass stars may form by gravitational fragmentation of relatively massive (a few 0.1 Msun), extended (a few hundred AU) discs around Sun-like stars. We present an ensemble of radiative hydrodynamic simulations that examine the conditions for disc fragmentation. Read More

We study analytically the development of gravitational instability in an expanding shell having finite thickness. We consider three models for the radial density profile of the shell: (i) an analytic uniform-density model, (ii) a semi-analytic model obtained by numerical solution of the hydrostatic equilibrium equation, and (iii) a 3D hydrodynamic simulation. We show that all three profiles are in close agreement, and this allows us to use the first model to describe fragments in the radial direction of the shell. Read More

2010May
Affiliations: 1Argelander Institute for Astronomy, Bonn, Germany, 2Argelander Institute for Astronomy, Bonn, Germany, 3Sheffield University, UK, 4Cardiff University, UK, 5Cardiff University, UK

Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar discs and therefore the formation of planets and brown dwarfs. Recent findings suggest that tidal perturbations of typical circumstellar discs due to close encounters may inhibit rather than trigger disc fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disc models in these calculations were restricted to disc radii of 40 AU and disc masses below 0. Read More

2009Nov
Affiliations: 1Cardiff University, UK, 2Cardiff University, UK

A large fraction of the observed brown dwarfs may form by gravitational fragmentation of unstable discs. This model reproduces the brown dwarf desert, and provides an explanation the existence of planetary-mass objects and for the binary properties of low-mass objects. We have performed an ensemble of radiative hydrodynamic simulations and determined the statistical properties of the low-mass objects produced by gravitational fragmentation of discs. Read More

2009Aug
Affiliations: 1School of Physics and Astronomy, Cardiff University, 2School of Physics and Astronomy, Cardiff University

Thermodynamics play an important role in determining the way a protostellar disc fragments to form planets, brown dwarfs and low-mass stars. We explore the effect that different treatments of radiative transfer have in simulations of fragmenting discs. Three prescriptions for the radiative transfer are used, (i) the diffusion approximation of Stamatellos et al. Read More

We investigate the gravitational fragmentation of expanding shells in the context of the linear thin--shell analysis. We make use of two very different numerical schemes; the FLASH Adaptive Mesh Refinement code and a version of the Benz Smoothed Particle Hydrodynamics code. We find that the agreement between the two codes is excellent. Read More

2008Dec
Affiliations: 1Institute for Astronomy, University of Edinburgh, 2Institute for Astronomy, University of Edinburgh, 3School for Physics and Astronomy, Cardiff University, 4School for Physics and Astronomy, Cardiff University
Category: Astrophysics

A new means of incorporating radiative transfer into smoothed particle hydrodynamics (SPH) is introduced, which builds on the success of two previous methods - the polytropic cooling approximation as devised by Stamatellos et al (2007), and flux limited diffusion (e.g. Mayer et al 2007). Read More

2008Oct
Affiliations: 1School of Physics & Astronomy, Cardiff University, 2School of Physics & Astronomy, Cardiff University
Category: Astrophysics

We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive extended discs around Sun-like stars. Such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs fragment within a few thousand years, and produce mainlybrown dwarf (BDs) stars, but also planetary mass (PM) stars and very low-mass hydrogen-burning (HB) stars. Read More

2008Sep
Affiliations: 1School of Physics & Astronomy, Cardiff University, 2School of Physics & Astronomy, Cardiff University
Category: Astrophysics

We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive, extended discs around Sun-like stars. We argue that such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs typically fragment within a few thousand years to produce mainly brown dwarfs (including planetary-mass brown dwarfs) and low-mass hydrogen-burning stars. Read More

We present a new method of incorporating radiative transfer into Smoothed Particle Hydrodynamics (SPH). There have been many recent attempts at radiative transfer in SPH (Stamatellos et al 2005, 2005, Mayer et al 2007, Whitehouse and Bate 2006), however these are becoming increasingly complex, with some methods requiring the photosphere to be mapped (which is often of non-trivial geometric shape), and extra conditions to be applied there (matching atmospheres as in Cai et al (2008), or specifying cooling at the photosphere as in Mayer et al (2007)). The method of identifying the photosphere is usually a significant addition to the total simulation runtime, and often requires extra free parameters, the changing of which will affect the final results. Read More

2008Feb
Affiliations: 1AIP, Potsdam, Germany, 2School of Physics & Astronomy, Cardiff University, U.K., 3ITA, University of Heidelberg, Germany
Category: Astrophysics

We present results of hydrodynamic simulations of star formation triggered by cloud-cloud collisions. During the early stages of star formation, low-mass objects form by gravitational instabilities in protostellar discs. A number of these low-mass objects are in the sub-stellar mass range, including a few objects of planetary mass. Read More

Stars form from dense molecular cores, and the mass function of these cores (the CMF) is often found to be similar to the form of the stellar initial mass function (IMF). This suggests that the form of the IMF is the result of the form of the CMF. However, most stars are thought to form in binary and multiple systems, therefore the relationship between the IMF and the CMF cannot be trivial. Read More

2007Sep
Affiliations: 1School of Physics and Astronomy, Cardiff University, UK, 2School of Physics and Astronomy, Cardiff University, UK
Category: Astrophysics

Gravitational fragmentation has been proposed as a mechanism for the formation of giant planets in close orbits around solar-type stars. However, it is debatable whether this mechanism can function in the inner regions (R<40 AU) of real discs. We use a newly developed method for treating the energy equation and the equation of state, which accounts for radiative transfer effects in SPH simulations of circumstellar discs. Read More

2007Aug
Affiliations: 1School of Physics & Astronomy, Cardiff University, UK, 2School of Physics & Astronomy, Cardiff University, UK, 3School of Physics & Astronomy, Cardiff University, UK
Category: Astrophysics

We suggest that low-mass hydrogen-burning stars like the Sun should sometimes form with massive extended discs; and we show, by means of radiation hydrodynamic simulations, that the outer parts of such discs (R>100 AU) are likely to fragment on a dynamical timescale (10^3 to $10^4 yr), forming low-mass companions: principally brown dwarfs (BDs), but also very low-mass hydrogen-burning stars and planetary-mass objects. A few of the BDs formed in this way remain attached to the primary star, orbiting at large radii. The majority are released into the field, by interactions amongst themselves; in so doing they acquire only a low velocity dispersion (<2 km/s), and therefore they usually retain small discs, capable of registering an infrared excess and sustaining accretion. Read More

We review five mechanisms for forming brown dwarfs: (i) turbulent fragmentation of molecular clouds, producing very low-mass prestellar cores by shock compression; (ii) collapse and fragmentation of more massive prestellar cores; (iii) disc fragmentation; (iv) premature ejection of protostellar embryos from their natal cores; and (v) photo-erosion of pre-existing cores overrun by HII regions. These mechanisms are not mutually exclusive. Their relative importance probably depends on environment, and should be judged by their ability to reproduce the brown-dwarf IMF, the distribution and kinematics of newly formed brown dwarfs, the binary statistics of brown dwarfs, the ability of brown dwarfs to retain discs, and hence their ability to sustain accretion and outflows. Read More

We examine the spatial distribution of brown dwarfs produced by the decay of small-N stellar systems as expected from the embryo ejection scenario. We model a cluster of several hundred stars grouped into 'cores' of a few stars/brown dwarfs. These cores decay, preferentially ejecting their lowest-mass members. Read More

2005May
Affiliations: 1School of Physics & Astronomy, Cardiff University, 2School of Physics & Astronomy, Cardiff University
Category: Astrophysics

We combine a Monte Carlo radiative transfer code with an SPH code, so that -- assuming thermal equilibrium -- we can calculate dust-temperature fields, spectral energy distributions, and isophotal maps, for the individual time-frames generated by an SPH simulation. On large scales, the radiative transfer cells (RT cells) are borrowed from the tree structure built by the SPH code, and are chosen so that their size -- and hence the resolution of the calculated temperature field -- is comparable with the resolution of the density field. We refer collectively to these cubic RT cells as the 'global grid'. Read More

The Monte Carlo method is a powerful tool for performing radiative equilibrium calculations, even in complex geometries. The main drawback of the standard Monte Carlo radiative equilibrium methods is that they require iteration, which makes them numerically very demanding. Bjorkman & Wood recently proposed a frequency distribution adjustment scheme, which allows radiative equilibrium Monte Carlo calculations to be performed without iteration, by choosing the frequency of each re-emitted photon such that it corrects for the incorrect spectrum of the previously re-emitted photons. Read More

We review and evaluate four mechanisms for forming brown dwarves: (i) dynamical ejection of a stellar embryo from its placental prestellar core; (ii) opacity-limited fragmentation of a shock-compressed layer; (iii) gravitational instabilities in discs, triggered by impulsive interactions with other discs or naked stars; and (iv) photo-erosion of pre-existing cores. All these mechanisms can produce free-floating brown dwarves, but only (ii) and (iii) are likely to produce brown dwarves in multiple systems, and (i) has difficulty delivering brown dwarves with discs. Read More

2004May
Affiliations: 1Cardiff University, UK, 2Cardiff University, UK, 3Cardiff University, UK
Category: Astrophysics

(Abridged) We explore, by means of a large ensemble of SPH simulations, how the level of turbulence affects the collapse and fragmentation of a star-forming core. All our simulated cores have the same, except that we vary (a) the initial level of turbulence (as measured by the ratio of turbulent to gravitational energy, $\alpha_{\rm turb} \equiv U_{\rm turb}/|\Omega| = 0, 0.01, 0. Read More

We review a range of stastistical methods for analyzing the structures of star clusters, and derive a new measure ${\cal Q}$ which both quantifies, and distinguishes between, a (relatively smooth) large-scale radial density gradient and multi-scale (fractal) sub-clustering. Q is derived from the normalised correlation length and the normalised edge length of the minimal spanning tree for each cluster. Read More

In order to study the capabilities of SPH in self-gravitating hydrodynamical problems, we have performed a series of three-dimensional numerical simulations of the collapse of a rotating homogeneous cloud, varying the number of particles and the artificial viscosity prescription. Read More