Publications


Discovery of a nearby 1700 km/s star ejected from the Milky Way by Sgr A*

Monthly Notices of the Royal Astronomical Society Oxford University Press 491 (2019) 2465-2480

D Boubert, TS Li, D Erkal, GS Da Costa, DB Zucker, AP Ji, K Kuehn, GF Lewis, D Mackey, JD Simpson, N Shipp, Z Wan, V Belokurov, J Bland-Hawthorn, SL Martell, T Nordlander, D De Silva, M-Y Wang

<jats:title>Abstract</jats:title> <jats:p>We present the serendipitous discovery of the fastest Main Sequence hyper-velocity star (HVS) by the Southern Stellar Stream Spectroscopic Survey (S5). The star S5-HVS1 is a ∼2.35 M⊙ A-type star located at a distance of ∼9 kpc from the Sun and has a heliocentric radial velocity of 1017 ± 2.7  km s−1 without any signature of velocity variability. The current 3-D velocity of the star in the Galactic frame is 1755 ± 50  km s−1. When integrated backwards in time, the orbit of the star points unambiguously to the Galactic Centre, implying that S5-HVS1 was kicked away from Sgr A* with a velocity of ∼1800  km s−1 and travelled for 4.8 Myr to its current location. This is so far the only HVS confidently associated with the Galactic Centre. S5-HVS1 is also the first hyper-velocity star to provide constraints on the geometry and kinematics of the Galaxy, such as the Solar motion Vy, ⊙ = 246.1 ± 5.3  km s−1 or position R0 = 8.12 ± 0.23 kpc. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disk of young stars at the Galactic centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hyper-velocity stars previously associated with the Galactic Centre, we question whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time.</jats:p>


Trapped orbits and solar-neighbourhood kinematics

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 495 (2020) 895-904

J Binney


Angle-action variables for orbits trapped at a Lindblad resonance

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 495 (2020) 886-894

J Binney


Impact of main ion pressure anisotropy on stellarator impurity transport

Nuclear Fusion IOP Publishing (2019)

I Calvo, FI Parra, JL Velasco, JM García-Regaña

Main ions influence impurity dynamics through a variety of mechanisms; in particular, via impurity-ion collisions. To lowest order in an expansion in the main ion mass over the impurity mass, the impurity-ion collision operator only depends on the component of the main ion distribution that is odd in the parallel velocity. These lowest order terms give the parallel friction of the impurities with the main ions, which is typically assumed to be the main cause of collisional impurity transport. Next-order terms in the mass ratio expansion of the impurity-ion collision operator, proportional to the component of the main ion distribution that is even in the parallel velocity, are usually neglected. However, in stellarators, the even component of the main ion distribution can be very large. In this article, such next-order terms in the mass ratio expansion of the impurity-ion collision operator are retained, and analytical expressions for the neoclassical radial flux of trace impurities are calculated in the Pfirsch-Schl\"uter, plateau and $1/\nu$ regimes. The new terms provide a drive for impurity transport that is physically very different from parallel friction: they are associated to anisotropy in the pressure of the main ions, which translates into impurity pressure anisotropy. It is argued that main ion pressure anisotropy must be taken into account for a correct description of impurity transport in certain realistic stellarator plasmas. Examples are given by numerically evaluating the analytical expressions for the impurity flux.


Cosmic Ray Acceleration in Hydromagnetic Flux Tubes

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2019)

A BELL, J Matthews, K Blundell, A Araudo

We find that hydromagnetic flux tubes in back-flows in the lobes of radio galaxies offer a suitable environment for the acceleration of cosmic rays (CR) to ultra-high energies. We show that CR can reach the Hillas (1984) energy even if the magnetised turbulence in the flux tube is not sufficiently strong for Bohm diffusion to apply. First-order Fermi acceleration by successive weak shocks in a hydromagnetic flux tube is shown to be equivalent to second-order Fermi acceleration by strong turbulence.


Action-based models for dwarf spheroidal galaxies and globular clusters

Monthly Notices of the Royal Astronomical Society Oxford University Press 488 (2019) 2423-2439

R Pascale, J Binney, C Nipoti, L Posti

A new family of self-consistent distribution function (DF)-based models of stellar systems is explored. The stellar component of the models is described by a DF depending on the action integrals, previously used to model the Fornax dwarf spheroidal galaxy (dSph). The stellar component may cohabit with either a dark halo, also described by a DF, or with a massive central black hole. In all cases we solve for the models self-consistent potential. Focussing on spherically symmetric models, we show how the stellar observables vary with the anisotropy prescribed by the DF, with the dominance and nature of the dark halo, and with the mass of the black hole. We show that precise fits to the observed surface brightness profiles of four globular clusters can be obtained for a wide range of prescribed velocity anisotropies. We also obtain precise fits to the observed projected densities of four dSphs. Finally, we present a three-component model of the Sculptor dSph with distinct DFs for the red and blue horizontal branch stars and the dark matter halo.


Extreme Plasma Astrophysics

Bulletin of the American Astronomical Society American Astronomical Society 51 (2019) 362

D Uzdensky, M Begelman, A Beloborodov, R Blandford, S Boldyrev, B Cerutti, F Fiuza, D Giannios, T Grismayer, M Kunz, N Loureiro, M Lyutikov, M Medvedev, M Petropoulou, A Philippov, E Quataert, A Schekochihin, K Schoeffler, L Silva, L Sironi, A Spitkovsky, G Werner, V Zhdankin, J Zrake, E Zweibel

This is a science white paper submitted to the Astro-2020 and Plasma-2020 Decadal Surveys. The paper describes the present status and emerging opportunities in Extreme Plasma Astrophysics -- a study of astrophysically-relevant plasma processes taking place under extreme conditions that necessitate taking into account relativistic, radiation, and QED effects.


Relaxation of spherical stellar systems

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 490 (2019) 478-490

JY Lau, J Binney


stella: An operator-split, implicit-explicit delta f-gyrokinetic code for general magnetic field configurations

JOURNAL OF COMPUTATIONAL PHYSICS 391 (2019) 365-380

M Barnes, FI Parra, M Landreman


Validation of gyrokinetic simulations of a National Spherical Torus eXperiment H-mode plasma and comparisons with a high-k scattering synthetic diagnostic

Plasma Physics and Controlled Fusion IOP Publishing 61 (2019) 115015-115015

JR Ruiz, W Guttenfelder, AE White, NT Howard, J Candy, Y Ren, DR Smith, NF Loureiro, C Holland, CW Domier


Overview of recent TJ-II stellarator results

NUCLEAR FUSION 59 (2019) ARTN 112019

E Ascasibar, D Alba, D Alegre, A Alonso, J Alonso, F de Aragon, A Baciero, JM Barcala, E Blanco, J Botija, L Bueno, S Cabrera, E de la Cal, I Calvo, A Cappa, D Carralero, R Carrasco, B Carreras, F Castejon, R Castro, A de Castro, G Catalan, AA Chmyga, M Chamorro, AW Cooper, A Dinklage, L Eliseev, T Estrada, M Ezzat, F Fernandez-Marina, JM Fontdecaba, L Garcia, I Garcia-Cortes, R Garcia-Gomez, JM Garcia-Regana, A Gonzalez-Jerez, G Grenfell, J Guasp, J Hernandez-Sanchez, J Hernanz, C Hidalgo, E Hollmann, A Jimenez-Denche, P Khabanov, N Kharchev, I Kirpitchev, R Kleiber, AS Kozachek, L Krupnik, F Lapayese, M Liniers, B Liu, D Lopez-Bruna, A Lopez-Fraguas, B Lopez-Miranda, J Lopez-Razola, U Losada, E de la Luna, A Martin de Aguilera, F Martin-Diaz, M Martinez-Fuentes, G Martin-Gomez, AB Martin-Rojo, J Martinez-Fernandez, KJ McCarthy, F Medina, M Medrano, L Melon, AV Melnikov, P Mendez, R Merino, FJ Miguel, B van Milligen, A Molinero, B Momo, P Monreal, S Mulas, Y Narushima, M Navarro, M Ochando, S Ohshima, J Olivares, E Oyarzabal, JL de Pablos, L Pacios, N Panadero, F Parra, I Pastor, A de la Pena, A Pereira, JR Pinzon, AB Portas, E Poveda, JA Quintana, FJ Ramos, GA Ratta, M Redondo, E Rincon, L Rios, C Rodriguez-Fernandez, L Rodriguez-Rodrigo, B Rojo, A Ros, E Rosa, E Sanchez, J Sanchez, M Sanchez, E Sanchez-Sarabia, S Satake, JA Sebastian, R Sharma, C Silva, ER Solano, A Soleto, BJ Sun, FL Tabares, D Tafalla, H Takahashi, N Tamura, A Tolkachev, J Vega, G Velasco, JL Velasco, S Yamamoto, B Zurro, TJ-Team


Suppressed effective viscosity in the bulk intergalactic plasma

Nature Astronomy Nature Research 3 (2019) 832–837-

I Zhuravleva, E Churazov, A Schekochihin, SW Allen, A Vikhlinin, N Werner

<p>Transport properties, such as viscosity and thermal conduction, of the hot intergalactic plasma in clusters of galaxies are largely unknown. Whereas for laboratory plasmas these characteristics are derived from the gas density and temperature<sup>1</sup>, such recipes can be fundamentally different for the intergalactic plasma<sup>2</sup> owing to a low rate of particle collisions and a weak magnetic field<sup>3</sup>. In numerical simulations, these unknowns can often be avoided by modelling these plasmas as hydrodynamic fluids<sup>4,5,6</sup>, even though local, non-hydrodynamic features observed in clusters contradict this assumption<sup>7,8,9</sup>. Using deep Chandra observations of the Coma Cluster<sup>10,11</sup>, we probe gas fluctuations in intergalactic medium down to spatial scales where the transport processes should prominently manifest themselves—provided that hydrodynamic models<sup>12</sup> with pure Coulomb collision rates are indeed adequate. We do not find evidence of such transport processes, implying that the effective isotropic viscosity is orders of magnitude smaller than naively expected. This indicates either an enhanced collision rate in the plasma due to particle scattering off microfluctuations caused by plasma instabilities<sup>2,13,14</sup> or that the transport processes are anisotropic with respect to the local magnetic field<sup>15</sup>. This also means that numerical models with high Reynolds number appear more consistent with observations. Our results demonstrate that observations of turbulence in clusters<sup>16,17</sup> are giving rise to a branch of astrophysics that can sharpen theoretical views on galactic plasmas.</p>


Spectroscopy of the Young Stellar Association Price-Whelan 1: origin in the magellanic leading arm and constraints on the Milky Way Hot Halo

Astrophysical Journal American Astronomical Society 887 (2019) 115

DL Nidever, AM Price-Whelan, Y Choi, RL Beaton, TT Hansen, D Boubert, D Aguado, R Ezzeddine, S Oh, NW Evans

We report spectroscopic measurements of stars in the recently discovered young stellar association Price-Whelan 1 (PW 1), which was found in the vicinity of the Leading Arm (LA) of the Magellanic Stream (MS). We obtained Magellan+MIKE high-resolution spectra of the 28 brightest stars in PW 1 and used The Cannon to determine their stellar parameters. We find that the mean metallicity of PW 1 is [Fe/H] = −1.23 with a small scatter of 0.06 dex and the mean RV is ${V}_{\mathrm{hel}}$ = 276.7 $\mathrm{km}\,{{\rm{s}}}^{-1}\,$ with a dispersion of $11.0$ $\mathrm{km}\,{{\rm{s}}}^{-1}$. Our results are consistent in ${\text{}}{T}_{\mathrm{eff}}$, $\mathrm{log}g$, and [Fe/H] with the young and metal-poor characteristics (116 Myr and [Fe/H] = −1.1) determined for PW 1 from our discovery paper. We find a strong correlation between the spatial pattern of the PW 1 stars and the LA II gas with an offset of −10fdg15 in ${L}_{\mathrm{MS}}$ and +1fdg55 in ${B}_{\mathrm{MS}}$. The similarity in metallicity, velocity, and spatial patterns indicates that PW 1 likely originated in LA II. We find that the spatial and kinematic separation between LA II and PW 1 can be explained by ram pressure from Milky Way (MW) gas. Using orbit integrations that account for the LMC and MW halo and outer disk gas, we constrain the halo gas density at the orbital pericenter of PW 1 to be ${{\boldsymbol{n}}}_{\mathrm{halo}}(17\,\mathrm{kpc})={2.7}_{-2.0}^{+3.4}\times {10}^{-3}\,\mathrm{atoms}\,{\mathrm{cm}}^{-3}$ and the disk gas density at the midplane at $20\,\mathrm{kpc}$ to be ${{\boldsymbol{n}}}_{\mathrm{disk}}(20\,\mathrm{kpc},0)={6.0}_{-2.0}^{+1.5}\times {10}^{-2}\,\mathrm{atoms}\,{\mathrm{cm}}^{-3}$. We, therefore, conclude that PW 1 formed from the LA II of the MS, making it a powerful constraint on the MW–Magellanic interaction.


Cosmic ray acceleration by shocks: spectral steepening due to turbulent magnetic field amplification

Monthly Notices of the Royal Astronomical Society Oxford University Press 488* (2019) 2466-2472

A Bell, J Matthews, K Blundell

We show that the energy required to turbulently amplify magnetic field during cosmic ray (CR) acceleration by shocks extracts energy from the CR and steepens the CR energy spectrum.


Hot, dense He II outflows during the 2017 outburst of the X-ray transient Swift J1357.2−0933

Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press 489 (2019) L47–L52-

P Charles, J Matthews, D Buckley, P Gandhi, E Kotze, J Paice

Time-resolved SALT spectra of the short-period, dipping X-ray transient, Swift J1357.2−0933, during its 2017 outburst has revealed broad Balmer and He II λ4686 absorption features, blueshifted by ∼600 km s−1. Remarkably these features are also variable on the ∼500 s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (≳30 000 K) outflowing wind seen at very high inclination, and draw comparisons with other accretion disc corona sources. We argue against previous distance estimates of 1.5 kpc and favour a value ≳6 kpc, implying an X-ray luminosity LX ≳ 4 × 1036 erg s−1. Hence it is not a very faint X-ray transient. Our preliminary 1D Monte Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic LX, a column density NH ≳ 1024 cm−2, and a low covering factor for the wind. Our study shows that Swift J1357.2−0933 is truly remarkable amongst the cohort of luminous, Galactic X-ray binaries, showing the first example of He  II λ4686 absorption, the first (and only) variable dip period and is possibly the first black hole ‘accretion disc corona’ candidate.


Dependence on ion temperature of shallow-angle magnetic presheaths with adiabatic electrons

Journal of Plasma Physics Cambridge University Press 85 (2019) 795850601

A Geraldini, F Parra, F Militello


Overview of new MAST physics in anticipation of first results from MAST Upgrade

Nuclear Fusion IOP Science 59 (2019) 112011

Harrison, RJ Akers, SY Allan, JS Allcock, JO Allen, L Appel, M Barnes, NB Ben Ayedl, W Boeglin, C Bowman, J Bradley, P Browning, P Bryant, M Carr, M Cecconello, CD Challis, S Chapman, IT Chapman, GJ Colyer, S Conroy, NJ Conway, M Cox, G Cunningham, RO Dendy, W Dorland

The mega amp spherical tokamak (MAST) was a low aspect ratio device (R/a  =  0.85/0.65 ~ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics issues for the operation of ITER, design of DEMO and future spherical tokamaks by utilising high resolution diagnostic measurements closely coupled with theory and modelling to significantly advance our understanding. An empirical scaling of the energy confinement time that favours higher power, lower collisionality devices is consistent with gyrokinetic modelling of electron scale turbulence. Measurements of ion scale turbulence with beam emission spectroscopy and gyrokinetic modelling in up-down symmetric plasmas find that the symmetry of the turbulence is broken by flow shear. Near the non-linear stability threshold, flow shear tilts the density fluctuation correlation function and skews the fluctuation amplitude distribution. Results from fast particle physics studies include the observation that sawteeth are found to redistribute passing and trapped fast particles injected from neutral beam injectors in equal measure, suggesting that resonances between the m  =  1 perturbation and the fast ion orbits may be playing a dominant role in the fast ion transport. Measured D–D fusion products from a neutron camera and a charged fusion product detector are 40% lower than predictions from TRANSP/NUBEAM, highlighting possible deficiencies in the guiding centre approximation. Modelling of fast ion losses in the presence of resonant magnetic perturbations (RMPs) can reproduce trends observed in experiments when the plasma response and charge-exchange losses are accounted for. Measurements with a neutral particle analyser during merging-compression start-up indicate the acceleration of ions and electrons. Transport at the plasma edge has been improved through reciprocating probe measurements that have characterised a geodesic acoustic mode at the edge of an ohmic L-mode plasma and particle-in-cell modelling has improved the interpretation of plasma potential estimates from ball-pen probes. The application of RMPs leads to a reduction in particle confinement in L-mode and H-mode and an increase in the core ionization source. The ejection of secondary filaments following type-I ELMs correlates with interactions with surfaces near the X-point. Simulations of the interaction between pairs of filaments in the scrape-off layer suggest this results in modest changes to their velocity, and in most cases can be treated as moving independently. A stochastic model of scrape-off layer profile formation based on the superposition of non-interacting filaments is in good agreement with measured time-average profiles. Transport in the divertor has been improved through fast camera imaging, indicating the presence of a quiescent region devoid of filament near the X-point, extending from the separatrix to ψ n ~ 1.02. Simulations of turbulent transport in the divertor show that the angle between the divertor leg on the curvature vector strongly influences transport into the private flux region via the interchange mechanism. Coherence imaging measurements show counter-streaming flows of impurities due to gas puffing increasing the pressure on field lines where the gas is ionised. MAST Upgrade is based on the original MAST device, with substantially improved capabilities to operate with a Super-X divertor to test extended divertor leg concepts. SOLPS-ITER modelling predicts the detachment threshold will be reduced by more than a factor of 2, in terms of upstream density, in the Super-X compared with a conventional configuration and that the radiation front movement is passively stabilised before it reaches the X-point. 1D fluid modelling reveals the key role of momentum and power loss mechanisms in governing detachment onset and evolution. Analytic modelling indicates that long legs placed at large major radius, or equivalently low at the target compared with the X-point are more amenable to external control. With MAST Upgrade experiments expected in 2019, a thorough characterisation of the sources of the intrinsic error field has been carried out and a mitigation strategy developed.


Field reconstruction from proton radiography of intense laser driven magnetic reconnection

Physics of Plasmas AIP Publishing 26 (2019)

CAJ Palmer, PT Campbell, Y Ma, L Antonelli, AFA Bott, G Gregori, J Halliday, Y Katzir, P Kordell, K Krushelnick, SV Lebedev, E Montgomery, M Notley, DC Carroll, CP Ridgers, A Schekochihin, MJV Streeter, AGR Thomas, ER Tubman, N Woolsey, L Willingale

Magnetic reconnection is a process that contributes significantly to plasma dynamics and energy transfer in a wide range of plasma and magnetic field regimes, including inertial confinement fusion experiments, stellar coronae, and compact, highly magnetized objects like neutron stars. Laboratory experiments in different regimes can help refine, expand, and test the applicability of theoretical models to describe reconnection. Laser-plasma experiments exploring magnetic reconnection at a moderate intensity (IL ∼1014 W cm-2) have been performed previously, where the Biermann battery effect self-generates magnetic fields and the field dynamics studied using proton radiography. At high laser intensities (ILλL2&gt;1018 Wcm-2μm2), relativistic surface currents and the time-varying electric sheath fields generate the azimuthal magnetic fields. Numerical modeling of these intensities has shown the conditions that within the magnetic field region can reach the threshold where the magnetic energy can exceed the rest mass energy such that σcold = B2/(μ0nemec2) &gt; 1 [A. E. Raymond et al., Phys. Rev. E 98, 043207 (2018)]. Presented here is the analysis of the proton radiography of a high-intensity (∼1018 W cm-2) laser driven magnetic reconnection geometry. The path integrated magnetic fields are recovered using a "field-reconstruction algorithm" to quantify the field strengths, geometry, and evolution.


Cosmic ray acceleration to ultrahigh energy in radio galaxies

EPJ Web of Conferences EDP Sciences (2019)

JH Matthews, AR Bell, AT Araudo, KM Blundell

The origin of ultrahigh energy cosmic rays (UHECRs) is an open question. In this proceeding, we first review the general physical requirements that a source must meet for acceleration to 10-100 EeV, including the consideration that the shock is not highly relativistic. We show that shocks in the backflows of radio galaxies can meet these requirements. We discuss a model in which giant-lobed radio galaxies such as Centaurus A and Fornax A act as slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a more powerful past episode. We also show that Centaurus A, Fornax A and other radio galaxies may explain the observed anisotropies in data from the Pierre Auger Observatory, before examining some of the difficulties in associating UHECR anisotropies with astrophysical sources.


Do reverberation mapping analyses provide an accurate picture of the broad-line region?

Monthly Notices of the Royal Astronomical Society Oxford University Press 488 (2019) 2780–2799-

SW Mangham, C Knigge, P Williams, K Horne, A Pancoast, J Matthews, KS Long, N Higginbottom

Reverberation mapping (RM) is a powerful approach for determining the nature of the broad-line region (BLR) in active galactic nuclei. However, inferring physical BLR properties from an observed spectroscopic time series is a difficult inverse problem. Here, we present a blind test of two widely used RM methods: MEMECHO (developed by Horne) and CARAMEL (developed by Pancoast and collaborators). The test data are simulated spectroscopic time series that track the Hα emission line response to an empirical continuum light curve. The underlying BLR model is a rotating, biconical accretion disc wind, and the synthetic spectra are generated via self-consistent ionization and radiative transfer simulations. We generate two mock data sets, representing Seyfert galaxies and QSOs. The Seyfert model produces a largely negative response, which neither method can recover. However, both fail ‘gracefully', neither generating spurious results. For the QSO model both CARAMEL and expert interpretation of MEMECHOś output both capture the broadly annular, rotation-dominated nature of the line-forming region, though MEMECHO analysis overestimates its size by 50 per cent, but CARAMEL is unable to distinguish between additional inflow and outflow components. Despite fitting individual spectra well, the CARAMEL velocity-delay maps and RMS line profiles are strongly inconsistent with the input data. Finally, since the Hα line-forming region is rotation dominated, neither method recovers the disc wind nature of the underlying BLR model. Thus considerable care is required when interpreting the results of RM analyses in terms of physical models.

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