# 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>

## Transport of high-energy charged particles through spatially-intermittent turbulent magnetic fields

Astrophysical Journal American Astronomical Society 892 (2020) 114

LE Chen, AFA Bott, P Tzeferacos, A Rigby, A Bell, R Bingham, C Graziani, J Katz, R Petrasso, G Gregori, F Miniati

Identifying the sources of the highest energy cosmic rays requires understanding how they are deflected by the stochastic, spatially intermittent intergalactic magnetic field. Here we report measurements of energetic charged-particle propagation through a laser-produced magnetized plasma with these properties. We characterize the diffusive transport of the particles experimentally. The results show that the transport is diffusive and that, for the regime of interest for the highest-energy cosmic rays, the diffusion coefficient is unaffected by the spatial intermittency of the magnetic field.

## 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

## Intermediate-mass Black Holes' Effects on Compact Object Binaries

ASTROPHYSICAL JOURNAL American Astronomical Society 892 (2020) ARTN 130

B Deme, Y Meiron, B Kocsis

Although their existence is not yet confirmed observationally, intermediate mass black holes (IMBHs) may play a key role in the dynamics of galactic nuclei. In this paper, we neglect the effect of the nuclear star cluster itself and investigate only how a small reservoir of IMBHs influences the secular dynamics of stellar-mass black hole binaries, using N-body simulations. We show that our simplifications are valid and that the IMBHs significantly enhance binary evaporation by pushing the binaries into the Hill-unstable region of parameter space, where they are separated by the SMBH's tidal field. For binaries in the S-cluster region of the Milky Way, IMBHs drive the binaries to merge in up to 1-6% of cases, assuming five IMBHs within 5 pc of mass 10,000 solar masses each. Observations of binaries in the Galactic center may strongly constrain the population of IMBHs therein.

## Making a Supermassive Star by Stellar Bombardment

ASTROPHYSICAL JOURNAL American Astronomical Society 892 (2020) ARTN 36

H Tagawa, Z Haiman, B Kocsis

Approximately two hundred supermassive black holes (SMBHs) have been discovered within the first $\sim$Gyr after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation, gas accretion, dynamical friction from stars and gas, stellar collisions, and gas ejection. We find that when the collapsing gas fragments at a density of $\lesssim 3\times 10^{10}\,\mathrm{cm^{-3}}$, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a $\lesssim 10^4~{\rm M_\odot}$ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with $Z\lesssim10^{-5}\,{\rm Z_\odot}$ with suppressed ${\rm H_2}$ abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as $\sim 10^5-10^6\,{\rm M_\odot}$. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.

## Cosmic Evolution of Stellar-mass Black Hole Merger Rate in Active Galactic Nuclei

ASTROPHYSICAL JOURNAL American Astronomical Society 896 (2020) ARTN 138

Y Yang, I Bartos, Z Haiman, B Kocsis, S Marka, H Tagawa

## GW170817A as a Hierarchical Black Hole Merger

ASTROPHYSICAL JOURNAL LETTERS American Astronomical Society 890 (2020) ARTN L20

V Gayathri, I Bartos, Z Haiman, S Klimenko, B Kocsis, S Marka, Y Yang

Despite the rapidly growing number of stellar-mass binary black hole mergers discovered through gravitational waves, the origin of these binaries is still not known. In galactic centers, black holes can be brought to each others' proximity by dynamical processes, resulting in mergers. It is also possible that black holes formed in previous mergers encounter new black holes, resulting in so-called hierarchical mergers. Hierarchical events carry signatures such as higher-than usual black hole mass and spin. Here we show that the recently reported gravitational-wave candidate, GW170817A, could be the result of such a hierarchical merger. In particular, its chirp mass $\sim40$ M$_\odot$ and effective spin of $\chi_{\rm eff}\sim0.5$ are the typically expected values from hierarchical mergers within the disks of active galactic nuclei. We find that the reconstructed parameters of GW170817A strongly favor a hierarchical merger origin over having been produced by an isolated binary origin (with an Odds ratio of $&gt;10^3$, after accounting for differences between the expected rates of hierarchical versus isolated mergers)

## Spin Evolution of Stellar-mass Black Hole Binaries in Active Galactic Nuclei

The Astrophysical Journal American Astronomical Society 899 (2020) 26-26

H Tagawa, Z Haiman, I Bartos, B Kocsis

## Formation and Evolution of Compact-object Binaries in AGN Disks

ASTROPHYSICAL JOURNAL American Astronomical Society 898 (2020) ARTN 25

H Tagawa, Z Haiman, B Kocsis

## Effective spin distribution of black hole mergers in triples

Monthly Notices of the Royal Astronomical Society 493 (2020) 3920-3931

G Fragione, B Kocsis

&#xA9; 2020 The Author(s). Many astrophysical scenarios have been proposed to explain the several black hole (BH) and neutron star binary mergers observed via gravitational waves (GWs) by the LIGO-Virgo collaboration. Contributions from various channels can be statistically disentangled by mass, spin, eccentricity, and redshift distributions of merging binaries. In this paper,we investigate the signatures of BH-BH binary mergers induced by a third companion through the Lidov-Kozai mechanism in triple systems. We adopt different prescriptions for the supernovae natal kicks and consider different progenitor metallicities and initial orbital parameters. We show that the typical eccentricity in the LIGO band is 0.01-0.1 and that the merger rate is in the range 0.008-9Gpc-3 yr-1, depending on the natal kick prescriptions and progenitor metallicity. Furthermore, we find that the typical distribution of effective projected spin is peaked at Xeff ~ 0 with significant tails. We show that the triple scenario could reproduce the distribution of Xeff. We find that the triple channel may be strongly constrained by the misalignment angle between the binary component spins in future detections with spin precession.

## Fluctuation dynamo in a weakly collisional plasma

Journal of Plasma Physics Cambridge University Press (CUP) 86 (2020) 905860503

DA St-Onge, MW Kunz, J Squire, AA Schekochihin

<jats:p>The turbulent amplification of cosmic magnetic fields depends upon the material properties of the host plasma. In many hot, dilute astrophysical systems, such as the intracluster medium (ICM) of galaxy clusters, the rarity of particle–particle collisions allows departures from local thermodynamic equilibrium. These departures – pressure anisotropies – exert anisotropic viscous stresses on the plasma motions that inhibit their ability to stretch magnetic-field lines. We present an extensive numerical study of the fluctuation dynamo in a weakly collisional plasma using magnetohydrodynamic (MHD) equations endowed with a field-parallel viscous (Braginskii) stress. When the stress is limited to values consistent with a pressure anisotropy regulated by firehose and mirror instabilities, the Braginskii-MHD dynamo largely resembles its MHD counterpart, particularly when the magnetic field is dynamically weak. If instead the parallel viscous stress is left unabated – a situation relevant to recent kinetic simulations of the fluctuation dynamo and, we argue, to the early stages of the dynamo in a magnetized ICM – the dynamo changes its character, amplifying the magnetic field while exhibiting many characteristics reminiscent of the saturated state of the large-Prandtl-number (<jats:inline-formula> <jats:alternatives> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0022377820000860_inline1.png" /> <jats:tex-math>${Pm}\gtrsim {1}$</jats:tex-math> </jats:alternatives> </jats:inline-formula>) MHD dynamo. We construct an analytic model for the Braginskii-MHD dynamo in this regime, which successfully matches simulated dynamo growth rates and magnetic-energy spectra. A prediction of this model, confirmed by our numerical simulations, is that a Braginskii-MHD plasma without pressure-anisotropy limiters will not support a dynamo if the ratio of perpendicular and parallel viscosities is too small. This ratio reflects the relative allowed rates of field-line stretching and mixing, the latter of which promotes resistive dissipation of the magnetic field. In all cases that do exhibit a viable dynamo, the generated magnetic field is organized into folds that persist into the saturated state and bias the chaotic flow to acquire a scale-dependent spectral anisotropy.</jats:p>

## 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.

## Electromagnetic transients and gravitational waves from white dwarf disruptions by stellar black holes in triple systems

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2020)

G Fragione, BD Metzger, R Perna, NWC Leigh, B Kocsis

Mergers of binaries comprised of compact objects can give rise to explosive transient events, heralding the birth of exotic objects which cannot be formed through single star evolution. Using a large number of direct N-body simulations, we explore the possibility that a white dwarf (WD) is dynamically driven to tidal disruption by a stellar-mass black hole (BH) as a consequence of the joint effects of gravitational wave (GW) emission and Lidov-Kozai oscillations imposed by the tidal field of a outer tertiary companion orbiting the inner BH-WD binary. We explore the sensitivity of our results to the distributions of natal kick velocities imparted to the BH and WD upon formation, adiabatic mass loss, semi-major axes and eccentricities of the triples, and stellar mass ratios. We find rates of WD-TDEs in the range $1.2\times 10^{-3}-1.4$ Gpc$^{-3}$ yr$^{-1}$ for $z\leq 0.1$, rarer than stellar TDEs in triples by a factor of $\sim 3$--$30$. The uncertainty in the TDE rates may be greatly reduced in the future using gravitational wave (GW) observations of Galactic binaries and triples with LISA. WD-TDEs may give rise to high energy X-ray or gamma-ray transients of duration similar to long gamma-ray bursts but lacking the signatures of a core-collapse supernova, while being accompanied by a supernova-like optical transient which lasts for only days. WD--BH and WD--NS binaries will also emit GWs in the LISA band before the TDE. The discovery and identification of triple-induced WD-TDE events by future time domain surveys and/or GWs could enable the study of the demographics of BHs in nearby galaxies.

## Binary Intermediate-mass Black Hole Mergers in Globular Clusters

The Astrophysical Journal American Astronomical Society 899 (2020) 149-149

A Rasskazov, G Fragione, B Kocsis

## Relaxation of spherical stellar systems

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

JY Lau, J Binney

## 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

## Anisotropic Mass Segregation in Rotating Globular Clusters

ASTROPHYSICAL JOURNAL American Astronomical Society 887 (2019) ARTN 123

A Szolgyen, Y Meiron, B Kocsis

We investigate the internal dynamics of anisotropic, rotating globular clusters with a multimass stellar population by performing new direct N-body simulations. In addition to the well-known radial mass segregation effect, where heavy stars and stellar remnants sink toward the center of the cluster, we find a mass segregation in the distribution of orbital inclinations as well. This newly discovered anisotropic mass segregation leads to the formation of a disk-like structure of massive objects near the equatorial plane of a rotating cluster. This result has important implications on the expected spatial distribution of black holes in globular clusters.

## 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.

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

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

A Geraldini, FI Parra, F Militello

The magnetic presheath is a boundary layer occurring when magnetized plasma is in contact with a wall and the angle α between the wall and the magnetic field B is oblique. Here, we consider the fusion-relevant case of a shallow-angle, α 1, electron-repelling sheath, with the electron density given by a Boltzmann distribution, valid for α/√τ + 1 √me/mi, where me is the electron mass, mi is the ion mass, τ = Ti/ZTe,Te is the electron temperature, Ti is the ion temperature and Z is the ionic charge state. The thickness of the magnetic presheath is of the order of a few ion sound Larmor radii ρs = √mi(ZTe + Ti)/ZeB, where e is the proton charge and B = |B| is the magnitude of the magnetic field. We study the dependence on τ of the electrostatic potential and ion distribution function in the magnetic presheath by using a set of prescribed ion distribution functions at the magnetic presheath entrance, parameterized by τ . The kinetic model is shown to be asymptotically equivalent to Chodura’s fluid model at small ion temperature, τ 1, for |ln α| > 3|ln τ | 1. In this limit, despite the fact that fluid equations give a reasonable approximation to the potential, ion gyroorbits acquire a spatial extent that occupies a large portion of the magnetic presheath. At large ion temperature, τ 1, relevant because Ti is measured to be a few times larger than Te near divertor targets of fusion devices, ions reach the Debye sheath entrance (and subsequently the wall) at a shallow angle whose size is given by √α or 1/√τ, depending on which is largest.