11 research outputs found
Bondi-Hoyle-Lyttleton Accretion onto Star Clusters
An isolated star moving supersonically through a uniform gas accretes
material from its gravitationally-induced wake. The rate of accretion is set by
the accretion radius of the star and is well-described by classical
Bondi-Hoyle-Lyttleton theory. Stars, however, are not born in isolation. They
form in clusters where they accrete material that is influenced by all the
stars in the cluster. We perform three-dimensional hydrodynamic simulations of
clusters of individual accretors embedded in a uniform-density wind in order to
study how the accretion rates experienced by individual cluster members are
altered by the properties of the ambient gas and the cluster itself. We study
accretion as a function of number of cluster members, mean separation between
them, and size of their individual accretion radii. We determine the effect of
these key parameters on the aggregate and individual accretion rates, which we
compare to analytic predictions. We show that when the accretion radii of the
individual objects in the cluster substantially overlap, the surrounding gas is
effectively accreted into the collective potential of the cluster prior to
being accreted onto the individual stars. We find that individual cluster
members can accrete drastically more than they would in isolation, in
particular when the flow is able to cool efficiently. This effect could
potentially modify the luminosity of accreting compact objects in star clusters
and could lead to the rejuvenation of young star clusters as well as globular
clusters with low-inclination and low-eccentricity.Comment: 15 pages, 12 figures. Accepted to Ap
Illuminating black hole subsystems in young star clusters
There is increasing evidence that globular clusters retain sizeable black
hole populations at present day. This is supported by dynamical simulations of
cluster evolution, which have unveiled the spatial distribution and mass
spectrum of black holes in clusters across cosmic age. However, black hole
populations of young, high metallicity clusters remain unconstrained. Black
holes hosted by these clusters mass segregate early in their evolutionary
history, forming central subsystems of hundreds to thousands of black holes. We
argue that after supernova feedback has subsided (), the
host cluster can accumulate gas from its dense surroundings, from which the
black hole subsystem accretes at highly enhanced rates. The collective
accretion luminosity can be substantial and provides a novel observational
constraint for young massive clusters. We test this hypothesis by performing 3D
hydrodynamic simulations where we embed discretized potentials, representing
our black holes, within the potential of a massive cluster. This system moves
supersonically with respect to a gaseous medium from which it accretes. We
study the accretion of this black hole subsystem for different subsystem
populations and determine the integrated accretion luminosity of the black hole
subsystem. We apply our results to the young massive clusters of the Antennae
Galaxies and find that a typical subsystem accretion luminosity should be in
excess of . We argue that no strong
candidates of this luminous signal have been observed and constrain the
subsystem population of a typical cluster in the Antennae Galaxies to
black holes, given that feedback doesn't
significantly impede accretion and that the gas remains optically thin.Comment: 15 pages, 8 figures. Accepted by Ap
Lessons from HAWC PWNe observations: the diffusion constant is not a constant; Pulsars remain the likeliest sources of the anomalous positron fraction; Cosmic rays are trapped for long periods of time in pockets of inefficient diffusion
Recent TeV observations of nearby pulsars with the HAWC telescope have been
interpreted as evidence that the diffusion of high-energy electrons and
positrons within pulsar wind nebulae is highly inefficient compared to the rest
of the interstellar medium. If the diffusion coefficient well outside the
nebula is close to the value inferred for the region inside the nebula,
high-energy electrons and positrons produced by the two observed pulsars could
not contribute significantly to the local measured cosmic-ray flux. The HAWC
Collaboration thus concluded that, under the assumption of isotropic and
homogeneous diffusion, the two pulsars are ruled out as sources of the
anomalous high-energy positron flux. Here, we argue that since the diffusion
coefficient is likely not spatially homogeneous, the assumption leading to this
conclusion is flawed. We solve the diffusion equation with a radially dependent
diffusion coefficient, and show that the pulsars observed by HAWC produce
potentially perfect matches to the observed high-energy positron fluxes. We
also study the implications of inefficient diffusion within pulsar wind nebulae
on Galactic scales, and show that cosmic rays are likely to have very long
residence times in regions of inefficient diffusion. We describe how this
prediction can be tested with studies of the diffuse Galactic emission.Comment: 12 pages, 7 figures, published in PR
A neutral analogue of a phosphamethine cyanine
Reaction of an imidazolio-phosphide with an N-heterocyclic bromo-borane and NaH afforded a neutral analogue of a phosphamethine cyanine cation. DFT studies were used to analyse the dative bonding across P-C/B bonds and the conformational preferences and imply that the observed conformation is imposed by sterics.Peer reviewe
The hydrodynamic evolution of binary black holes embedded within the vertically stratified disks of active galactic nuclei
Stellar-mass black holes can become embedded within the gaseous disks of
active galactic nuclei (AGNs). Afterwards, their interactions are mediated by
their gaseous surroundings. In this work, we study the evolution of
stellar-mass binary black holes (BBHs) embedded within AGN disks using a
combination of three-dimensional hydrodynamic simulations and analytic methods,
focusing on environments in which the AGN disk scale height is
the BBH sphere of influence. We model the local surroundings of the embedded
BBHs using a wind tunnel formalism and characterize different accretion regimes
based on the local properties of the disk, which range from wind-dominated to
quasi-spherical. We use our simulations to develop prescriptions for mass
accretion and drag for embedded BBHs. We use these prescriptions, along with
AGN disk models that can represent the Toomre-unstable outer regions of AGN
disks, to study the long-term evolution of the BBHs as they migrate through the
disk. We find that BBHs typically merge within ,
increasing their mass significantly in the process, allowing BBHs to enter (or
cross) the pair-instability supernova mass gap. The rate at which gas is
supplied to these BBHs often exceeds the Eddington limit, sometimes by several
orders of magnitude. We conclude that most embedded BBHs will merge before
migrating significantly in the disk. Depending on the conditions of the ambient
gas and the distance to the system, LISA can detect the transition between the
gas-dominated and gravitational wave dominated regime for inspiraling BBHs that
are formed sufficiently close to the AGN ( 0.1 pc). We also discuss
possible electromagnetic signatures during and following the inspiral, finding
that it is generally unlikely but not inconceivable for the bolometric
luminosity of the BBH to exceed that of the host AGN.Comment: 19 pages, 8 figures, submitted to Ap
Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks
The conventional accretion disk lore is that magnetized turbulence is the
principal angular momentum transport process that drives accretion. However,
when dynamically important magnetic fields thread an accretion disk, they can
produce mass and angular momentum outflows that also drive accretion. Yet, the
relative importance of turbulent and wind-driven angular momentum transport is
still poorly understood. To probe this question, we analyze a long-duration
() simulation of a rapidly rotating ()
black hole (BH) feeding from a thick (), adiabatic, magnetically
arrested disk (MAD), whose dynamically-important magnetic field regulates mass
inflow and drives both uncollimated and collimated outflows (e.g., "winds" and
"jets", respectively). By carefully disentangling the various angular momentum
transport processes occurring within the system, we demonstrate the novel
result that both disk winds and disk turbulence extract roughly equal amounts
of angular momentum from the disk. We find cumulative angular momentum and mass
accretion outflow rates of and , respectively. This result suggests that understanding both turbulent
and laminar stresses is key to understanding the evolution of systems where
geometrically thick MADs can occur, such as the hard state of X-ray binaries,
low-luminosity active galactic nuclei, some tidal disruption events, and
possibly gamma ray bursts.Comment: 15 pages, 6 figures. Submitted to ApJ. Comments welcom
Recommended from our members
Lessons from HAWC pulsar wind nebulae observations: The diffusion constant is not a constant; pulsars remain the likeliest sources of the anomalous positron fraction; cosmic rays are trapped for long periods of time in pockets of inefficient diffusion
Recent TeV observations of nearby pulsars with the HAWC telescope have been
interpreted as evidence that the diffusion of high-energy electrons and
positrons within pulsar wind nebulae is highly inefficient compared to the rest
of the interstellar medium. If the diffusion coefficient well outside the
nebula is close to the value inferred for the region inside the nebula,
high-energy electrons and positrons produced by the two observed pulsars could
not contribute significantly to the local measured cosmic-ray flux. The HAWC
Collaboration thus concluded that, under the assumption of isotropic and
homogeneous diffusion, the two pulsars are ruled out as sources of the
anomalous high-energy positron flux. Here, we argue that since the diffusion
coefficient is likely not spatially homogeneous, the assumption leading to this
conclusion is flawed. We solve the diffusion equation with a radially dependent
diffusion coefficient, and show that the pulsars observed by HAWC produce
potentially perfect matches to the observed high-energy positron fluxes. We
also study the implications of inefficient diffusion within pulsar wind nebulae
on Galactic scales, and show that cosmic rays are likely to have very long
residence times in regions of inefficient diffusion. We describe how this
prediction can be tested with studies of the diffuse Galactic emission
Recommended from our members
Lessons from HAWC pulsar wind nebulae observations: The diffusion constant is not a constant; pulsars remain the likeliest sources of the anomalous positron fraction; cosmic rays are trapped for long periods of time in pockets of inefficient diffusion
Recent TeV observations of nearby pulsars with the HAWC telescope have been
interpreted as evidence that the diffusion of high-energy electrons and
positrons within pulsar wind nebulae is highly inefficient compared to the rest
of the interstellar medium. If the diffusion coefficient well outside the
nebula is close to the value inferred for the region inside the nebula,
high-energy electrons and positrons produced by the two observed pulsars could
not contribute significantly to the local measured cosmic-ray flux. The HAWC
Collaboration thus concluded that, under the assumption of isotropic and
homogeneous diffusion, the two pulsars are ruled out as sources of the
anomalous high-energy positron flux. Here, we argue that since the diffusion
coefficient is likely not spatially homogeneous, the assumption leading to this
conclusion is flawed. We solve the diffusion equation with a radially dependent
diffusion coefficient, and show that the pulsars observed by HAWC produce
potentially perfect matches to the observed high-energy positron fluxes. We
also study the implications of inefficient diffusion within pulsar wind nebulae
on Galactic scales, and show that cosmic rays are likely to have very long
residence times in regions of inefficient diffusion. We describe how this
prediction can be tested with studies of the diffuse Galactic emission
Jet Formation in 3D GRMHD Simulations of Bondi-Hoyle-Lyttleton Accretion
A black hole (BH) travelling through a uniform, gaseous medium is described
by Bondi-Hoyle-Lyttleton (BHL) accretion. If the medium is magnetized, then the
black hole can produce relativistic outflows. We performed the first 3D,
general-relativistic magnetohydrodynamics simulations of BHL accretion onto
rapidly rotating black holes using the code H-AMR, where we mainly varied the
strength of a background magnetic field that threads the medium. We found that
the ensuing accretion continuously drags to the BH the magnetic flux, which
accumulates near the event horizon until it becomes dynamically important.
Depending on the strength of the background magnetic field, the BHs can
sometimes launch relativistic jets with high enough power to drill out of the
inner accretion flow, become bent by the headwind, and escape to large
distances. While for stronger background magnetic fields the jets are
continuously powered, at weaker field strengths they are intermittent, turning
on and off depending on the fluctuating gas and magnetic flux distributions
near the event horizon. We find that our jets reach extremely high efficiencies
of , even in the absence of an accretion disk. We also
calculated the drag forces exerted by the gas onto to the BH, finding that the
presence of magnetic fields causes drag forces to be much less efficient than
in unmagnetized BHL accretion, and sometimes become negative, accelerating the
BH rather than slowing it down. Our results extend classical BHL accretion to
rotating BHs moving through magnetized media and demonstrate that accretion and
drag are significantly altered in this environment.Comment: Comments welcome! 16 pages, 8 figures, submitted to Ap
Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks
The conventional accretion disk lore is that magnetized turbulence is the principal angular momentum transport process that drives accretion. However, when dynamically important large-scale magnetic fields thread an accretion disk, they can produce mass and angular momentum outflows, known as winds , that also drive accretion. Yet, the relative importance of turbulent and wind-driven angular momentum transport is still poorly understood. To probe this question, we analyze a long-duration (1.2 × 10 ^5 r _g / c ) simulation of a rapidly rotating ( a = 0.9) black hole feeding from a thick ( H / r ∼ 0.3), adiabatic, magnetically arrested disk (MAD), whose dynamically important magnetic field regulates mass inflow and drives both uncollimated and collimated outflows (i.e., winds and jets, respectively). By carefully disentangling the various angular momentum transport processes within the system, we demonstrate the novel result that disk winds and disk turbulence both extract roughly equal amounts of angular momentum from the disk. We find cumulative angular momentum and mass accretion outflow rates of and , respectively. This result suggests that understanding both turbulent and laminar stresses is key to understanding the evolution of systems where geometrically thick MADs can occur, such as the hard state of X-ray binaries, low-luminosity active galactic nuclei, some tidal disruption events, and possibly gamma-ray bursts