Frontiers in High Energy Density Physics The X-Games of Contemporary Science (2003) / Chapter Skim
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2. High Energy Density Astrophysics
2. High Energy Density Astrophysics
Pages 34-70
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From page 34... ...
The twin engines of gravitational collapse and nuclear fusion power phenomena on a nearly unimaginable scale. Giant black holes consume the fiery hearts of galaxies, sweeping entire star systems into their immense accretion disks; relativistic particle jets, powered by unknown acceleration mechanisms, focus their extreme energies with incredible precision across millions of light years; supernovae shocks sweep up turbulent plasma and dust, creating the seeds for stellar rebirth; neutron stars the size of Manhattan spin at kilohertz rates, weaving their huge magnetic fields through the surrounding plasma and creating brilliant x-ray lighthouses.
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From page 35... ...
, astrophysical jets (see Figure 2.2) , radiatively driven molecular clouds, accreting black holes, planetary interiors, and gamma-ray bursts.
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From page 36... ...
1054, is a beautiful example of a relatively young remnant, whose appearance is thought to be largely driven by particle acceleration processes tied to the Crab Pulsar; the observed radiation is due to synchrotron emission from highly relativistic electrons accelerated within the structure seen in this image. The precise nature of the connection between the pulsar and its magnetosphere, and the surroundings, remains uncertain.
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From page 37... ...
The "engine" producing these jets is most likely one of the most extreme physical environments encountered in astrophysics, namely, the vicinity of a massive black hole lying near the core of active galaxies (see Figure 2.6 in this chapter)
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From page 38... ...
In order to solve the many fundamental questions of planetary formation, evolution, and structure, it is essential to improve our understanding of hydrogen in the ultrahigh-pressure environment found in the interior of brown dwarfs and giant planets. This environment is characterized by moderate temperatures of order 0.1 to 1 eV, and extreme pressures of order 1 to 10 Mbar.
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From page 39... ...
of NASA and Space Telescope Science Institute, and (bottom)
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From page 40... ...
is important because convection of pressure-ionized, metallic hydrogen is thought to create the 10 to 15 gauss magnetic field of Jupiter. Of particular concern is whether a first-order plasma phase transition exists, as this critically affects the internal structure in the important convection zone and the degree of gravitational energy release due to sedimentation of helium (He)
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From page 41... ...
On these future facilities, quasi-isentropic compressions of well over 10 Mbar should be possible. These laboratory conditions correspond to the interiors of terrestrial and giant gas planets, brown dwarfs, average mass stars, and the envelopes of white dwarfs.
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From page 42... ...
. The Opacities of Stellar Matter Historically, one of the oldest areas of overlap between HED plasma physics and astronomy has been the radiative opacity of matter.
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From page 43... ...
HED experiments promise to allow the study of a nuclear-burning plasma, that is, the source of stellar energy. Accelerator experiments have provided means to study nuclear reactions in a piecemeal way; HED experiments will allow us to place the phenomena into the context of a burning plasma.
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From page 44... ...
For example, by heating or compressing a gas of atoms, one eventually forms a plasma in which the nuclei become stripped of the electrons, which go into continuum states forming an electron gas. Similarly, when nuclei are squeezed (as happens in the formation of neutron stars in supernovae where the matter is compressed by gravitational collapse)
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From page 45... ...
The order of the quark-gluon phase transition may have important implications for gravitational-wave radiation from stellar collapse. A third astrophysical situation in which these issues arise is the formation of neutron stars and black holes by the gravitational collapse of stars.
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From page 46... ...
However, we do not understand the nature of this turbulence or even if it is stationary. Laser and beam instability experiments carried out in large chambers filled with magnetized plasma are quite capable of creating collisionless shocks, the properties of which can be studied.
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From page 47... ...
These UHECR events have an unexpectedly high flux and a surprisingly isotropic arrival directional distribution. If these particles are protons, the galactic magnetic field is not strong enough to contain them, and the sources are most certainly extragalactic.
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From page 48... ...
The origin of these magnetic fields remains a mystery, including especially the origins of magnetic fields in astronomical objects specifically relevant to HED astrophysics the magnetic fields of neutron stars and giant gaseous planets.
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From page 49... ...
A similarly detailed understanding of the origin of Jupiter's magnetic fields, or of the magnetic fields of neutron stars, is still absent. In these cases, models have been proposed, but the detailed calculations and measurements that would allow an understanding of how these magnetic fields came to be present have not been carried out.
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From page 50... ...
In the incompressible limit, the governing principle appears to be that as energy cascades to smaller scales, the field gradient becomes more strongly directed perpendicular to the mean magnetic field, with implications for reconnection. Applying these ideas to understanding the nature of dissipation in
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From page 51... ...
While the first development of accretion theory began a long time ago, intensive development of this theory began after the discovery of the first x-ray sources. Accretion onto stars, including neutron stars, terminates at an inner 51
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From page 52... ...
The radiative efficiency of accretion is not known a priori, in contrast with the case of accretion onto a star, and depends strongly on factors such as the angular momentum of the incoming matter and the magnetic field embedded in it. For spherical accretion of nonmagnetized gas, the efficiency of radiation may be as small as 10-8 for low mass accretion rates; the presence of a magnetic field in the accreting matter can increase the efficiency up to it_ at.*
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From page 53... ...
A magnetohydrodynamic instability as a source of the turbulence in accretion disks has been studied extensively by linear stability studies and MHD simulations during the past several years. The linear instability the magnetorotational instability was originally discovered in the late 1 950s, but rediscovered and applied for the first time to accretion disks by astrophysicists in the 1 980s; it occurs for a weak magnetic field in a differentially rotating disk (Alfven speed < sound speed)
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From page 54... ...
This instability could be important in the absence of any other source of turbulence and if the magnetic fields are sufficiently weak. Stronger fields probably occur in actual disks, and unstable singular modes have been recently discovered in a strongly magnetized accretion disk.
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From page 55... ...
Recent observations, such as the Hubble Space Telescope images of HH 30, and theoretical and simulation studies support models where the twisting of an ordered magnetic field threading an accretion disk acts to magnetically accelerate the jets. The power in the jets is thought to come from matter accretion in the disk, but it may include power extracted electromagnetically from a spinning black hole.
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From page 56... ...
Further, an initial dipolelike poloidal magnetic field can be supplied by a coil in the z = 0 plane. A collimated Poynting jet is predicted to occur under conditions where Bz(O)
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From page 57... ...
Many ot these are binary systems in which one of the members is a collapsed object such as a neutron star or black hole, in the case of an x-ray binary, or a white dwarf in the case of a "cataclysmic variable." At the extreme end lie the active galactic nuclei and quasars. Their enormous luminosities are thought to result from the energy conversion of matter falling into supermassive black holes at the center of galaxies.
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From page 58... ...
These new HED laboratory capabilities will allow complex x-ray photoionization theories and models to be tested under relevant conditions, thereby serving as a critical component in the effort to understand the dynamics of accreting black holes. Gamma-Ray Bursters Gamma-ray bursts are among the greatest enigmas in contemporary astrophysics.
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From page 59... ...
The rest-mass densities that characterize the inner portions of the accretion disk and the jet are p 2 1044 g cm-3, comparable to those inside nuclei and in neutron stars. As the fireball expands, inelastic nuclear collisions are expected when the n and p fluids decouple and their relative drift velocity becomes comparable to the speed of light.
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From page 60... ...
Another intriguing observation in these ultraintense laser experiments was the generation of ultrastrong magnetic fields. Strong magnetic-field generation (>100 MG = 104 T)
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From page 61... ...
an example of a proposed laboratory experiment exploring aspects of the astrophysical model. SOURCES: Images (a)
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From page 62... ...
This pair fireball can be made to collide with another pair fireball to mimic the internal shock model of gamma-ray bursts, allowing us to study how the expansion energy of pair fireballs can be converted into internal energy and gamma rays. Introduction of external equipartition magnetic fields (~10 T)
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From page 63... ...
Radiative blast waves with radiatively preheated upstream gas and radiative blast waves with magnetic fields are also being investigated analytically and numerically. They will be priorities for future experiments.
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From page 64... ...
Experimentally, aspects of the dynamics of radiatively driven molecular clouds, and therefore aspects of these questions, can be tested in the laboratory. Using the radiation emitted from tiny radiation cavity "point sources" on large lasers and pulsed-power facilities, it appears possible to reproduce the dominant photoevaporation-front hydrodynamics of radiatively driven molecular clouds.
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From page 65... ...
High-Density Plasma in Strong Magnetic Fields and the Study of Surface Emission from Isolated Neutron Stars Background The study of thermal radiation from isolated neutron stars can provide important information on the interior physics, magnetic fields, surface composition, and other properties of neutron stars. Such study has been a long-term goal of neutron star physics/astrophysics since early theoretical works indicated that neutron stars should remain detectable as soft x-ray sources for approximately 105 to 1 o6 years after thei r hi rth.
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From page 66... ...
plasma in the outer layers of neutron stars in the presence of intense magnetic fields (B ~ 1044 to 1 ox G) , and to calculate the emergent thermal radiation spectra from the neutron star surfaces.
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From page 67... ...
For sufficiently high magnetic fields and low temperatures (butstill realistic for neutron stars) , the condensed phase is more stable than the atoms and molecules, and there is a first-order phase transition from the nondegenerate gas to the macroscopic condensed state.
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From page 68... ...
/Eb 2 1, where ACOC is the energy of the electron at the cyclotron frequency due to the magnetic field, and Eb is the electron coulomb binding energy. The details of plasma emission and absorption spectra will be significantly affected, which could be relevant to emission spectra from neutron star atmospheres.
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From page 69... ...
In many instances, physical processes arising under astrophysical conditions have counterparts under laboratory conditions mixing instabilities such as Rayleigh-Taylor exemplify such processes. In such cases, laboratory experiments can be used to validate astrophysics simulation codes; this is a necessary condition for ensuring that such simulations properly describe the astrophysical situation (albeit not a sufficient condition)
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From page 70... ...
Areas of promising overlap include the physics of supernova explosions and supernova remnant evol ution, h igh-Mach-number astrophysical jets, planetary interiors, photoevaporation-front hydrodynamics of molecular clouds, photoionized plasmas around accreting black holes, and relativistic plasmas in gamma-ray burster fireballs. This selection of topics may well be just the "tip of the iceberg" as more experience is obtained in carrying out both large-scale numerical simulations and scaled H ED experiments i n support of astrophysics.
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