Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session X12: General Astrophysics and Compact Objects |
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Sponsoring Units: DAP Chair: Christopher Fryer, Los Alamos National Laboratory Room: Plaza Court 2 |
Tuesday, May 5, 2009 1:30PM - 1:42PM |
X12.00001: The Physics of Stars W. David Arnett John von Neumann speculated that computers might become sufficiently powerful that they could be used to solve analytically intractable problems numerically (he gave turbulence as an example), and that those ``numerical experiments'' could be used to provide the insight necessary to develop analytic solutions. A case study will be presented in which we attempt in this way to use computer simulations of 3D turbulent flow in presupernova stars. We find that we can reproduce the simulations surprisingly well---on average---if we replace the viscous term with an effective damping which turns out to be similar to that inferred by Kolmogorov for a turbulent cascade. Stars are gravitationally-controlled thermonuclear reactors. Abundance change (and hence evolution) occurs because of nuclear burning, and mixing. It is now possible to treat this coupled problem in a self-consistent way, free of astronomically calibrated parameters. Implications for stellar evolution, nucleosynthesis yields, core collapse, supernova explosions, helio-seismology, and solar neutrinos will be discussed. It is argued that advances in the treatment of stellar fluid dynamics, along with new developments in laboratory astrophysics, now allow far more reliable predictions of how stars behave. [Preview Abstract] |
Tuesday, May 5, 2009 1:42PM - 1:54PM |
X12.00002: Black hole spacetimes and pulsar timing Richard Price, Teviet Creighton, Fredrick Jenet, Yan Wang A pulsar in a relativistic orbit around a supermassive black hole will exhibit potentially observable strong field effects in the times of arrival of its pulses. We present a simple formalism for computing these effects. This formalism is applied to illustrate the several types of strong field effects, to give explicit examples for the simple case of equatorial pulse beaming and to present a more general discussion for nonequatorial beaming. [Preview Abstract] |
Tuesday, May 5, 2009 1:54PM - 2:06PM |
X12.00003: Magnetohydrodynamic Bondi--Hoyle Accretion Andrew Penner In relativistic Bondi--Hoyle accretion, a massive body travels through a uniform space background. As this body accretes matter it is known to develop either a bow shock or a tail shock. For the parameter space where a bow shock develops, there is the potential for a flip-flop instability to develop. Previous studies of the relativistic extension by Font etal. revealed that the flip-flop instability did not develop; however, they caution that their parameter space, when taken in the Newtonian limit, was outside of the regime that a Newtonian flip-flop would occur. In my research, I extend the works of Font etal. to include the effects of ideal magnetohydrodynamics. First, I continue the search for the flip-flop instability by analyzing lower asymptotic speeds of sound, which correspond closer to the Newtonian region in which a flip-flop will occur. I then include the magnetic fields to determine if these extra fields can trigger further instabilities and possibly enhance the likelihood of developing a flip-flop instability. [Preview Abstract] |
Tuesday, May 5, 2009 2:06PM - 2:18PM |
X12.00004: 3D-Spirals Emerging from Plasma Disks and High Frequency QPOs* P. Rebusco, B. Coppi, M. Bursa An interpretation based on a novel kind of plasma modes\footnote{B. Coppi, MIT/LNS Report 08/08, submitted to Astronomy and Astrophysics (2008).} emerging from axisymmetric disks is proposed for High-Frequency Quasi-Periodic Oscillations (HFQPOs) in low mass X-ray binaries as QPOs can be a probe of strong field gravity. Tri-dimensional, tightly wound spirals are considered that co-rotate with the magnetized plasma disk surrounding a black hole in the vicinity of the marginally stable orbit. These modes can be excited by the combined effects of the differential rotation and the vertical gradients of the plasma density and temperature. The spirals are localized over relatively narrow radial widths and have frequencies that are multiples of the plasma rotation frequency. The high toroidal number $m_{\phi}$ modes are considered to decay into $m_{\phi}=2$ and $m_{\phi}=3$ modes, explaining the observed twin peak QPOs spectra with the 3:2 ratio. The modulation of the observed radiation associated with general relativistic effects is analyzed, considering different emission processes. These are connected to strong variations of the runaway electric field corresponding to a local rarefaction and heating, or to a local increase of plasma density and cooling due to the considered spirals. *Sponsored in part by the U.S. DOE and the Pappalardo Fellowship program. [Preview Abstract] |
Tuesday, May 5, 2009 2:18PM - 2:30PM |
X12.00005: Kinetic simulations of turbulent magnetic-field growth by streaming cosmic rays Tom Stroman, Martin Pohl, Jacek Niemiec The acceleration of cosmic rays to high energies is thought to occur at supernova-remnant shocks via the mechanism of diffusive shock acceleration. Efficient acceleration requires turbulent, amplified magnetic fields in the shock's upstream region. I will present results of multidimensional particle-in-cell simulations aimed at observing the magnetic field amplification that is expected to arise from the cosmic-ray current ahead of the shock. We find that the initial structure and peak strength of the amplified field is somewhat sensitive to the choice of parameters, but that the field growth saturates in a similar manner in all cases: the back-reaction on the cosmic rays leads to net transfer of momentum to the interstellar medium, substantially weakening their relative drift while also implying the development of a modified shock. [Preview Abstract] |
Tuesday, May 5, 2009 2:30PM - 2:42PM |
X12.00006: Effects of recoiling black holes on surrounding disks Miguel Megevand, Luis Lehner, Matthew Anderson, Eric Hirschmann, Steven Liebling, David Neilsen The dynamics of a binary black hole can give rise to a final black hole with a recoil velocity. When a circumbinary disk is present, it will be disturbed by the recoiling black hole, possibly producing shocks and heating the gas. The hot gas can produce electromagnetic radiation through a variety of processes. In this work we study the effects of a recoiling black hole on a thick gaseous disk for different magnitude and orientations of the recoil velocity. [Preview Abstract] |
Tuesday, May 5, 2009 2:42PM - 2:54PM |
X12.00007: Measurement of the OH-He and OH-D$_2$ absolute collision cross-sections from 85 to 460 K Benjamin Stuhl While recent surveys have yielded a large increase in the number of known OH megamasers, detailed simulations of their mechanism have so far been strongly hampered by a lack of knowledge of the collisional properties of the OH-H$_2$ and OH-He systems. Using a Stark-decelerated and magnetically trapped OH sample and a cryogenic crossed beam, we have measured the absolute total OH-He scattering cross-section over a temperature range of 85-331 K and that of the OH-D$_2$ system over 209-460 K. These measurements should provide strong guidance to efforts to calculate the cross-sections at lower temperatures. [Preview Abstract] |
Tuesday, May 5, 2009 2:54PM - 3:06PM |
X12.00008: Nuclear Bremsstrahlung and Radiation Effects: Implications for Nuclear Engineering and Astrophysics Nie Luo, Magdi Ragheb, George Miley, Barclay Jones We consider the effect of nuclear bremsstrahlung in proton-neutron and proton-deuteron reactions. The average center-of-mass energy for such reactions is around keV in a number of nuclear engineering problems. At this low energy, the reacting nucleons are in an s-wave state in terms of their relative angular momentum. The single-gamma radiation process is thus strongly suppressed due to conservation laws. Instead the gamma ray released is likely to be accompanied by soft X-ray photons from a nuclear bremsstrahlung process. The release of soft phonons invalidates the impulse approximation assumed in the calculation of a number of nuclear processes. Its effects on a few nuclear cross-sections at the epithermal energy are discussed. The generated soft X-ray has a continuous spectrum and peaks around a few hundred eV to a few keV. The average photon energy and spectrum properties of such a process are calculated with a semiclassical approach. The experimental observation of this phenomenon is complicated by the presence of electron bremsstrahlung in a fusion tokamak. However, its unique spectrum also opens up the possibility of new plasma diagnostics more sensitive to the ionic or nuclear degree of freedom. This rediation is also linked to some of the X-ray observations in nuclear astrophysics. [Preview Abstract] |
Tuesday, May 5, 2009 3:06PM - 3:18PM |
X12.00009: Hydrogen and Helium atoms in strong magnetic fields Anand Thirumalai, Jeremy Heyl The energy levels of hydrogen and helium atoms in strong magnetic fields are calculated in this study. The current work contains accurate estimates of the binding energies of the first few low-lying states of these systems that are improvements upon previous estimates. The methodology involves computing the eigenvalues and eigenvectors of the generalized two-dimensional Hartree-Fock partial differential equations for these one- and two-electron systems in a self-consistent manner. The method described herein is applicable to calculations of atomic structure in magnetic fields of arbitrary strength as it exploits the natural symmetries of the problem without assumptions of any basis functions for expressing the wave functions of the electrons or the commonly employed adiabatic approximation. The method is found to be readily extendable to systems with more than two electrons. [Preview Abstract] |
Tuesday, May 5, 2009 3:18PM - 3:30PM |
X12.00010: ABSTRACT WITHDRAWN |
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