Bulletin of the American Physical Society
APS April Meeting 2013
Volume 58, Number 4
Saturday–Tuesday, April 13–16, 2013; Denver, Colorado
Session Y8: Galactic Astronomy |
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Sponsoring Units: DAP Chair: Cynthia Froning, University of Colorado at Boulder Room: Governor's Square 10 |
Tuesday, April 16, 2013 1:30PM - 1:42PM |
Y8.00001: Transport Properties and Radiation Production in Plasmas with Sub-Larmor-Scale Magnetic Turbulence B. Keenan, M.V. Medvedev Kinetic streaming instabilities, such as the Weibel instability, occur in various astrophysical systems, e.g., collisionless shocks and reconnection sites. Such instabilities generate strong (sub-equipartition) magnetic fields which reside at small, sub-Larmor spatial scales. Efficient electron acceleration to relativistic energies is not uncommon in such environments. Spectra of radiation emitted by these relativistic electrons can deliver wealth of information about the internal structure of such ``Weibel turbulence.'' The small-scale fields simultaneously drive the particle transport via pitch-angle diffusion. Both effects are related and can be used to diagnose the plasma state. We study such a relation between transport and radiation in sub-Larmor-scale turbulence via numerical simulations and analysis. [Preview Abstract] |
Tuesday, April 16, 2013 1:42PM - 1:54PM |
Y8.00002: On mass and linear momentum conservation for the evolution of magnetized plasma in a highly dilute plasma environment Daniel Berdichevsky A non-force-free analytical 3-D magnetohydrodynamic (MHD) solution of a magnetic flux-rope (FR) is presented. This FR solution may explain the uniform propagation, beyond \textit{$\sim$0.05 AU from the Sun}, of transients. i.e., coronal mass ejections commonly observed by today's missions like \textit{The Solar Terrestrial Relations Observatory (STEREO)}, tracked in some cases up to 1 AU, and/or beyond. We present arguments on the transient evolution based on its mass and linear momentum conservation. For astrophysical observations --using, e.g., the Hubble telescope-- of remnants in nebulae of pass supper-novae, as well other interstellar events this MHD solution(s) may further be a good way beyond gas-dynamics in the development of a coherent picture of shock and its driver, as they are becoming a current interpretation. Here, we require that the gravitational and magnetic forces balance each other, in the framework of the MHD theory for a simple model of the evolution of the transient magnetized plasma, assuming it interacts weakly with surrounding `steady flows' in the presence of a point-like, gravitational field. When satisfying these ans\"{a}tze we identify a relation between the transported mechanical mass by the transient with its geometrical parameters and the intensity of the magnetic field carried by the structure, in a way consistent with current observations. [Preview Abstract] |
Tuesday, April 16, 2013 1:54PM - 2:06PM |
Y8.00003: High Energy Particle Populations and Relevant Plasma Processes Associated with Shining Black Holes* Bruno Coppi Shining Black Holes are identified as collapsed objects surrounded by plasmas whose radiation emission can be associated, in most cases, with non-thermal high energy particle populations. When this feature can be represented by a non-isotropic pressure tensor in a fluid-like description of the relevant plasmas, new collective modes [1,2] whose excitation depend on the plasma pressure anisotropy have been found. By the same kind of theory new plasma and field stationary configurations (e.g. ``solitary rings'') associated with the plasma pressure anisotropy have been identified. This is a first step toward dealing with the plasmas surrounding black holes by theories involving phase space that do not have to rely on having nearly isotropic Maxwellian particle distributions in momentum space. The considered collective modes and the equilibrium configurations, that do not require the presence of a seed magnetic field for their derivation and where the magnetic energy density is of the order of the gravitationally confined plasma pressure, provide a clear theoretical framework to describe how magnetic field can be generated and amplified around black holes [1]. *Sponsored in part by the U.S. D.O.E.\\[4pt] [1] B. Coppi, A\&A 548, A84 (2012)\\[0pt] [2] B. Coppi, Phys. Plasmas 18, 032901 (2011) [Preview Abstract] |
Tuesday, April 16, 2013 2:06PM - 2:18PM |
Y8.00004: Do R Coronae Borealis Stars Form from Double White Dwarf Mergers? Patrick Motl, Jan Staff, Athira Menon, Falk Herwig, Wesley Even, Chris Fryer, Tom Geballe, Marco Pignatari, Geoffrey Clayton, Joel Tohline A leading formation scenario for the irregular variable R Coronae Borealis (RCB) stars invokes the merger of a degenerate Helium white dwarf with a Carbon-Oxygen white dwarf in a binary. The observed ratio of $^{16}\textrm{O} / ^{18}\textrm{O}$ for RCB stars is in the range of 0.3 - 20, much smaller than the solar value of ~500. We report on our investigations into whether such a low oxygen isotope ratio can be obtained in simulations of double white dwarf mergers. We identify a ``shell of fire'' feature in the simulations surrounding the merged object where temperatures and densities are favorable for forming $^{18}\textrm{O}$ for binaries with initial mass ratios near 0.7. However, the accretion stream's impact dredges up $^{16}\textrm{O}$ from the Carbon-Oxygen white dwarf which forms a competing process that raises the oxygen isotope ratio. We present the most favorable scenarios we have identified for creating RCB stars in light of these competing processes and outline steps for future progress. [Preview Abstract] |
Tuesday, April 16, 2013 2:18PM - 2:30PM |
Y8.00005: The effect of host cluster gravitational tidal forces on the internal dynamics of spiral galaxies Alexander Mayer New empirical observation by Bidin, Carraro, Mendez \& Smith finds ``a lack of dark matter in the Solar neighborhood" (2012 \emph{ApJ} \textbf{751}, 30). This, and the discovery of a vast polar structure of Milky Way satellites by Pawlowski, Pflamm-Altenburg \& Kroupa (2012 \emph{MNRAS} \textbf{423}, 1109), conflict with the prevailing interpretation of the measured Galactic rotation curve. Simulating the dynamical effects of host cluster tidal forces on galaxy disks reveals radial migration in a spiral structure and an orbital velocity that accelerates with increasing galactocentric radial coordinate. A virtual ``toy model,'' which is based on an Earth-orbiting system of particles and is physically realizable in principle, is available at \textbf{GravitySim.net}. Given the perturbing gravitational effect of the host cluster on a spiral galaxy disk and that a similar effect does not exist for the Solar System, the two systems represent distinct classes of gravitational dynamical systems. The observed `flat' and accelerating rotation curves of spiral galaxies can be attributed to gravitational interaction with the host cluster; no `dark matter halo' is required to explain the observable. [Preview Abstract] |
Tuesday, April 16, 2013 2:30PM - 2:42PM |
Y8.00006: Abundances of Elements in Nebulae and Chemical Evolution of the Universe Sultana Nahar, M. Dance, E. Palay, A. Pradhan Ionized gasous nebulae are associated with both the birth of stars and the endpoints of stellar evolution.\footnote{A.K. Pradhan and S.N. Naha} Their chemical enrichment is therefore a chronometer of the life of the universe itself. But there are vast uncertainties in determining nebular abundances of common elements such as C, N, O, Ne, Fe etc. One of the most fundamental astrophysical problem is the discordant abundances obtained from collisionally excited lines and those from recombination lines. Part of the reason is the accuracy of atomic physics. With inclusion of higher order relativistic effects in the Breit-Pauli R-matrix codes$^1$, high accuracy collision strengths are being computed. We find significant effects due to fine structure and resonances in the low energy region. Results are presented for the forbidden optical and far-infrared transitions in two of the most important ions in nebular temperature and density diagnostics: [Ne~V] and [O~III]. We also compare and contrast collisional excitation with electron-ion recombination. The far-infrared lines are of particular interest since they spectroscopically reveal Ultra Luminous Infrared Galaxies buried in heavily dust obscured regsions at high redshift, observed by three space observatories: SPITZER, HERSCHEL, SOFIA. [Preview Abstract] |
Tuesday, April 16, 2013 2:42PM - 2:54PM |
Y8.00007: HAWC sensitivity to Galactic TeV gamma-ray sources Michelle Hui The High Altitude Water Cherenkov (HAWC) Observatory is a second generation detector of TeV gamma rays based on the water Cherenkov technique. It will comprise an array of 300 water Cherenkov detectors. It is an all-sky surveying instrument with greater than 90\% duty cycle, a field of view of 2 sr, and angular resolution of 0.1 degrees for energies above 10 TeV. The HAWC Observatory is currently under construction in Sierra Negra in the state of Puebla, Mexico. The site is at a latitude of 19 degrees North, and an altitude of 4100 m. Ten percent of the array started data taking in September, 2012, and one third of the full array will be operational by Summer 2013. I will present the sensitivity of the HAWC Observatory to known Galactic gamma-ray sources, including the complex Cygnus region, and regions with unidentified source associations. [Preview Abstract] |
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