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- White Dwarfs (3)
- Binaries: Close (2)
- Cataclysmic Variables (2)
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- Novae (2)
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- Stars: Abundances (1)
- Stars: Activity (1)
- Stars: Individual (DQ Her) (1)
- Stars: Individual (HD 149499 B (1)
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Articles 1 - 12 of 12
Full-Text Articles in Physical Sciences and Mathematics
Galactic Cosmic-Ray Modulation Using A Solar Minimum Mhd Heliosphere: A Stochastic Particle Approach, Bryan M. Ball, Ming Zhang, Hamid K. Rassoul, Timur J. Linde
Galactic Cosmic-Ray Modulation Using A Solar Minimum Mhd Heliosphere: A Stochastic Particle Approach, Bryan M. Ball, Ming Zhang, Hamid K. Rassoul, Timur J. Linde
Aerospace, Physics, and Space Science Faculty Publications
An example of Galactic cosmic-ray modulation in a fully three-dimensional heliosphere is presented here. We use a stochastic particle method to solve for modulation without requiring symmetric boundaries or fields. We include all typical modulation terms, including full three-dimensional drift. We have applied this to an MHD heliosphere appropriate for solar minimum conditions. This field includes nonradial solar wind velocity components, as well as a built-in nonspherical termination shock. Parameters that are of interest in modulation can be analyzed in detail, particularly the momentum change of cosmic rays during their transport through the heliosphere. We show radial profiles of modulation …
Dq Herculis In Profile: Whole Earth Telescope Observations And Smoothed Particle Hydrodynamics Simulations Of An Edge-On Cataclysmic Variable System, Matt A. Wood
Aerospace, Physics, and Space Science Faculty Publications
The old nova DQ Herculis was the Whole Earth Telescope Northern Hemisphere target for the 1997 July campaign and was observed on four nights with the SARA 0.9 m telescope during 2003 June. We present updated ephemerides for the eclipse and 71 s timings. The Fourier transform displays power at the presumed white dwarf spin period of 71.0655 s, but no significant power at either 142 or 35.5 s. The mean pulsed light curve is obtained by folding on the orbital period modulus the mean ephemeris of the 71 s period, and from this we calculate an O - C …
Abundance Of Elements Beyond The Iron Group In Cool Do White Dwarfs, Pierre Chayer, Stéphane Vennes, Jean Jean François, Jeffrey W. Kruk
Abundance Of Elements Beyond The Iron Group In Cool Do White Dwarfs, Pierre Chayer, Stéphane Vennes, Jean Jean François, Jeffrey W. Kruk
Aerospace, Physics, and Space Science Faculty Publications
We report the presence of elements beyond the iron group in the atmospheres of the cool DO white dwarfs HD 149499 B and HZ 21. Photospheric lines of germanium (Z = 32), arsenic (33), selenium (34), tin (50), tellurium (52), iodine (53), and perhaps bromine (35) are observed in ultraviolet spectra of HD 149499 B obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Goddard High Resolution Spectrograph (GHRS), and the International Ultraviolet Explorer. Germanium, arsenic, and tellurium are also observed in FUSE and GHRS spectra of HZ 21. Light elements such as carbon, silicon, phosphorus, and sulfur are present …
Cosmic Rays From Gamma-Ray Bursts In The Galaxy, Charles Dennison Dermer, Jeremy M. Holmes
Cosmic Rays From Gamma-Ray Bursts In The Galaxy, Charles Dennison Dermer, Jeremy M. Holmes
Aerospace, Physics, and Space Science Faculty Publications
The rate of terrestrial irradiation events by Galactic gamma-ray bursts (GRBs) is estimated using recent standard-energy results. We assume that GRBs accelerate high-energy cosmic rays, and we present results of three-dimensional simulations of cosmic rays moving in the Galactic magnetic field and diffusing through pitch-angle scattering. An on-axis GRB extinction event begins with a powerful prompt γ-ray and neutron pulse, followed by a longer lived phase from cosmic-ray protons and neutron-decay protons that diffuse toward Earth. Our results force a reinterpretation of reported ∼10¹⁸ eV cosmic-ray anisotropies and offer a rigorous test of the model in which high-energy cosmic rays …
The Model Dependence Of Solar Energetic Particle Mean Free Paths Under Weak Scattering, Gang Qin, Joseph R. Dwyer, Hamid K. Rassoul, Glenn M. Mason
The Model Dependence Of Solar Energetic Particle Mean Free Paths Under Weak Scattering, Gang Qin, Joseph R. Dwyer, Hamid K. Rassoul, Glenn M. Mason
Aerospace, Physics, and Space Science Faculty Publications
The mean free path is widely used to measure the level of solar energetic particles' diffusive transport. We model a solar energetic particle event observed by Wind STEP at 0.31-0.62 MeV nucleonˉ¹, by solving the focused transport equation using the Markov stochastic process theory. With different functions of the pitch angle diffusion coefficient D μμ, we obtain different parallel mean free paths for the same event. We show that the different values of the mean free path are due to the high anisotropy of the solar energetic particles. This makes it problematic to use just the mean free path to …
The Ccompton-Getting Effect Of Energetic Particles With An Anisotropic Pitch-Angle Distribution: An Application To Voyager 1 Results At ∼85 Au, Ming Zhang
Aerospace, Physics, and Space Science Faculty Publications
This paper provides a theoretical simulation of anisotropy measurements by the Low-Energy Charged Particle (LECP) experiment on Voyager. The model starts with an anisotropic pitch-angle distribution function in the solar wind plasma reference frame. It includes the effects of both Compton-Getting anisotropy and a perpendicular diffusion anisotropy that possibly exists in the upstream region of the termination shock. The calculation is directly applied to the measurements during the late 2002 particle event seen by Voyager 1. It is shown that the data cannot rule out either the model with zero solar wind speed or the one with a finite speed …
The Chromospheric Activity And Ages Of M Dwarf Stars In Wide Binary Systems, Nicole M. Silvestri, Suzanne L. Hawley, Terry D. Oswalt
The Chromospheric Activity And Ages Of M Dwarf Stars In Wide Binary Systems, Nicole M. Silvestri, Suzanne L. Hawley, Terry D. Oswalt
Aerospace, Physics, and Space Science Faculty Publications
We investigate the relationship between age and chromospheric activity for 139 M dwarf stars in wide binary systems with white dwarf companions. The age of each system is determined from the cooling age of its white dwarf component. The current limit for activity-age relations found for M dwarfs in open clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for the exploration of this relationship at ages older than 4 Gyr. The general trend of stars remaining active for a longer time at a later spectral type is confirmed. However, our larger sample and greater …
Where Are The Magnetic White Dwarfs With Detached, Nondegenerate Companions?, James Liebert, Terry D. Oswalt
Where Are The Magnetic White Dwarfs With Detached, Nondegenerate Companions?, James Liebert, Terry D. Oswalt
Aerospace, Physics, and Space Science Faculty Publications
The Sloan Digital Sky Survey has already more than doubled the sample of white dwarfs with spectral classi- fications, the subset with detached M dwarf companions, and the subset of magnetic white dwarfs. In the course of assessing these new discoveries, we have noticed a curious, unexpected property of the total lists of magnetic white dwarfs and of white dwarf plus main-sequence binaries: there appears to be virtually zero overlap between the two samples! No confirmed magnetic white dwarf has yet been found in such a pairing with a main-sequence star. The same statement can be made for the samples …
X-Ray Bursts Associated With Leader Steps In Cloud-To-Ground Lightning, Joseph R. Dwyer, Hamid K. Rassoul, M. Al-Dayeh, Lee Caraway, A. Chrest, Brian F. Wright, E. Kozak
X-Ray Bursts Associated With Leader Steps In Cloud-To-Ground Lightning, Joseph R. Dwyer, Hamid K. Rassoul, M. Al-Dayeh, Lee Caraway, A. Chrest, Brian F. Wright, E. Kozak
Aerospace, Physics, and Space Science Faculty Publications
X-ray and electric field measurements were made during five nearby negative natural lightning strikes in north central Florida during the summer of 2004. The observed X-ray emission typically was detected ∼1 ms before the first return stroke, during the stepped-leader phase, and had energies extending up to a few hundred keV. The X rays were produced in discrete, intense bursts emitted in coincidence with the formation of the leader steps, demonstrating unambiguously that the source of lightning X rays is closely related to the stepping process. The X-ray emission from lightning stepped leaders is found to be remarkably similar to …
A Comparison Between Monte Carlo Simulations Of Runaway Breakdown And Terrestrial Gamma-Ray Flash Observations, Joseph R. Dwyer, David M. Smith
A Comparison Between Monte Carlo Simulations Of Runaway Breakdown And Terrestrial Gamma-Ray Flash Observations, Joseph R. Dwyer, David M. Smith
Aerospace, Physics, and Space Science Faculty Publications
Monte Carlo simulations of the runaway breakdown of air are used to calculate the spectra of terrestrial gamma-ray flashes (TGFs), which are then compared with RHESSI and CGRO/BATSE observations. It is found that the recent RHESSI spectrum is not consistent with a source altitude above 24 km but can be well fit by a source in the range of 15-21 km, depending upon the electric field geometry of the source. Because 15 km is not unusual for the tops of thunderstorms, especially at low latitudes, and is lower than typical minimum sprite altitudes, the RHESSI data imply that thunderstorms and …
The Initiation Of Lightning By Runaway Air Breakdown, Joseph R. Dwyer
The Initiation Of Lightning By Runaway Air Breakdown, Joseph R. Dwyer
Aerospace, Physics, and Space Science Faculty Publications
A mechanism for lightning initiation by the sustained runaway breakdown of air is presented. Unlike earlior models that rely upon large cosmic-ray air showers, this mechanism uses the runaway electrons produced by the steady background of atmospheric cosmic-rays to amplify non-uniformities in the electric field. The ionization of air from the runaway electrons creates a region of discharge that propagates in the opposite direction of the electrons, enhancing the electric field in front of it to the point where a conventional breakdown can occur. As the discharged region grows, positron feedback can become important, dramatically increasing the flux of runaway …
X-Ray Bursts Produced By Laboratory Sparks In Air, Joseph R. Dwyer, Hamid K. Rassoul, Ziad H. Saleh, Martin A. Uman, J. Jerauld, J. Anderson Plumer
X-Ray Bursts Produced By Laboratory Sparks In Air, Joseph R. Dwyer, Hamid K. Rassoul, Ziad H. Saleh, Martin A. Uman, J. Jerauld, J. Anderson Plumer
Aerospace, Physics, and Space Science Faculty Publications
X-ray observations were made during fourteen 1.5 to 2.0 m high-voltage discharges in air produced by a 1.5 MV Marx circuit. All 14 discharges generated x-rays in the ∼30 to 150 keV range. The x-rays, which arrived in discrete bursts, less than 0.5 microseconds in duration, occurred from both positive and negative polarity rod-to-plane discharges as well as from small, 5-10 cm series spark gaps within the Marx generator. The x-ray bursts usually occurred when either the voltages across the gaps were the largest or were in the process of collapsing. The bursts are remarkably similar to the x-ray bursts …