2,047 research outputs found
Effect of the Milky Way on Magellanic Cloud structure
A combination of analytic models and n-body simulations implies that the
structural evolution of the Large Magellanic Cloud (LMC) is dominated by its
dynamical interaction with the Milky Way. Although expected at some level, the
scope of the involvement has significant observational consequences. First, LMC
disk orbits are torqued out of the disk plane, thickening the disk and
populating a spheroid. The torque results from direct forcing by the Milky Way
tide and, indirectly, from the drag between the LMC disk and its halo resulting
from the induced precession of the LMC disk. The latter is a newly reported
mechanism that can affect all satellite interations. However, the overall
torque can not isotropize the stellar orbits and their kinematics remains
disk-like. Such a kinematic signature is observed for nearly all LMC
populations. The extended disk distribution is predicted to increase the
microlensing toward the LMC. Second, the disk's binding energy slowly decreases
during this process, puffing up and priming the outer regions for subsequent
tidal stripping. Because the tidally stripped debris will be spatially
extended, the distribution of stripped stars is much more extended than the HI
Magellanic Stream. This is consistent with upper limits to stellar densities in
the gas stream and suggests a different strategy for detecting the stripped
stars. And, finally, the mass loss over several LMC orbits is predicted by
n-body simulation and the debris extends to tens of kiloparsecs from the tidal
boundary. Although the overall space density of the stripped stars is low,
possible existence of such intervening populations have been recently reported
and may be detectable using 2MASS.Comment: 15 pages, color Postscript figures, uses emulateapj.sty. Also
available from http://www-astro.phast.umass.edu/~weinberg/weinberg-pubs.htm
Production of Milky Way structure by the Magellanic Clouds
Previous attempts at disturbing the galactic disk by the Magellanic Clouds
relied on direct tidal forcing. However, by allowing the halo to actively
respond rather than remain a rigid contributor to the rotation curve, the
Clouds may produce a wake in the halo which then distorts the disk. Recent work
reported here suggests that the Magellanic Clouds use this mechanism to produce
disk distortions sufficient to account for both the radial location, position
angle and sign of the HI warp and observed anomalies in stellar kinematics
towards the galactic anticenter and LSR motion.Comment: 8 pages, uuencoded compressed PostScript, no figures, html version
with figures and mpeg simulations available at
http://www-astro.phast.umass.edu/Preprints/martin/martin1/lmc_online.htm
Using GEANT to Model Calrimeter Response for Electromagnetic Cascades from Nucleus-Nucleus Interactions in a Cosmic Ray Detector
Ascintillating optical fiber calorimeter (SOFCAL) is being developed by NASA/Marshall Space Flight Center for use in balloon-borne experiments to study the spectrum of high-energy cosmic rays and gamma rays. SOFCAL will not saturate for long exposures and the calorimeter willbe useful in emulsion chambers to study primary cosmic-ray nuclei with energies from 100 GeV to 1,000 TeV. The event generator FRITIOF was used to model the collision of a cosmic-ray projectile with a target nucleus inan emulsion chamber. The measurements of charged particles from the interaction in the emulsions are related to the energy of the primary cosmic ray nucleus-nucleus interaction, computer simulations of electromagnetic cascades allow computation of the energy ££y deposited indifferent regions of the calorimeter. The Monte Carlo program GEANT was used to model SOFCAL response to incident gamma rays and to compute the measure of energy deposition X£y in different layers ofthe calorimeter within the emulsion chamber. The partial coefficient o finelasticity kr defined by 1JE Y = kY E0 ,was computed for different energies Eo of primary cosmic rays. The were computed and compared with existing calorimeter data. Funding was provided by the NASA/University Joint Venture (JOVE) Program
Using FRITIOF to Model Nucleus-Nucleus Interactions in a Cosmic Ray Detector
Ascintillating optical fiber calorimeter (SOFCAL) isbeing developed by NASA/Marshall Space Flight Center for use in experiments to study the spectrum of high-energy cosmic rays and gamma rays from 100 GeV to 1,000 TeV. SOFCAL willnot saturate for long exposures and this calorimeter inthese balloon-borne emulsion chambers willbe helpful for the study of the composition of primary cosmic-ray nuclei. For primary nuclei with energies much greater than 1014 eV, nucleus-nucleus interactions are likely to exhibit characteristics of a quark-gluon plasma (QGP). Aparticle event generator was used tomodel the collision of a cosmic-ray nucleus with a target nucleus inan emulsion chamber. FRITIOF withLUCIAE was chosen tomodel collisions of primary cosmic rays inan emulsion chamber with SOFCAL. Pseudo-rapidity distributions were computed for protons on lead at 200 GeV/c and compared with experimental data. Pseudo-rapidity distributions were computed for protons or iron incident on a carbon or silver nucleus. For gamma-rays from nucleus-nucleus interactions, the total energy of the electromagnetic component Z£y was computed. The partial coefficient ofinelasticity kr defined by L£ y = kY E0 ,was computed from the primary energy Eo of the cosmic rays. The f(ky )-distributions were computed and compared with existing calorimeter data. Funding was provided by the NASA/University Joint Venture (JOVE) Program
Using FRITIOF to Model Nucleus-Nucleus Interactions in a Cosmic Ray Detector
Ascintillating optical fiber calorimeter (SOFCAL) isbeing developed by NASA/Marshall Space Flight Center for use in experiments to study the spectrum of high-energy cosmic rays and gamma rays from 100 GeV to 1,000 TeV. SOFCAL willnot saturate for long exposures and this calorimeter inthese balloon-borne emulsion chambers willbe helpful for the study of the composition of primary cosmic-ray nuclei. For primary nuclei with energies much greater than 1014 eV, nucleus-nucleus interactions are likely to exhibit characteristics of a quark-gluon plasma (QGP). Aparticle event generator was used tomodel the collision of a cosmic-ray nucleus with a target nucleus inan emulsion chamber. FRITIOF withLUCIAE was chosen tomodel collisions of primary cosmic rays inan emulsion chamber with SOFCAL. Pseudo-rapidity distributions were computed for protons on lead at 200 GeV/c and compared with experimental data. Pseudo-rapidity distributions were computed for protons or iron incident on a carbon or silver nucleus. For gamma-rays from nucleus-nucleus interactions, the total energy of the electromagnetic component Z£y was computed. The partial coefficient ofinelasticity kr defined by L£ y = kY E0 ,was computed from the primary energy Eo of the cosmic rays. The f(ky )-distributions were computed and compared with existing calorimeter data. Funding was provided by the NASA/University Joint Venture (JOVE) Program
Tidal Streams as Probes of the Galactic Potential
We explore the use of tidal streams from Galactic satellites to recover the
potential of the Milky Way. Our study is motivated both by the discovery of the
first lengthy stellar stream in the halo (\cite{it98}) and by the prospect of
measuring proper motions of stars brighter than 20th magnitude in such a stream
with an accuracy of yr, as will be possible with the Space
Interferometry Mission (SIM). We assume that the heliocentric radial velocities
of these stars can be determined from supporting ground-based spectroscopic
surveys, and that the mass and phase-space coordinates of the Galactic
satellite with which they are associated will also be known to SIM accuracy.
Using results from numerical simulations as trial data sets, we find that, if
we assume the correct form for the Galactic potential, we can predict the
distances to the stars as a consequence of the narrow distribution of energy
expected along the streams. We develop an algorithm to evaluate the accuracy of
any adopted potential by requiring that the satellite and stars recombine
within a Galactic lifetime when their current phase-space coordinates are
integrated backwards. When applied to a four-dimensional grid of triaxial
logarithmic potentials, with varying circular velocities, axis ratios and
orientation of the major-axis in the disk plane, the algorithm can recover the
parameters used for the Milky Way in a simulated data set to within a few
percent using only 100 stars in a tidal stream.Comment: Revised version - original algorithm generalised to be applicable to
any potential shape. LaTeX, 12 pages including 3 figures. To be published in
ApJ Letter
The Magellanic Stream and the density of coronal gas in the Galactic halo
The properties of the Magellanic Stream constrain the density of coronal gas
in the distant Galactic halo. We show that motion through ambient gas can
strongly heat Stream clouds, driving mass loss and causing evaporation. If the
ambient gas density is too high, then evaporation occurs on unreasonably short
timescales. Since heating dominates drag, tidal stripping appears to be
responsible for producing the Stream. Requiring the survival of the cloud MS IV
for 500 Myr sets an upper limit on the halo gas density n_H< 10^{-5} cm^{-3} at
50 kpc, roughly a factor of 10 lower than that estimated from the drag model of
Moore & Davis (1994). Implications for models of the evolution of gas in galaxy
halos are discussed.Comment: 4 pages, 1 figure, in press, ApJ
Analyses of intrinsic magnetoelectric properties in spin-valve-type tunnel junctions with high magnetoresistance and low resistance
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