60,836 research outputs found
A Color-Magnitude Diagram for a Globular Cluster In the Giant Elliptical Galaxy NGC 5128
The Hubble Space Telescope has been used to obtain WFPC2 (V,I) photometry for
a large sample of stars in the outer halo of the giant elliptical NGC 5128 (d =
4 Mpc). The globular cluster N5128-C44, at the center of the Planetary Camera
field, is well enough resolved to permit the construction of a color-magnitude
diagram (CMD) for it which covers the brightest two magnitudes of the giant
branch. The CMD is consistent with that of a normal old, moderately
low-metallicity ([Fe/H] = -1.30 globular cluster, distinctly more metal-poor
than most of the field halo stars at the same projected location (which average
[Fe/H] ~ -0.5). This is the most distant globular cluster in which direct
color-magnitude photometry has been achieved to date, and the first one
belonging to a giant E galaxy.Comment: 12 pages, LaTeX, including 5 postscript figures; submitted to
Astronomical Journa
Coherent energy migration in solids: Determination of the average coherence length in oneâdimensional systems using tunable dye lasers
The coherent nature of energy propagation in solids at low temperatures was established from the time resolved response of the crystal to short optical pulses obtained from a dye laser (pumped by a nitrogen gas laser). The trapping and detrapping of the energy by shallow defects (x traps) was evident in the spectra and enabled us to extract the coherence length: lâł700 Ă
=186 molecules for the oneâdimensional triplet excitons of 1,2,4,5âtetrachlorobenzene crystals at T<4.2° K. This length which clearly exceeds the stochastic random walk limit is related to the thermalization mechanisms in this coupled excitonâtrap system, and its magnitude supports the notion that excitonâphonon coupling is responsible for the loss of coherence on very long molecular chains (trap concentration is 1/256â000)
Superrotation planetary atmospheres: Mechanical analogy, angular momentum budget and simulation of the spin up process
Superrotation rates observed in planetary atmospheres are analyzed based on the concept of a thermally driven zonally symmetric circulation. Specifically, how this superrotation is produced and maintained against the tendency for friction to oppose differential motions between the atmosphere and the underlying planet is addressed. The time evolution of a fluid leading from corotation under uniform heating to superrotation under globally nonuniform heating is simulated using a three dimensional zonally symmetric spectral model and Laplace transformation. The increased tendency toward geostrophy combined with the increase of surface pressure toward the poles (due to meridional mass transport), induces the atmosphere to subrotate temporarily at lower altitudes. The resulting viscous shear near the surface thus permits angular momentum to flow from the planet into the atmosphere where it propagates upwards and, combined with the change in moment of inertia, produces large superrotation rates at higher viscosities
HST Photometry for the Halo Stars in the Leo Elliptical NGC 3377
We have used the ACS camera on HST to obtain (V,I) photometry for 57,000
red-giant stars in the halo of the Leo elliptical NGC 3377. We use this sample
of stars to derive the metallicity distribution function (MDF) for its halo
field stars, and comment on its chemical evolution history compared with both
larger and smaller E galaxies. Our ACS/WFC field spans a radial range extending
from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers
a significant portion of this galaxy's halo. We find that the MDF is broad,
reaching a peak at [m/H] ~ -0.6. It may, in addition, have relatively few
stars more metal-rich than [m/H] = -0.3$, although interpretation of the
high-metallicity end of the MDF is limited by photometric completeness that
affects the detection of the reddest, most metal-rich stars. NGC 3377 appears
to have an enrichment history intermediate between those of normal dwarf
ellipticals and the much larger giants. As yet, we find no clear evidence that
the halo of NGC 3377 contains a significant population of ``young'' (< 3 Gy)
stars.Comment: 40 pages, 17 figure
The Large Scale X-ray Emission from M87
We describe asymmetrical features in a long exposure X-ray map of M87 made
with the ROSAT High Resolution Imager (HRI). A bright triangular region is
marked by a linear `spur' along one edge. The structure of this spur suggests
an interpretation of a tangential view of a shock front 18 kpc long. None of
the brighter features are spatially coincident with radio or optical structures
so we concur with earlier investigators that most of the emission arises from
thermal processes.Comment: 6 pages latex, including 3 postscript figures. Uses psfig and
LAMUPHYS (Springer) macro. To be published in 'The M87 Ringberg Workshop',
September 1997, Springer Lecture Notes in Physics Series, Roeser and
Meisenheimer, ed
Lunar surface exploration by satellite. An integrated experiment package to perform measurements of the composition of the lunar surface Final report
Integrated experiment package to measure lunar surface composition - gamma ray, alpha particle, X ray fluorescence, and neutron albedo experiment
Finite-Difference Solution for Laminar or Turbulent Boundary Layer Flow over Axisymmetric Bodies with Ideal Gas, CF4, or Equilibrium Air Chemistry
A computer code was developed that uses an implicit finite-difference technique to solve nonsimilar, axisymmetric boundary layer equations for both laminar and turbulent flow. The code can treat ideal gases, air in chemical equilibrium, and carbon tetrafluoride (CF4), which is a useful gas for hypersonic blunt-body simulations. This is the only known boundary layer code that can treat CF4. Comparisons with experimental data have demonstrated that accurate solutions are obtained. The method should prove useful as an analysis tool for comparing calculations with wind tunnel experiments and for making calculations about flight vehicles where equilibrium air chemistry assumptions are valid
Fluctuations of the heat flux of a one-dimensional hard particle gas
Momentum-conserving one-dimensional models are known to exhibit anomalous
Fourier's law, with a thermal conductivity varying as a power law of the system
size. Here we measure, by numerical simulations, several cumulants of the heat
flux of a one-dimensional hard particle gas. We find that the cumulants, like
the conductivity, vary as power laws of the system size. Our results also
indicate that cumulants higher than the second follow different power laws when
one compares the ring geometry at equilibrium and the linear case in contact
with two heat baths (at equal or unequal temperatures). keywords: current
fluctuations, anomalous Fourier law, hard particle gasComment: 5 figure
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