287 research outputs found
Effect of Target Material Yield Strength on Hypervelocity Perforation and Ballistic Limit
Viscoplastic flow theory in hypervelocity projectile perforation analyses of thin plate
Monitoring All Sky for Variability
A few percent of all stars are variable, yet over 90% of variables brighter
than 12 magnitude have not been discovered yet. There is a need for an all sky
search and for the early detection of any unexpected events: optical flashes
from gamma-ray bursts, novae, dwarf novae, supernovae, `killer asteroids'. The
ongoing projects like ROTSE, ASAS, TASS, and others, using instruments with
just 4 inch aperture, have already discovered thousands of new variable stars,
a flash from an explosion at a cosmological distance, and the first partial
eclipse of a nearby star by its Jupiter like planet. About one million
variables may be discovered with such small instruments, and many more with
larger telescopes. The critical elements are software and full automation of
the hardware. A complete census of the brightest eclipsing binaries is needed
to select objects for a robust empirical calibration of the the accurate
distance determination to the Magellanic Clouds, the first step towards the
Hubble constant. An archive to be generated by a large number of small
instruments will be very valuable for data mining projects. The real time
alerts will provide great targets of opportunity for the follow-up observations
with the largest telescopes.Comment: 6 pages, latex, minor changes, published in the Publications of the
Astronomical Society of the Pacific as one of Millennium Essays: 2000, PASP,
112, 1281-128
The ESO-Sculptor Faint Galaxy Redshift Survey: The Photometric Sample
We present the photometric sample of a faint galaxy survey carried out in the
southern hemisphere, using CCDs on the 3.60m and NTT-3.5m telescopes at La
Silla (ESO). The survey area is a continuous strip of 0.2 deg x 1.53 deg
located at high galactic latitude (-83 deg) in the Sculptor constellation. The
photometric survey provides total magnitudes in the bands B, V (Johnson) and R
(Cousins) to limiting magnitudes of 24.5, 24.0, 23.5 respectively. To these
limits, the catalog contains about 9500, 12150, 13000 galaxies in B, V, R bands
respectively and is the first large digital multi-colour photometric catalog at
this depth. This photometric survey also provides the entry catalog for a
fully-sampled redshift survey of ~ 700 galaxies with R < 20.5 (Bellanger et al.
1995). In this paper, we describe the photometric observations and the steps
used in the data reduction. The analysis of objects and the star-galaxy
separation with a neural network are performed using SExtractor, a new
photometric software developed by E. Bertin (1996). The photometric accuracy of
the resulting catalog is ~ 0.05 mag for R < 22. The differential galaxy number
counts in B, V, R are in good agreement with previously published CCD studies
and confirm the evidence for significant evolution at faint magnitudes as
compared to a standard non evolving model (by factors 3.6, 2.6, 2.1). The
galaxy colour distributions B-R, B-V of our sample show a blueing trend of ~
0.5 mag between 21 < R < 23.5 in contrast to the V-R colour distribution where
no significant evolution is observed.Comment: LATEX, 18 Postscript figures, 20 pages. To appear July 1997. Modified
version of article. Abstract corrected for missing lin
Cross-Sectional Deformations of Monocoque Beams and Their Effects on the Natural Vibration Frequencies
The variational principle, differential equations, and boundary conditions governing the cross-sectional distortions due to inertia loading of a two-dimensional model of a thin monocoque wing are shown. A theoretical analysis of this simplified model is made in order to determine the nature of the coupling between the cross-sectional modes and the spanwise deformation modes. General solutions are obtained in finite-difference form for arbitrary cross sections and an exact solution is presented for a parabolic-arc cross section of constant cover thickness. The application of these results in evaluating the coupled frequencies of the actual structure is discussed. Frequencies evaluated for a parabolic-arc monocoque beam show good agreement with experimental values
Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6
We present multi-epoch, diffraction-limited images of the nebula around the
carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the
Keck-I telescope. The near-IR nebula is resolved into two main components, an
elongated, bright feature showing time-variable asymmetry and a fainter
component about 60 milliarcseconds away with a cooler color temperature. These
images were precisely registered (~35 milliarcseconds) with respect to recent
visible images from the Hubble Space Telescope (Trammell et al. 2000), which
showed a bipolar structure in scattered light. The dominant near-IR feature is
associated with the northern lobe of this scattering nebula, and the
multi-wavelength dataset can be understood in terms of a bipolar dust shell
around CIT 6. Variability of the near-IR morphology is qualitatively consistent
with previously observed changes in red polarization, caused by varying
illumination geometry due to non-uniform dust production. The blue emission
morphology and polarization properties can not be explained by the above model
alone, but require the presence of a wide binary companion in the vicinity of
the southern polar lobe. The physical mechanisms responsible for the breaking
of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216,
remain uncertain.Comment: 18 pages, 5 figures (one in color), to appear in the Astrophysical
Journa
Evaluation of the Levy Method as Applied to Vibrations of a 45 deg Delta Wing
The Levy method which deals with an idealized structure was used to obtain the natural modes and frequencies of a large-scale built-up 45 deg. delta wing. The results from this approach, both with and without the effects of transverse shear, were compared with the results obtained experimentally and also with those calculated by the Stein-Sanders method. From these comparisons it was concluded that the method as proposed by Levy gives excellent results for thin-skin delta wings, provided that corrections are made for the effect of transverse shear
Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
We investigate the secular evolution of the orbital semi-major axis and
eccentricity due to mass transfer in eccentric binaries, assuming conservation
of total system mass and orbital angular momentum. Assuming a delta function
mass transfer rate centered at periastron, we find rates of secular change of
the orbital semi-major axis and eccentricity which are linearly proportional to
the magnitude of the mass transfer rate at periastron. The rates can be
positive as well as negative, so that the semi-major axis and eccentricity can
increase as well as decrease in time. Adopting a delta-function mass-transfer
rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution
timescales ranging from a few Myr to a Hubble time or more, depending on the
binary mass ratio and orbital eccentricity. Comparison with orbital evolution
timescales due to dissipative tides furthermore shows that tides cannot, in all
cases, circularize the orbit rapidly enough to justify the often adopted
assumption of instantaneous circularization at the onset of mass transfer. The
formalism presented can be incorporated in binary evolution and population
synthesis codes to create a self-consistent treatment of mass transfer in
eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa
Photometric Solutions for Detached Eclipsing Binaries: selection of ideal distance indicators in the SMC
Detached eclipsing binary stars provide a robust one-step distance
determination to nearby galaxies. As a by-product of Galactic microlensing
searches, catalogs of thousands of variable stars including eclipsing binaries
have been produced by the OGLE, MACHO and EROS collaborations. We present
photometric solutions for detached eclipsing binaries in the Small Magellanic
Cloud (SMC) discovered by the OGLE collaboration. The solutions were obtained
with an automated version of the Wilson-Devinney program. By fitting mock
catalogs of eclipsing binaries we find that the normalized stellar radii
(particularly their sum) and the surface brightness ratio are accurately
described by the fitted parameters and estimated standard errors, despite
various systematic uncertainties. In many cases these parameters are well
constrained. In addition we find that systems exhibiting complete eclipses can
be reliably identified where the fractional standard errors in the radii are
small. We present two quantitatively selected sub-samples of eclipsing binaries
that will be excellent distance indicators. These can be used both for
computation of the distance to the SMC and to probe its structure. One
particularly interesting binary has a very well determined solution, exhibits
complete eclipses, and is comprised of well detached G-type, class giants.Comment: 29 pages, 12 figures. To be published in Ap
In situ stress database of the greater Ruhr region (Germany) derived from hydrofracturing tests and borehole logs
Between 1986 and 1995, 429 hydrofracturing tests have been carried out in six now-abandoned coal mines and two coal bed methane boreholes at depths between 600 and 1950 m within the greater Ruhr region in western Germany. From these tests, stress magnitudes and orientations of the stress tensor are derived. The majority of hydrofracturing tests were carried out from mine galleries away from mine workings in a relatively undisturbed rock mass. These data along with detailed information have been disclosed recently. In combination with already published material, we provide the first comprehensive stress database of the greater Ruhr region. Our study summarises the results of the extensive in situ stress test campaign and assigns quality to each data record using the established quality ranking schemes of the World Stress Map project. The stress magnitudes suggest predominantly strike-slip stress regime, where the magnitude of the minimum horizontal stress, Shmin, is half of the magnitude of the maximum horizontal stress, SHmax, implying that the horizontal differential stress is high. We observe no particular change in the stress gradient at depth throughout the Carboniferous layers and no significant difference between tests carried out in coal mines and deep boreholes. The mean SHmax orientation varies between 133 ± 13∘ in the westernmost located Friedrich Heinrich coal mine and 168 ± 23∘ in the easternmost located Westphalia coal mine. The mean SHmax orientation, based on 87 data records from this and already published studies, of 161 ± 43∘ is in good agreement with the regional stress orientation observed in northwestern Europe. The presented public database provides in situ stress magnitude and stress orientation data records that are essential for the calibration of geomechanical numerical models on regional and/or reservoir scales for, among others, assessing stability issues of borehole trajectories, caverns, and georeservoirs in general. For an application example of this database, we estimate slip and dilation tendencies of major geological discontinuities, discovered during the 700-year-long coal mining activities in the region. The result, although burdened by high uncertainties, shows that the discontinuities striking in the N–S and NW–SE directions have a higher slip tendency compared to the ones striking ENE–WSW and NNW–SSE, whereas a high dilation tendency is observed for discontinuities striking NNW–SSE and a low dilation tendency for the ones striking ENE–WSW. The stress orientation database is available under https://doi.org/10.24406/fordatis/200 (Kruszewski et al., 2022a), the stress magnitude database is available under https://doi.org/10.24406/fordatis/201 (Kruszewski et al., 2022b), whereas the hydrofracturing test reports are available under https://doi.org/10.24406/fordatis/222 (Kruszewski et al., 2022c).</p
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