1,356 research outputs found
Evaluation of scanning earth sensor mechanism on engineering test satellite 4
The results of the analysis and the evaluation of flight data obtained from the horizon sensor test project are described. The rotary mechanism of the scanning earth sensor composed of direct drive motor and bearings using solid lubricant is operated satisfactorily. The transmitted flight data from Engineering Test Satellite IV was evaluated in comparison with the design value
Supernova Explosions in the Early Universe: Evolution of Radiative Remnants and the Halo Destruction Efficiency
We study the evolution of supernova (SN) remnants of the first stars, taking
proper account of the radiative feedback of the progenitor stars on the
surroundings. We carry out a series of one-dimensional hydrodynamic simulations
with radiative cooling, starting from initial configurations that are drawn
from the results of our earlier radiation hydrodynamic simulations of the first
HII regions. In low-mass (< 10^6 M_sun) halos, the stellar radiation
significantly reduces the ambient gas density prior to the SN explosion. The
blastwave quickly propagates over the halo's virial radius, leading to complete
evacuation of the gas even with the input energy of 10^50 erg. We find that a
large fraction of the remnant's thermal energy is lost in 0.1-10 Myr by line
cooling, whereas, for larger explosion energies, the remnant expands even more
rapidly with decreasing interior density, and cools predominantly via inverse
Compton process. In higher mass halos, the gas density near the explosion site
remains high and the SN shock is heavily confined; the thermal energy of the
remnant is quickly radiated away by free-free emission, even if the total input
energy exceeds the binding energy of halos by two orders of magnitude. We show
that the efficiency of halo destruction is determined not only by the explosion
energy but also by the gas density profile, and thus controlled by radiative
feedback prior to the explosion. Several implications of our results for the
formation of first quasars and second-generation stars in the universe are also
discussed.Comment: 13 pages, 11 embedded figures. Accepted for publication in Ap
A perception and manipulation system for collecting rock samples
An important part of a planetary exploration mission is to collect and analyze surface samples. As part of the Carnegie Mellon University Ambler Project, researchers are investigating techniques for collecting samples using a robot arm and a range sensor. The aim of this work is to make the sample collection operation fully autonomous. Described here are the components of the experimental system, including a perception module that extracts objects of interest from range images and produces models of their shapes, and a manipulation module that enables the system to pick up the objects identified by the perception module. The system was tested on a small testbed using natural terrain
Formation of Sub-galactic Clouds under UV Background Radiation
The effects of the UV background radiation on the formation of sub-galactic
clouds are studied by means of one-dimensional hydrodynamical simulations. The
radiative transfer of the ionizing photons due to the absorption by HI, HeI and
HeII, neglecting the emission, is explicitly taken into account. We find that
the complete suppression of collapse occurs for the clouds with circular
velocities typically in the range V_c \sim 15-40 km/s and the 50% reduction in
the cooled gas mass with V_c \sim 20-55 km/s. These values depend most
sensitively on the collapse epoch of the cloud, the shape of the UV spectrum,
and the evolution of the UV intensity. Compared to the optically thin case,
previously investigated by Thoul & Weinberg (1996), the absorption of the
external UV photon by the intervening medium systematically lowers the above
threshold values by \Delta V_c \sim 5 km/s. Whether the gas can contract or
keeps expanding is roughly determined by the balance between the gravitational
force and the thermal pressure gradient when it is maximally exposed to the
external UV flux. Based on our simulation results, we discuss a number of
implications on galaxy formation, cosmic star formation history, and the
observations of quasar absorption lines. In Appendix, we derive analytical
formulae for the photoionization coefficients and heating rates, which
incorporate the frequency/direction-dependent transfer of external photons.Comment: 38 pages, 16 figures, accepted for publication in Ap
Bond stretching phonon softening and angle-resolved photoemission kinks in optimally doped Bi2Sr1.6La0.4Cu2O6 superconductors
We report the first measurement of the optical phonon dispersion in optimally
doped single layer Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering.
We found a strong softening of the Cu-O bond stretching phonon at about
q=(0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates.
A direct comparison with angle-resolved photoemission spectroscopy measurements
taken on the same sample, revealed an excellent agreement in terms of energy
and momentum between the ARPES nodal kink and the soft part of the bond
stretching phonon. Indeed, we find that the momentum space where a 63 meV kink
is observed can be connected with a vector q=(xi,0,0) with xi~0.22, which
corresponds exactly to the soft part of the bond stretching phonon mode. This
result supports an interpretation of the ARPES kink in terms of electron-phonon
coupling.Comment: submited to PR
The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
We use the Gaussian-fit results of Paper I to investigate the properties of
interstellar HI in the Solar neighborhood. The Warm and Cold Neutral Media (WNM
and CNM) are physically distinct components. The CNM spin temperature histogram
peaks at about 40 K. About 60% of all HI is WNM. At z=0, we derive a volume
filling fraction of about 0.50 for the WNM; this value is very rough. The
upper-limit WNM temperatures determined from line width range upward from about
500 K; a minimum of about 48% of the WNM lies in the thermally unstable region
500 to 5000 K. The WNM is a prominent constituent of the interstellar medium
and its properties depend on many factors, requiring global models that include
all relevant energy sources, of which there are many. We use Principal
Components Analysis, together with a form of least squares fitting that
accounts for errors in both the independent and dependent parameters, to
discuss the relationships among the four CNM Gaussian parameters. The spin
temperature T_s and column density N(HI) are, approximately, the two most
important eigenvectors; as such, they are sufficient, convenient, and
physically meaningful primary parameters for describing CNM clouds. The Mach
number of internal macroscopic motions for CNM clouds is typically 2.5, but
there are wide variations. We discuss the historical tau-T_s relationship in
some detail and show that it has little physical meaning. We discuss CNM
morphology using the CNM pressure known from UV stellar absorption lines.
Knowing the pressure allows us to show that CNM structures cannot be isotropic
but instead are sheetlike, with length-to-thickness aspect ratios ranging up to
about 280. We present large-scale maps of two regions where CNM lies in very
large ``blobby sheets''.Comment: Revised submission to Ap.J. Changes include: (1) correction of
turbulent Mach number in equation 16 and figure 12; the new typical value is
1.3 versus the old, incorrect value 2.5. (2) smaller typeface for the
astro-ph version to conserve paper. 60 pages, 16 figure
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