48,509 research outputs found
Operational performance of the photovoltaic-powered grain mill and water pump at Tangaye, Burkina Faso (formerly Upper Volta)
A photovoltaic (PV) system powering a grain mill and water pump was installed in the remote African village of Tangaye, Burkina Faso (formerly Upper Volta) under the sponsorship of the U.S. Agency for International Development (AID) and by the National Aeronautics and Space Administration (NASA) Lewis Research Center (LeRC) in early 1979. The presence reports covers the second two years of operation from April 1981 through June 1983. During this time, the grain mill and water pump were operational 96 and 88 percent of the time respectively, and the PV system generated sufficient electricity to enable the grinding of about 111 metric tons of finely ground flow and the pumping of over 5000 cm sq of water from the 10 m deep well. The report includes a description of the current configuration of the system, a review of system performance, a discussion of the socioeconomic impact of the system on the villagers and a summary of results and conclusions covering the entire four-year period
On the transmission of light through a single rectangular hole
In this Letter we show that a single rectangular hole exhibits transmission
resonances that appear near the cutoff wavelength of the hole waveguide. For
light polarized with the electric field pointing along the short axis, it is
shown that the normalized-to-area transmittance at resonance is proportional to
the ratio between the long and short sides, and to the dielectric constant
inside the hole. Importantly, this resonant transmission process is accompanied
by a huge enhancement of the electric field at both entrance and exit
interfaces of the hole. These findings open the possibility of using
rectangular holes for spectroscopic purposes or for exploring non-linear
effects.Comment: Submitted to PRL on Feb. 9th, 200
Proper Motions of Ionized Gas at the Galactic Center: Evidence for Unbound Orbiting Gas
We present radio continuum observations of the spiral-shaped ionized feature
(Sgr A West) within the inner pc of the Galactic center at three epochs
spanning 1986 to 1995. The VLA A-configuration was used at 2cm
(resolution of 0\dasec10\dasec2). We detect proper motions of a number
of features in the Northern and Eastern Arms of Sgr A West including the
ionized gas associated with IRS 13 with V(RA)= 113 \pm 10, V(Dec)=150 \pm15
km/s, IRS 2 with V(RA)= 122 \pm 11, V(Dec)=24 \pm 34 km/s and the Norther Arm
V(RA)= 126 \pm 30, V(Dec)=--207 \pm 58 km/s. We also report the detection of
features having transverse velocities > 1000 km/s including a head-tail radio
structure, the ``Bullet'', northwest of Sgr A with V(RA)= 722
\pm 156, V(Dec)=832 \pm 203 km/s, exceeding the escape velocity at the Galactic
center.
The proper motion measurements when combined with previous H92 radio
recombination line data suggest an unambiguous direction of the flow of ionized
gas orbiting the Galactic center. The measured velocity distribution suggests
that the ionized gas in the Northern Arm is not bound to the Galactic center
assuming a 2.5 million solar mass of dark matter residing at the Galactic
center. This implies that the stellar and ionized gas systems are not
dynamically coupled, thus, supporting a picture in which the gas features in
the Northern Arm and its extensions are the result of an energetic phenomenon
that has externally driven a cloud of gas cloud into the Galactic center.Comment: 11 pages, three figures (one color) and one table. Astrophysical
Journal Letters in pres
Desorption From Interstellar Ices
The desorption of molecular species from ice mantles back into the gas phase
in molecular clouds results from a variety of very poorly understood processes.
We have investigated three mechanisms; desorption resulting from H_2 formation
on grains, direct cosmic ray heating and cosmic ray induced photodesorption.
Whilst qualitative differences exist between these processes (essentially
deriving from the assumptions concerning the species-selectivity of the
desorption and the assumed threshold adsorption energies, E_t) all three
processes are found to be potentially very significant in dark cloud
conditions. It is therefore important that all three mechanisms should be
considered in studies of molecular clouds in which freeze-out and desorption
are believed to be important.
Employing a chemical model of a typical static molecular core and using
likely estimates for the quantum yields of the three processes we find that
desorption by H_2 formation probably dominates over the other two mechanisms.
However, the physics of the desorption processes and the nature of the dust
grains and ice mantles are very poorly constrained. We therefore conclude that
the best approach is to set empirical constraints on the desorption, based on
observed molecular depletions - rather than try to establish the desorption
efficiencies from purely theoretical considerations. Applying this method to
one such object (L1689B) yields upper limits to the desorption efficiencies
that are consistent with our understanding of these mechanisms.Comment: 11 pages, 5 figures, accepted by MNRAS subject to minor revision
which has been carried ou
Extended OH(1720 MHz) Maser Emission from Supernova Remnants
Compact OH(1720 MHz) masers have proven to be excellent signposts for the
interaction of supernova remnants with adjacent molecular clouds. Less
appreciated has been the weak, extended OH(1720 MHz) emission which accompanies
strong compact maser sources. Recent single-dish and interferometric
observations reveal the majority of maser-emitting supernova remnants have
accompanying regions of extended maser emission. Enhanced OH abundance created
by the passing shock is observed both as maser emission and absorption against
the strong background of the remnant. Modeling the observed OH profiles gives
an estimate of the physical conditions in which weak, extended maser emission
arises. I will discuss how we can realize the utility of this extended maser
emission, particularly the potential to measure the strength of the post-shock
magnetic field via Zeeman splitting over these large-scales.Comment: 5 Pages, 2 Figures, To appear in IAU 242, Astrophysical Masers and
Their Environments, eds. J. Chapman & W. Baa
Resonant transmission of light through finite chains of subwavelength holes
In this paper we show that the extraordinary optical transmission phenomenon
found before in 2D hole arrays is already present in a linear chain of
subwavelength holes, which can be considered as the basic geometrical unit
showing this property. In order to study this problem we have developed a new
theoretical framework, able to analyze the optical properties of finite
collections of subwavelength apertures and/or dimples (of any shape and placed
in arbitrary positions) drilled in a metallic film.Comment: Accepted for publication in Phys. Rev. Let
Radio Continuum Observations of the Galactic Center: Photoevaporative Proplyd-like Objects near Sgr A*
We present radio images within 30 of Sgr A* based on recent VLA
observations at 34 GHz with 7.8 microJy sensitivity and resolution
milliarcseconds (mas). We report 44 partially resolved compact
sources clustered in two regions in the E arm of ionized gas that orbits Sgr
A*. These sources have size scales ranging between ~50 and 200 mas (400 to 1600
AUs), and a bow-shock appearance facing the direction of Sgr A*. Unlike the
bow-shock sources previously identified in the near-IR but associated with
massive stars, these 34 GHz sources do not appear to have near-IR counterparts
at 3.8 m. We interpret these sources as a candidate population of
photoevaporative protoplanetary disks (proplyds) that are associated with newly
formed low mass stars with mass loss rates ~10^{-7} - 10^{-6} solar mass per
year and are located at the edge of a molecular cloud outlined by ionized gas.
The disks are externally illuminated by strong Lyman continuum radiation from
the ~100 OB and WR massive stars distributed within 10'' of Sgr A*. The
presence of proplyds implies current in-situ star formation activity near Sgr
A* and opens a window for the first time to study low mass star, planetary and
brown dwarf formations near a supermassive black hole.Comment: 13 pages, 4 figures, ApJL (in press
Hot Electron Capture Dissociation Distinguishes Leucine from Isoleucine in a Novel Hemoglobin Variant, Hb Askew, β54(D5)ValâIle
Population migration has led to the global dispersion of human hemoglobinopathies and has precipitated a need for their identification. An effective mass spectrometry-based procedure involves analysis of the intact Îą- and β-globin chains to determine their mass, followed by location of the variant amino acid residue by direct analysis of the enzymatically digested chains and low-energy collision induced dissociation of the variant peptide. Using this procedure, a variant was identified as either β54ValâLeu or β54ValâIle, since the amino acids leucine and isoleucine cannot be distinguished using low-energy collisions. Here, we describe how hot electron capture dissociation on a Fourier transform-ion cyclotron resonance mass spectrometer was used to distinguish isoleucine from leucine and identify the mutation as β54(D5)ValâIle. This is a novel variant, and we have named it Hb Askew
High-resolution Observations of OH(1720 MHz) Masers Toward the Galactic Center
High-resolution VLA observations of 1720 MHz OH maser emission from Sgr A
East and the circumnuclear disk with spatial and spectral resolutions of
2\dasec5 1\dasec3 and 0.27 \kms are reported. This follow-up
observational study focuses on the recent discovery of a number of such OH
maser features and their intense circularly polarized maser lines detected
toward these Galactic center sources. The 1720 MHz maser line of OH arises from
collisionally excited gas behind a C-type shock and is an important diagnostic
of the interaction process that may occur between molecular clouds and
associated X-ray emitting shell-type supernova remnants. The present
observations have confirmed that the observed Stokes signal is due to
Zeeman splitting and that the OH masers are angularly broadened by the
scattering medium toward the Galactic center. The scale length of the magnetic
field fluctuations in the scattering medium toward the Galactic center is
estimated to be greater than 0.1-0.2 pc using the correlation of the position
angles of the scatter-broadened maser spots. In addition, the kinematics of the
maser spots associated with Sgr A East are used to place a 5 pc displacement
between this extended radio structure and the Galactic center.Comment: 13 pages, 2 Tables and 2 figures, to be published in Ap
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